use this feature without a clearance from a patch
distributor. Removal (rmmod) of patch modules is permanently
disabled when the feature is used. See
- Documentation/livepatch/livepatch.txt for more information.
+ Documentation/livepatch/livepatch.rst for more information.
What: /sys/kernel/livepatch/<patch>/<object>
Date: Nov 2014
--- /dev/null
+======================
+(Un)patching Callbacks
+======================
+
+Livepatch (un)patch-callbacks provide a mechanism for livepatch modules
+to execute callback functions when a kernel object is (un)patched. They
+can be considered a "power feature" that extends livepatching abilities
+to include:
+
+ - Safe updates to global data
+
+ - "Patches" to init and probe functions
+
+ - Patching otherwise unpatchable code (i.e. assembly)
+
+In most cases, (un)patch callbacks will need to be used in conjunction
+with memory barriers and kernel synchronization primitives, like
+mutexes/spinlocks, or even stop_machine(), to avoid concurrency issues.
+
+Callbacks differ from existing kernel facilities:
+
+ - Module init/exit code doesn't run when disabling and re-enabling a
+ patch.
+
+ - A module notifier can't stop a to-be-patched module from loading.
+
+Callbacks are part of the klp_object structure and their implementation
+is specific to that klp_object. Other livepatch objects may or may not
+be patched, irrespective of the target klp_object's current state.
+
+Callbacks can be registered for the following livepatch actions:
+
+ * Pre-patch
+ - before a klp_object is patched
+
+ * Post-patch
+ - after a klp_object has been patched and is active
+ across all tasks
+
+ * Pre-unpatch
+ - before a klp_object is unpatched (ie, patched code is
+ active), used to clean up post-patch callback
+ resources
+
+ * Post-unpatch
+ - after a klp_object has been patched, all code has
+ been restored and no tasks are running patched code,
+ used to cleanup pre-patch callback resources
+
+Each callback is optional, omitting one does not preclude specifying any
+other. However, the livepatching core executes the handlers in
+symmetry: pre-patch callbacks have a post-unpatch counterpart and
+post-patch callbacks have a pre-unpatch counterpart. An unpatch
+callback will only be executed if its corresponding patch callback was
+executed. Typical use cases pair a patch handler that acquires and
+configures resources with an unpatch handler tears down and releases
+those same resources.
+
+A callback is only executed if its host klp_object is loaded. For
+in-kernel vmlinux targets, this means that callbacks will always execute
+when a livepatch is enabled/disabled. For patch target kernel modules,
+callbacks will only execute if the target module is loaded. When a
+module target is (un)loaded, its callbacks will execute only if the
+livepatch module is enabled.
+
+The pre-patch callback, if specified, is expected to return a status
+code (0 for success, -ERRNO on error). An error status code indicates
+to the livepatching core that patching of the current klp_object is not
+safe and to stop the current patching request. (When no pre-patch
+callback is provided, the transition is assumed to be safe.) If a
+pre-patch callback returns failure, the kernel's module loader will:
+
+ - Refuse to load a livepatch, if the livepatch is loaded after
+ targeted code.
+
+ or:
+
+ - Refuse to load a module, if the livepatch was already successfully
+ loaded.
+
+No post-patch, pre-unpatch, or post-unpatch callbacks will be executed
+for a given klp_object if the object failed to patch, due to a failed
+pre_patch callback or for any other reason.
+
+If a patch transition is reversed, no pre-unpatch handlers will be run
+(this follows the previously mentioned symmetry -- pre-unpatch callbacks
+will only occur if their corresponding post-patch callback executed).
+
+If the object did successfully patch, but the patch transition never
+started for some reason (e.g., if another object failed to patch),
+only the post-unpatch callback will be called.
+
+
+Example Use-cases
+=================
+
+Update global data
+------------------
+
+A pre-patch callback can be useful to update a global variable. For
+example, 75ff39ccc1bd ("tcp: make challenge acks less predictable")
+changes a global sysctl, as well as patches the tcp_send_challenge_ack()
+function.
+
+In this case, if we're being super paranoid, it might make sense to
+patch the data *after* patching is complete with a post-patch callback,
+so that tcp_send_challenge_ack() could first be changed to read
+sysctl_tcp_challenge_ack_limit with READ_ONCE.
+
+
+Support __init and probe function patches
+-----------------------------------------
+
+Although __init and probe functions are not directly livepatch-able, it
+may be possible to implement similar updates via pre/post-patch
+callbacks.
+
+48900cb6af42 ("virtio-net: drop NETIF_F_FRAGLIST") change the way that
+virtnet_probe() initialized its driver's net_device features. A
+pre/post-patch callback could iterate over all such devices, making a
+similar change to their hw_features value. (Client functions of the
+value may need to be updated accordingly.)
+
+
+Other Examples
+==============
+
+Sample livepatch modules demonstrating the callback API can be found in
+samples/livepatch/ directory. These samples were modified for use in
+kselftests and can be found in the lib/livepatch directory.
+++ /dev/null
-======================
-(Un)patching Callbacks
-======================
-
-Livepatch (un)patch-callbacks provide a mechanism for livepatch modules
-to execute callback functions when a kernel object is (un)patched. They
-can be considered a "power feature" that extends livepatching abilities
-to include:
-
- - Safe updates to global data
-
- - "Patches" to init and probe functions
-
- - Patching otherwise unpatchable code (i.e. assembly)
-
-In most cases, (un)patch callbacks will need to be used in conjunction
-with memory barriers and kernel synchronization primitives, like
-mutexes/spinlocks, or even stop_machine(), to avoid concurrency issues.
-
-Callbacks differ from existing kernel facilities:
-
- - Module init/exit code doesn't run when disabling and re-enabling a
- patch.
-
- - A module notifier can't stop a to-be-patched module from loading.
-
-Callbacks are part of the klp_object structure and their implementation
-is specific to that klp_object. Other livepatch objects may or may not
-be patched, irrespective of the target klp_object's current state.
-
-Callbacks can be registered for the following livepatch actions:
-
- * Pre-patch - before a klp_object is patched
-
- * Post-patch - after a klp_object has been patched and is active
- across all tasks
-
- * Pre-unpatch - before a klp_object is unpatched (ie, patched code is
- active), used to clean up post-patch callback
- resources
-
- * Post-unpatch - after a klp_object has been patched, all code has
- been restored and no tasks are running patched code,
- used to cleanup pre-patch callback resources
-
-Each callback is optional, omitting one does not preclude specifying any
-other. However, the livepatching core executes the handlers in
-symmetry: pre-patch callbacks have a post-unpatch counterpart and
-post-patch callbacks have a pre-unpatch counterpart. An unpatch
-callback will only be executed if its corresponding patch callback was
-executed. Typical use cases pair a patch handler that acquires and
-configures resources with an unpatch handler tears down and releases
-those same resources.
-
-A callback is only executed if its host klp_object is loaded. For
-in-kernel vmlinux targets, this means that callbacks will always execute
-when a livepatch is enabled/disabled. For patch target kernel modules,
-callbacks will only execute if the target module is loaded. When a
-module target is (un)loaded, its callbacks will execute only if the
-livepatch module is enabled.
-
-The pre-patch callback, if specified, is expected to return a status
-code (0 for success, -ERRNO on error). An error status code indicates
-to the livepatching core that patching of the current klp_object is not
-safe and to stop the current patching request. (When no pre-patch
-callback is provided, the transition is assumed to be safe.) If a
-pre-patch callback returns failure, the kernel's module loader will:
-
- - Refuse to load a livepatch, if the livepatch is loaded after
- targeted code.
-
- or:
-
- - Refuse to load a module, if the livepatch was already successfully
- loaded.
-
-No post-patch, pre-unpatch, or post-unpatch callbacks will be executed
-for a given klp_object if the object failed to patch, due to a failed
-pre_patch callback or for any other reason.
-
-If a patch transition is reversed, no pre-unpatch handlers will be run
-(this follows the previously mentioned symmetry -- pre-unpatch callbacks
-will only occur if their corresponding post-patch callback executed).
-
-If the object did successfully patch, but the patch transition never
-started for some reason (e.g., if another object failed to patch),
-only the post-unpatch callback will be called.
-
-
-Example Use-cases
-=================
-
-Update global data
-------------------
-
-A pre-patch callback can be useful to update a global variable. For
-example, 75ff39ccc1bd ("tcp: make challenge acks less predictable")
-changes a global sysctl, as well as patches the tcp_send_challenge_ack()
-function.
-
-In this case, if we're being super paranoid, it might make sense to
-patch the data *after* patching is complete with a post-patch callback,
-so that tcp_send_challenge_ack() could first be changed to read
-sysctl_tcp_challenge_ack_limit with READ_ONCE.
-
-
-Support __init and probe function patches
------------------------------------------
-
-Although __init and probe functions are not directly livepatch-able, it
-may be possible to implement similar updates via pre/post-patch
-callbacks.
-
-48900cb6af42 ("virtio-net: drop NETIF_F_FRAGLIST") change the way that
-virtnet_probe() initialized its driver's net_device features. A
-pre/post-patch callback could iterate over all such devices, making a
-similar change to their hw_features value. (Client functions of the
-value may need to be updated accordingly.)
-
-
-Other Examples
-==============
-
-Sample livepatch modules demonstrating the callback API can be found in
-samples/livepatch/ directory. These samples were modified for use in
-kselftests and can be found in the lib/livepatch directory.
--- /dev/null
+===================================
+Atomic Replace & Cumulative Patches
+===================================
+
+There might be dependencies between livepatches. If multiple patches need
+to do different changes to the same function(s) then we need to define
+an order in which the patches will be installed. And function implementations
+from any newer livepatch must be done on top of the older ones.
+
+This might become a maintenance nightmare. Especially when more patches
+modified the same function in different ways.
+
+An elegant solution comes with the feature called "Atomic Replace". It allows
+creation of so called "Cumulative Patches". They include all wanted changes
+from all older livepatches and completely replace them in one transition.
+
+Usage
+-----
+
+The atomic replace can be enabled by setting "replace" flag in struct klp_patch,
+for example::
+
+ static struct klp_patch patch = {
+ .mod = THIS_MODULE,
+ .objs = objs,
+ .replace = true,
+ };
+
+All processes are then migrated to use the code only from the new patch.
+Once the transition is finished, all older patches are automatically
+disabled.
+
+Ftrace handlers are transparently removed from functions that are no
+longer modified by the new cumulative patch.
+
+As a result, the livepatch authors might maintain sources only for one
+cumulative patch. It helps to keep the patch consistent while adding or
+removing various fixes or features.
+
+Users could keep only the last patch installed on the system after
+the transition to has finished. It helps to clearly see what code is
+actually in use. Also the livepatch might then be seen as a "normal"
+module that modifies the kernel behavior. The only difference is that
+it can be updated at runtime without breaking its functionality.
+
+
+Features
+--------
+
+The atomic replace allows:
+
+ - Atomically revert some functions in a previous patch while
+ upgrading other functions.
+
+ - Remove eventual performance impact caused by core redirection
+ for functions that are no longer patched.
+
+ - Decrease user confusion about dependencies between livepatches.
+
+
+Limitations:
+------------
+
+ - Once the operation finishes, there is no straightforward way
+ to reverse it and restore the replaced patches atomically.
+
+ A good practice is to set .replace flag in any released livepatch.
+ Then re-adding an older livepatch is equivalent to downgrading
+ to that patch. This is safe as long as the livepatches do _not_ do
+ extra modifications in (un)patching callbacks or in the module_init()
+ or module_exit() functions, see below.
+
+ Also note that the replaced patch can be removed and loaded again
+ only when the transition was not forced.
+
+
+ - Only the (un)patching callbacks from the _new_ cumulative livepatch are
+ executed. Any callbacks from the replaced patches are ignored.
+
+ In other words, the cumulative patch is responsible for doing any actions
+ that are necessary to properly replace any older patch.
+
+ As a result, it might be dangerous to replace newer cumulative patches by
+ older ones. The old livepatches might not provide the necessary callbacks.
+
+ This might be seen as a limitation in some scenarios. But it makes life
+ easier in many others. Only the new cumulative livepatch knows what
+ fixes/features are added/removed and what special actions are necessary
+ for a smooth transition.
+
+ In any case, it would be a nightmare to think about the order of
+ the various callbacks and their interactions if the callbacks from all
+ enabled patches were called.
+
+
+ - There is no special handling of shadow variables. Livepatch authors
+ must create their own rules how to pass them from one cumulative
+ patch to the other. Especially that they should not blindly remove
+ them in module_exit() functions.
+
+ A good practice might be to remove shadow variables in the post-unpatch
+ callback. It is called only when the livepatch is properly disabled.
+++ /dev/null
-===================================
-Atomic Replace & Cumulative Patches
-===================================
-
-There might be dependencies between livepatches. If multiple patches need
-to do different changes to the same function(s) then we need to define
-an order in which the patches will be installed. And function implementations
-from any newer livepatch must be done on top of the older ones.
-
-This might become a maintenance nightmare. Especially when more patches
-modified the same function in different ways.
-
-An elegant solution comes with the feature called "Atomic Replace". It allows
-creation of so called "Cumulative Patches". They include all wanted changes
-from all older livepatches and completely replace them in one transition.
-
-Usage
------
-
-The atomic replace can be enabled by setting "replace" flag in struct klp_patch,
-for example:
-
- static struct klp_patch patch = {
- .mod = THIS_MODULE,
- .objs = objs,
- .replace = true,
- };
-
-All processes are then migrated to use the code only from the new patch.
-Once the transition is finished, all older patches are automatically
-disabled.
-
-Ftrace handlers are transparently removed from functions that are no
-longer modified by the new cumulative patch.
-
-As a result, the livepatch authors might maintain sources only for one
-cumulative patch. It helps to keep the patch consistent while adding or
-removing various fixes or features.
-
-Users could keep only the last patch installed on the system after
-the transition to has finished. It helps to clearly see what code is
-actually in use. Also the livepatch might then be seen as a "normal"
-module that modifies the kernel behavior. The only difference is that
-it can be updated at runtime without breaking its functionality.
-
-
-Features
---------
-
-The atomic replace allows:
-
- + Atomically revert some functions in a previous patch while
- upgrading other functions.
-
- + Remove eventual performance impact caused by core redirection
- for functions that are no longer patched.
-
- + Decrease user confusion about dependencies between livepatches.
-
-
-Limitations:
-------------
-
- + Once the operation finishes, there is no straightforward way
- to reverse it and restore the replaced patches atomically.
-
- A good practice is to set .replace flag in any released livepatch.
- Then re-adding an older livepatch is equivalent to downgrading
- to that patch. This is safe as long as the livepatches do _not_ do
- extra modifications in (un)patching callbacks or in the module_init()
- or module_exit() functions, see below.
-
- Also note that the replaced patch can be removed and loaded again
- only when the transition was not forced.
-
-
- + Only the (un)patching callbacks from the _new_ cumulative livepatch are
- executed. Any callbacks from the replaced patches are ignored.
-
- In other words, the cumulative patch is responsible for doing any actions
- that are necessary to properly replace any older patch.
-
- As a result, it might be dangerous to replace newer cumulative patches by
- older ones. The old livepatches might not provide the necessary callbacks.
-
- This might be seen as a limitation in some scenarios. But it makes life
- easier in many others. Only the new cumulative livepatch knows what
- fixes/features are added/removed and what special actions are necessary
- for a smooth transition.
-
- In any case, it would be a nightmare to think about the order of
- the various callbacks and their interactions if the callbacks from all
- enabled patches were called.
-
-
- + There is no special handling of shadow variables. Livepatch authors
- must create their own rules how to pass them from one cumulative
- patch to the other. Especially that they should not blindly remove
- them in module_exit() functions.
-
- A good practice might be to remove shadow variables in the post-unpatch
- callback. It is called only when the livepatch is properly disabled.
--- /dev/null
+:orphan:
+
+===================
+Kernel Livepatching
+===================
+
+.. toctree::
+ :maxdepth: 1
+
+ livepatch
+ callbacks
+ cumulative-patches
+ module-elf-format
+ shadow-vars
+
+.. only:: subproject and html
+
+ Indices
+ =======
+
+ * :ref:`genindex`
--- /dev/null
+=========
+Livepatch
+=========
+
+This document outlines basic information about kernel livepatching.
+
+.. Table of Contents:
+
+ 1. Motivation
+ 2. Kprobes, Ftrace, Livepatching
+ 3. Consistency model
+ 4. Livepatch module
+ 4.1. New functions
+ 4.2. Metadata
+ 5. Livepatch life-cycle
+ 5.1. Loading
+ 5.2. Enabling
+ 5.3. Replacing
+ 5.4. Disabling
+ 5.5. Removing
+ 6. Sysfs
+ 7. Limitations
+
+
+1. Motivation
+=============
+
+There are many situations where users are reluctant to reboot a system. It may
+be because their system is performing complex scientific computations or under
+heavy load during peak usage. In addition to keeping systems up and running,
+users want to also have a stable and secure system. Livepatching gives users
+both by allowing for function calls to be redirected; thus, fixing critical
+functions without a system reboot.
+
+
+2. Kprobes, Ftrace, Livepatching
+================================
+
+There are multiple mechanisms in the Linux kernel that are directly related
+to redirection of code execution; namely: kernel probes, function tracing,
+and livepatching:
+
+ - The kernel probes are the most generic. The code can be redirected by
+ putting a breakpoint instruction instead of any instruction.
+
+ - The function tracer calls the code from a predefined location that is
+ close to the function entry point. This location is generated by the
+ compiler using the '-pg' gcc option.
+
+ - Livepatching typically needs to redirect the code at the very beginning
+ of the function entry before the function parameters or the stack
+ are in any way modified.
+
+All three approaches need to modify the existing code at runtime. Therefore
+they need to be aware of each other and not step over each other's toes.
+Most of these problems are solved by using the dynamic ftrace framework as
+a base. A Kprobe is registered as a ftrace handler when the function entry
+is probed, see CONFIG_KPROBES_ON_FTRACE. Also an alternative function from
+a live patch is called with the help of a custom ftrace handler. But there are
+some limitations, see below.
+
+
+3. Consistency model
+====================
+
+Functions are there for a reason. They take some input parameters, get or
+release locks, read, process, and even write some data in a defined way,
+have return values. In other words, each function has a defined semantic.
+
+Many fixes do not change the semantic of the modified functions. For
+example, they add a NULL pointer or a boundary check, fix a race by adding
+a missing memory barrier, or add some locking around a critical section.
+Most of these changes are self contained and the function presents itself
+the same way to the rest of the system. In this case, the functions might
+be updated independently one by one.
+
+But there are more complex fixes. For example, a patch might change
+ordering of locking in multiple functions at the same time. Or a patch
+might exchange meaning of some temporary structures and update
+all the relevant functions. In this case, the affected unit
+(thread, whole kernel) need to start using all new versions of
+the functions at the same time. Also the switch must happen only
+when it is safe to do so, e.g. when the affected locks are released
+or no data are stored in the modified structures at the moment.
+
+The theory about how to apply functions a safe way is rather complex.
+The aim is to define a so-called consistency model. It attempts to define
+conditions when the new implementation could be used so that the system
+stays consistent.
+
+Livepatch has a consistency model which is a hybrid of kGraft and
+kpatch: it uses kGraft's per-task consistency and syscall barrier
+switching combined with kpatch's stack trace switching. There are also
+a number of fallback options which make it quite flexible.
+
+Patches are applied on a per-task basis, when the task is deemed safe to
+switch over. When a patch is enabled, livepatch enters into a
+transition state where tasks are converging to the patched state.
+Usually this transition state can complete in a few seconds. The same
+sequence occurs when a patch is disabled, except the tasks converge from
+the patched state to the unpatched state.
+
+An interrupt handler inherits the patched state of the task it
+interrupts. The same is true for forked tasks: the child inherits the
+patched state of the parent.
+
+Livepatch uses several complementary approaches to determine when it's
+safe to patch tasks:
+
+1. The first and most effective approach is stack checking of sleeping
+ tasks. If no affected functions are on the stack of a given task,
+ the task is patched. In most cases this will patch most or all of
+ the tasks on the first try. Otherwise it'll keep trying
+ periodically. This option is only available if the architecture has
+ reliable stacks (HAVE_RELIABLE_STACKTRACE).
+
+2. The second approach, if needed, is kernel exit switching. A
+ task is switched when it returns to user space from a system call, a
+ user space IRQ, or a signal. It's useful in the following cases:
+
+ a) Patching I/O-bound user tasks which are sleeping on an affected
+ function. In this case you have to send SIGSTOP and SIGCONT to
+ force it to exit the kernel and be patched.
+ b) Patching CPU-bound user tasks. If the task is highly CPU-bound
+ then it will get patched the next time it gets interrupted by an
+ IRQ.
+
+3. For idle "swapper" tasks, since they don't ever exit the kernel, they
+ instead have a klp_update_patch_state() call in the idle loop which
+ allows them to be patched before the CPU enters the idle state.
+
+ (Note there's not yet such an approach for kthreads.)
+
+Architectures which don't have HAVE_RELIABLE_STACKTRACE solely rely on
+the second approach. It's highly likely that some tasks may still be
+running with an old version of the function, until that function
+returns. In this case you would have to signal the tasks. This
+especially applies to kthreads. They may not be woken up and would need
+to be forced. See below for more information.
+
+Unless we can come up with another way to patch kthreads, architectures
+without HAVE_RELIABLE_STACKTRACE are not considered fully supported by
+the kernel livepatching.
+
+The /sys/kernel/livepatch/<patch>/transition file shows whether a patch
+is in transition. Only a single patch can be in transition at a given
+time. A patch can remain in transition indefinitely, if any of the tasks
+are stuck in the initial patch state.
+
+A transition can be reversed and effectively canceled by writing the
+opposite value to the /sys/kernel/livepatch/<patch>/enabled file while
+the transition is in progress. Then all the tasks will attempt to
+converge back to the original patch state.
+
+There's also a /proc/<pid>/patch_state file which can be used to
+determine which tasks are blocking completion of a patching operation.
+If a patch is in transition, this file shows 0 to indicate the task is
+unpatched and 1 to indicate it's patched. Otherwise, if no patch is in
+transition, it shows -1. Any tasks which are blocking the transition
+can be signaled with SIGSTOP and SIGCONT to force them to change their
+patched state. This may be harmful to the system though. Sending a fake signal
+to all remaining blocking tasks is a better alternative. No proper signal is
+actually delivered (there is no data in signal pending structures). Tasks are
+interrupted or woken up, and forced to change their patched state. The fake
+signal is automatically sent every 15 seconds.
+
+Administrator can also affect a transition through
+/sys/kernel/livepatch/<patch>/force attribute. Writing 1 there clears
+TIF_PATCH_PENDING flag of all tasks and thus forces the tasks to the patched
+state. Important note! The force attribute is intended for cases when the
+transition gets stuck for a long time because of a blocking task. Administrator
+is expected to collect all necessary data (namely stack traces of such blocking
+tasks) and request a clearance from a patch distributor to force the transition.
+Unauthorized usage may cause harm to the system. It depends on the nature of the
+patch, which functions are (un)patched, and which functions the blocking tasks
+are sleeping in (/proc/<pid>/stack may help here). Removal (rmmod) of patch
+modules is permanently disabled when the force feature is used. It cannot be
+guaranteed there is no task sleeping in such module. It implies unbounded
+reference count if a patch module is disabled and enabled in a loop.
+
+Moreover, the usage of force may also affect future applications of live
+patches and cause even more harm to the system. Administrator should first
+consider to simply cancel a transition (see above). If force is used, reboot
+should be planned and no more live patches applied.
+
+3.1 Adding consistency model support to new architectures
+---------------------------------------------------------
+
+For adding consistency model support to new architectures, there are a
+few options:
+
+1) Add CONFIG_HAVE_RELIABLE_STACKTRACE. This means porting objtool, and
+ for non-DWARF unwinders, also making sure there's a way for the stack
+ tracing code to detect interrupts on the stack.
+
+2) Alternatively, ensure that every kthread has a call to
+ klp_update_patch_state() in a safe location. Kthreads are typically
+ in an infinite loop which does some action repeatedly. The safe
+ location to switch the kthread's patch state would be at a designated
+ point in the loop where there are no locks taken and all data
+ structures are in a well-defined state.
+
+ The location is clear when using workqueues or the kthread worker
+ API. These kthreads process independent actions in a generic loop.
+
+ It's much more complicated with kthreads which have a custom loop.
+ There the safe location must be carefully selected on a case-by-case
+ basis.
+
+ In that case, arches without HAVE_RELIABLE_STACKTRACE would still be
+ able to use the non-stack-checking parts of the consistency model:
+
+ a) patching user tasks when they cross the kernel/user space
+ boundary; and
+
+ b) patching kthreads and idle tasks at their designated patch points.
+
+ This option isn't as good as option 1 because it requires signaling
+ user tasks and waking kthreads to patch them. But it could still be
+ a good backup option for those architectures which don't have
+ reliable stack traces yet.
+
+
+4. Livepatch module
+===================
+
+Livepatches are distributed using kernel modules, see
+samples/livepatch/livepatch-sample.c.
+
+The module includes a new implementation of functions that we want
+to replace. In addition, it defines some structures describing the
+relation between the original and the new implementation. Then there
+is code that makes the kernel start using the new code when the livepatch
+module is loaded. Also there is code that cleans up before the
+livepatch module is removed. All this is explained in more details in
+the next sections.
+
+
+4.1. New functions
+------------------
+
+New versions of functions are typically just copied from the original
+sources. A good practice is to add a prefix to the names so that they
+can be distinguished from the original ones, e.g. in a backtrace. Also
+they can be declared as static because they are not called directly
+and do not need the global visibility.
+
+The patch contains only functions that are really modified. But they
+might want to access functions or data from the original source file
+that may only be locally accessible. This can be solved by a special
+relocation section in the generated livepatch module, see
+Documentation/livepatch/module-elf-format.rst for more details.
+
+
+4.2. Metadata
+-------------
+
+The patch is described by several structures that split the information
+into three levels:
+
+ - struct klp_func is defined for each patched function. It describes
+ the relation between the original and the new implementation of a
+ particular function.
+
+ The structure includes the name, as a string, of the original function.
+ The function address is found via kallsyms at runtime.
+
+ Then it includes the address of the new function. It is defined
+ directly by assigning the function pointer. Note that the new
+ function is typically defined in the same source file.
+
+ As an optional parameter, the symbol position in the kallsyms database can
+ be used to disambiguate functions of the same name. This is not the
+ absolute position in the database, but rather the order it has been found
+ only for a particular object ( vmlinux or a kernel module ). Note that
+ kallsyms allows for searching symbols according to the object name.
+
+ - struct klp_object defines an array of patched functions (struct
+ klp_func) in the same object. Where the object is either vmlinux
+ (NULL) or a module name.
+
+ The structure helps to group and handle functions for each object
+ together. Note that patched modules might be loaded later than
+ the patch itself and the relevant functions might be patched
+ only when they are available.
+
+
+ - struct klp_patch defines an array of patched objects (struct
+ klp_object).
+
+ This structure handles all patched functions consistently and eventually,
+ synchronously. The whole patch is applied only when all patched
+ symbols are found. The only exception are symbols from objects
+ (kernel modules) that have not been loaded yet.
+
+ For more details on how the patch is applied on a per-task basis,
+ see the "Consistency model" section.
+
+
+5. Livepatch life-cycle
+=======================
+
+Livepatching can be described by five basic operations:
+loading, enabling, replacing, disabling, removing.
+
+Where the replacing and the disabling operations are mutually
+exclusive. They have the same result for the given patch but
+not for the system.
+
+
+5.1. Loading
+------------
+
+The only reasonable way is to enable the patch when the livepatch kernel
+module is being loaded. For this, klp_enable_patch() has to be called
+in the module_init() callback. There are two main reasons:
+
+First, only the module has an easy access to the related struct klp_patch.
+
+Second, the error code might be used to refuse loading the module when
+the patch cannot get enabled.
+
+
+5.2. Enabling
+-------------
+
+The livepatch gets enabled by calling klp_enable_patch() from
+the module_init() callback. The system will start using the new
+implementation of the patched functions at this stage.
+
+First, the addresses of the patched functions are found according to their
+names. The special relocations, mentioned in the section "New functions",
+are applied. The relevant entries are created under
+/sys/kernel/livepatch/<name>. The patch is rejected when any above
+operation fails.
+
+Second, livepatch enters into a transition state where tasks are converging
+to the patched state. If an original function is patched for the first
+time, a function specific struct klp_ops is created and an universal
+ftrace handler is registered\ [#]_. This stage is indicated by a value of '1'
+in /sys/kernel/livepatch/<name>/transition. For more information about
+this process, see the "Consistency model" section.
+
+Finally, once all tasks have been patched, the 'transition' value changes
+to '0'.
+
+.. [#]
+
+ Note that functions might be patched multiple times. The ftrace handler
+ is registered only once for a given function. Further patches just add
+ an entry to the list (see field `func_stack`) of the struct klp_ops.
+ The right implementation is selected by the ftrace handler, see
+ the "Consistency model" section.
+
+ That said, it is highly recommended to use cumulative livepatches
+ because they help keeping the consistency of all changes. In this case,
+ functions might be patched two times only during the transition period.
+
+
+5.3. Replacing
+--------------
+
+All enabled patches might get replaced by a cumulative patch that
+has the .replace flag set.
+
+Once the new patch is enabled and the 'transition' finishes then
+all the functions (struct klp_func) associated with the replaced
+patches are removed from the corresponding struct klp_ops. Also
+the ftrace handler is unregistered and the struct klp_ops is
+freed when the related function is not modified by the new patch
+and func_stack list becomes empty.
+
+See Documentation/livepatch/cumulative-patches.rst for more details.
+
+
+5.4. Disabling
+--------------
+
+Enabled patches might get disabled by writing '0' to
+/sys/kernel/livepatch/<name>/enabled.
+
+First, livepatch enters into a transition state where tasks are converging
+to the unpatched state. The system starts using either the code from
+the previously enabled patch or even the original one. This stage is
+indicated by a value of '1' in /sys/kernel/livepatch/<name>/transition.
+For more information about this process, see the "Consistency model"
+section.
+
+Second, once all tasks have been unpatched, the 'transition' value changes
+to '0'. All the functions (struct klp_func) associated with the to-be-disabled
+patch are removed from the corresponding struct klp_ops. The ftrace handler
+is unregistered and the struct klp_ops is freed when the func_stack list
+becomes empty.
+
+Third, the sysfs interface is destroyed.
+
+
+5.5. Removing
+-------------
+
+Module removal is only safe when there are no users of functions provided
+by the module. This is the reason why the force feature permanently
+disables the removal. Only when the system is successfully transitioned
+to a new patch state (patched/unpatched) without being forced it is
+guaranteed that no task sleeps or runs in the old code.
+
+
+6. Sysfs
+========
+
+Information about the registered patches can be found under
+/sys/kernel/livepatch. The patches could be enabled and disabled
+by writing there.
+
+/sys/kernel/livepatch/<patch>/force attributes allow administrator to affect a
+patching operation.
+
+See Documentation/ABI/testing/sysfs-kernel-livepatch for more details.
+
+
+7. Limitations
+==============
+
+The current Livepatch implementation has several limitations:
+
+ - Only functions that can be traced could be patched.
+
+ Livepatch is based on the dynamic ftrace. In particular, functions
+ implementing ftrace or the livepatch ftrace handler could not be
+ patched. Otherwise, the code would end up in an infinite loop. A
+ potential mistake is prevented by marking the problematic functions
+ by "notrace".
+
+
+
+ - Livepatch works reliably only when the dynamic ftrace is located at
+ the very beginning of the function.
+
+ The function need to be redirected before the stack or the function
+ parameters are modified in any way. For example, livepatch requires
+ using -fentry gcc compiler option on x86_64.
+
+ One exception is the PPC port. It uses relative addressing and TOC.
+ Each function has to handle TOC and save LR before it could call
+ the ftrace handler. This operation has to be reverted on return.
+ Fortunately, the generic ftrace code has the same problem and all
+ this is handled on the ftrace level.
+
+
+ - Kretprobes using the ftrace framework conflict with the patched
+ functions.
+
+ Both kretprobes and livepatches use a ftrace handler that modifies
+ the return address. The first user wins. Either the probe or the patch
+ is rejected when the handler is already in use by the other.
+
+
+ - Kprobes in the original function are ignored when the code is
+ redirected to the new implementation.
+
+ There is a work in progress to add warnings about this situation.
+++ /dev/null
-=========
-Livepatch
-=========
-
-This document outlines basic information about kernel livepatching.
-
-Table of Contents:
-
-1. Motivation
-2. Kprobes, Ftrace, Livepatching
-3. Consistency model
-4. Livepatch module
- 4.1. New functions
- 4.2. Metadata
-5. Livepatch life-cycle
- 5.1. Loading
- 5.2. Enabling
- 5.3. Replacing
- 5.4. Disabling
- 5.5. Removing
-6. Sysfs
-7. Limitations
-
-
-1. Motivation
-=============
-
-There are many situations where users are reluctant to reboot a system. It may
-be because their system is performing complex scientific computations or under
-heavy load during peak usage. In addition to keeping systems up and running,
-users want to also have a stable and secure system. Livepatching gives users
-both by allowing for function calls to be redirected; thus, fixing critical
-functions without a system reboot.
-
-
-2. Kprobes, Ftrace, Livepatching
-================================
-
-There are multiple mechanisms in the Linux kernel that are directly related
-to redirection of code execution; namely: kernel probes, function tracing,
-and livepatching:
-
- + The kernel probes are the most generic. The code can be redirected by
- putting a breakpoint instruction instead of any instruction.
-
- + The function tracer calls the code from a predefined location that is
- close to the function entry point. This location is generated by the
- compiler using the '-pg' gcc option.
-
- + Livepatching typically needs to redirect the code at the very beginning
- of the function entry before the function parameters or the stack
- are in any way modified.
-
-All three approaches need to modify the existing code at runtime. Therefore
-they need to be aware of each other and not step over each other's toes.
-Most of these problems are solved by using the dynamic ftrace framework as
-a base. A Kprobe is registered as a ftrace handler when the function entry
-is probed, see CONFIG_KPROBES_ON_FTRACE. Also an alternative function from
-a live patch is called with the help of a custom ftrace handler. But there are
-some limitations, see below.
-
-
-3. Consistency model
-====================
-
-Functions are there for a reason. They take some input parameters, get or
-release locks, read, process, and even write some data in a defined way,
-have return values. In other words, each function has a defined semantic.
-
-Many fixes do not change the semantic of the modified functions. For
-example, they add a NULL pointer or a boundary check, fix a race by adding
-a missing memory barrier, or add some locking around a critical section.
-Most of these changes are self contained and the function presents itself
-the same way to the rest of the system. In this case, the functions might
-be updated independently one by one.
-
-But there are more complex fixes. For example, a patch might change
-ordering of locking in multiple functions at the same time. Or a patch
-might exchange meaning of some temporary structures and update
-all the relevant functions. In this case, the affected unit
-(thread, whole kernel) need to start using all new versions of
-the functions at the same time. Also the switch must happen only
-when it is safe to do so, e.g. when the affected locks are released
-or no data are stored in the modified structures at the moment.
-
-The theory about how to apply functions a safe way is rather complex.
-The aim is to define a so-called consistency model. It attempts to define
-conditions when the new implementation could be used so that the system
-stays consistent.
-
-Livepatch has a consistency model which is a hybrid of kGraft and
-kpatch: it uses kGraft's per-task consistency and syscall barrier
-switching combined with kpatch's stack trace switching. There are also
-a number of fallback options which make it quite flexible.
-
-Patches are applied on a per-task basis, when the task is deemed safe to
-switch over. When a patch is enabled, livepatch enters into a
-transition state where tasks are converging to the patched state.
-Usually this transition state can complete in a few seconds. The same
-sequence occurs when a patch is disabled, except the tasks converge from
-the patched state to the unpatched state.
-
-An interrupt handler inherits the patched state of the task it
-interrupts. The same is true for forked tasks: the child inherits the
-patched state of the parent.
-
-Livepatch uses several complementary approaches to determine when it's
-safe to patch tasks:
-
-1. The first and most effective approach is stack checking of sleeping
- tasks. If no affected functions are on the stack of a given task,
- the task is patched. In most cases this will patch most or all of
- the tasks on the first try. Otherwise it'll keep trying
- periodically. This option is only available if the architecture has
- reliable stacks (HAVE_RELIABLE_STACKTRACE).
-
-2. The second approach, if needed, is kernel exit switching. A
- task is switched when it returns to user space from a system call, a
- user space IRQ, or a signal. It's useful in the following cases:
-
- a) Patching I/O-bound user tasks which are sleeping on an affected
- function. In this case you have to send SIGSTOP and SIGCONT to
- force it to exit the kernel and be patched.
- b) Patching CPU-bound user tasks. If the task is highly CPU-bound
- then it will get patched the next time it gets interrupted by an
- IRQ.
-
-3. For idle "swapper" tasks, since they don't ever exit the kernel, they
- instead have a klp_update_patch_state() call in the idle loop which
- allows them to be patched before the CPU enters the idle state.
-
- (Note there's not yet such an approach for kthreads.)
-
-Architectures which don't have HAVE_RELIABLE_STACKTRACE solely rely on
-the second approach. It's highly likely that some tasks may still be
-running with an old version of the function, until that function
-returns. In this case you would have to signal the tasks. This
-especially applies to kthreads. They may not be woken up and would need
-to be forced. See below for more information.
-
-Unless we can come up with another way to patch kthreads, architectures
-without HAVE_RELIABLE_STACKTRACE are not considered fully supported by
-the kernel livepatching.
-
-The /sys/kernel/livepatch/<patch>/transition file shows whether a patch
-is in transition. Only a single patch can be in transition at a given
-time. A patch can remain in transition indefinitely, if any of the tasks
-are stuck in the initial patch state.
-
-A transition can be reversed and effectively canceled by writing the
-opposite value to the /sys/kernel/livepatch/<patch>/enabled file while
-the transition is in progress. Then all the tasks will attempt to
-converge back to the original patch state.
-
-There's also a /proc/<pid>/patch_state file which can be used to
-determine which tasks are blocking completion of a patching operation.
-If a patch is in transition, this file shows 0 to indicate the task is
-unpatched and 1 to indicate it's patched. Otherwise, if no patch is in
-transition, it shows -1. Any tasks which are blocking the transition
-can be signaled with SIGSTOP and SIGCONT to force them to change their
-patched state. This may be harmful to the system though. Sending a fake signal
-to all remaining blocking tasks is a better alternative. No proper signal is
-actually delivered (there is no data in signal pending structures). Tasks are
-interrupted or woken up, and forced to change their patched state. The fake
-signal is automatically sent every 15 seconds.
-
-Administrator can also affect a transition through
-/sys/kernel/livepatch/<patch>/force attribute. Writing 1 there clears
-TIF_PATCH_PENDING flag of all tasks and thus forces the tasks to the patched
-state. Important note! The force attribute is intended for cases when the
-transition gets stuck for a long time because of a blocking task. Administrator
-is expected to collect all necessary data (namely stack traces of such blocking
-tasks) and request a clearance from a patch distributor to force the transition.
-Unauthorized usage may cause harm to the system. It depends on the nature of the
-patch, which functions are (un)patched, and which functions the blocking tasks
-are sleeping in (/proc/<pid>/stack may help here). Removal (rmmod) of patch
-modules is permanently disabled when the force feature is used. It cannot be
-guaranteed there is no task sleeping in such module. It implies unbounded
-reference count if a patch module is disabled and enabled in a loop.
-
-Moreover, the usage of force may also affect future applications of live
-patches and cause even more harm to the system. Administrator should first
-consider to simply cancel a transition (see above). If force is used, reboot
-should be planned and no more live patches applied.
-
-3.1 Adding consistency model support to new architectures
----------------------------------------------------------
-
-For adding consistency model support to new architectures, there are a
-few options:
-
-1) Add CONFIG_HAVE_RELIABLE_STACKTRACE. This means porting objtool, and
- for non-DWARF unwinders, also making sure there's a way for the stack
- tracing code to detect interrupts on the stack.
-
-2) Alternatively, ensure that every kthread has a call to
- klp_update_patch_state() in a safe location. Kthreads are typically
- in an infinite loop which does some action repeatedly. The safe
- location to switch the kthread's patch state would be at a designated
- point in the loop where there are no locks taken and all data
- structures are in a well-defined state.
-
- The location is clear when using workqueues or the kthread worker
- API. These kthreads process independent actions in a generic loop.
-
- It's much more complicated with kthreads which have a custom loop.
- There the safe location must be carefully selected on a case-by-case
- basis.
-
- In that case, arches without HAVE_RELIABLE_STACKTRACE would still be
- able to use the non-stack-checking parts of the consistency model:
-
- a) patching user tasks when they cross the kernel/user space
- boundary; and
-
- b) patching kthreads and idle tasks at their designated patch points.
-
- This option isn't as good as option 1 because it requires signaling
- user tasks and waking kthreads to patch them. But it could still be
- a good backup option for those architectures which don't have
- reliable stack traces yet.
-
-
-4. Livepatch module
-===================
-
-Livepatches are distributed using kernel modules, see
-samples/livepatch/livepatch-sample.c.
-
-The module includes a new implementation of functions that we want
-to replace. In addition, it defines some structures describing the
-relation between the original and the new implementation. Then there
-is code that makes the kernel start using the new code when the livepatch
-module is loaded. Also there is code that cleans up before the
-livepatch module is removed. All this is explained in more details in
-the next sections.
-
-
-4.1. New functions
-------------------
-
-New versions of functions are typically just copied from the original
-sources. A good practice is to add a prefix to the names so that they
-can be distinguished from the original ones, e.g. in a backtrace. Also
-they can be declared as static because they are not called directly
-and do not need the global visibility.
-
-The patch contains only functions that are really modified. But they
-might want to access functions or data from the original source file
-that may only be locally accessible. This can be solved by a special
-relocation section in the generated livepatch module, see
-Documentation/livepatch/module-elf-format.txt for more details.
-
-
-4.2. Metadata
--------------
-
-The patch is described by several structures that split the information
-into three levels:
-
- + struct klp_func is defined for each patched function. It describes
- the relation between the original and the new implementation of a
- particular function.
-
- The structure includes the name, as a string, of the original function.
- The function address is found via kallsyms at runtime.
-
- Then it includes the address of the new function. It is defined
- directly by assigning the function pointer. Note that the new
- function is typically defined in the same source file.
-
- As an optional parameter, the symbol position in the kallsyms database can
- be used to disambiguate functions of the same name. This is not the
- absolute position in the database, but rather the order it has been found
- only for a particular object ( vmlinux or a kernel module ). Note that
- kallsyms allows for searching symbols according to the object name.
-
- + struct klp_object defines an array of patched functions (struct
- klp_func) in the same object. Where the object is either vmlinux
- (NULL) or a module name.
-
- The structure helps to group and handle functions for each object
- together. Note that patched modules might be loaded later than
- the patch itself and the relevant functions might be patched
- only when they are available.
-
-
- + struct klp_patch defines an array of patched objects (struct
- klp_object).
-
- This structure handles all patched functions consistently and eventually,
- synchronously. The whole patch is applied only when all patched
- symbols are found. The only exception are symbols from objects
- (kernel modules) that have not been loaded yet.
-
- For more details on how the patch is applied on a per-task basis,
- see the "Consistency model" section.
-
-
-5. Livepatch life-cycle
-=======================
-
-Livepatching can be described by five basic operations:
-loading, enabling, replacing, disabling, removing.
-
-Where the replacing and the disabling operations are mutually
-exclusive. They have the same result for the given patch but
-not for the system.
-
-
-5.1. Loading
-------------
-
-The only reasonable way is to enable the patch when the livepatch kernel
-module is being loaded. For this, klp_enable_patch() has to be called
-in the module_init() callback. There are two main reasons:
-
-First, only the module has an easy access to the related struct klp_patch.
-
-Second, the error code might be used to refuse loading the module when
-the patch cannot get enabled.
-
-
-5.2. Enabling
--------------
-
-The livepatch gets enabled by calling klp_enable_patch() from
-the module_init() callback. The system will start using the new
-implementation of the patched functions at this stage.
-
-First, the addresses of the patched functions are found according to their
-names. The special relocations, mentioned in the section "New functions",
-are applied. The relevant entries are created under
-/sys/kernel/livepatch/<name>. The patch is rejected when any above
-operation fails.
-
-Second, livepatch enters into a transition state where tasks are converging
-to the patched state. If an original function is patched for the first
-time, a function specific struct klp_ops is created and an universal
-ftrace handler is registered[*]. This stage is indicated by a value of '1'
-in /sys/kernel/livepatch/<name>/transition. For more information about
-this process, see the "Consistency model" section.
-
-Finally, once all tasks have been patched, the 'transition' value changes
-to '0'.
-
-[*] Note that functions might be patched multiple times. The ftrace handler
- is registered only once for a given function. Further patches just add
- an entry to the list (see field `func_stack`) of the struct klp_ops.
- The right implementation is selected by the ftrace handler, see
- the "Consistency model" section.
-
- That said, it is highly recommended to use cumulative livepatches
- because they help keeping the consistency of all changes. In this case,
- functions might be patched two times only during the transition period.
-
-
-5.3. Replacing
---------------
-
-All enabled patches might get replaced by a cumulative patch that
-has the .replace flag set.
-
-Once the new patch is enabled and the 'transition' finishes then
-all the functions (struct klp_func) associated with the replaced
-patches are removed from the corresponding struct klp_ops. Also
-the ftrace handler is unregistered and the struct klp_ops is
-freed when the related function is not modified by the new patch
-and func_stack list becomes empty.
-
-See Documentation/livepatch/cumulative-patches.txt for more details.
-
-
-5.4. Disabling
---------------
-
-Enabled patches might get disabled by writing '0' to
-/sys/kernel/livepatch/<name>/enabled.
-
-First, livepatch enters into a transition state where tasks are converging
-to the unpatched state. The system starts using either the code from
-the previously enabled patch or even the original one. This stage is
-indicated by a value of '1' in /sys/kernel/livepatch/<name>/transition.
-For more information about this process, see the "Consistency model"
-section.
-
-Second, once all tasks have been unpatched, the 'transition' value changes
-to '0'. All the functions (struct klp_func) associated with the to-be-disabled
-patch are removed from the corresponding struct klp_ops. The ftrace handler
-is unregistered and the struct klp_ops is freed when the func_stack list
-becomes empty.
-
-Third, the sysfs interface is destroyed.
-
-
-5.5. Removing
--------------
-
-Module removal is only safe when there are no users of functions provided
-by the module. This is the reason why the force feature permanently
-disables the removal. Only when the system is successfully transitioned
-to a new patch state (patched/unpatched) without being forced it is
-guaranteed that no task sleeps or runs in the old code.
-
-
-6. Sysfs
-========
-
-Information about the registered patches can be found under
-/sys/kernel/livepatch. The patches could be enabled and disabled
-by writing there.
-
-/sys/kernel/livepatch/<patch>/force attributes allow administrator to affect a
-patching operation.
-
-See Documentation/ABI/testing/sysfs-kernel-livepatch for more details.
-
-
-7. Limitations
-==============
-
-The current Livepatch implementation has several limitations:
-
- + Only functions that can be traced could be patched.
-
- Livepatch is based on the dynamic ftrace. In particular, functions
- implementing ftrace or the livepatch ftrace handler could not be
- patched. Otherwise, the code would end up in an infinite loop. A
- potential mistake is prevented by marking the problematic functions
- by "notrace".
-
-
-
- + Livepatch works reliably only when the dynamic ftrace is located at
- the very beginning of the function.
-
- The function need to be redirected before the stack or the function
- parameters are modified in any way. For example, livepatch requires
- using -fentry gcc compiler option on x86_64.
-
- One exception is the PPC port. It uses relative addressing and TOC.
- Each function has to handle TOC and save LR before it could call
- the ftrace handler. This operation has to be reverted on return.
- Fortunately, the generic ftrace code has the same problem and all
- this is handled on the ftrace level.
-
-
- + Kretprobes using the ftrace framework conflict with the patched
- functions.
-
- Both kretprobes and livepatches use a ftrace handler that modifies
- the return address. The first user wins. Either the probe or the patch
- is rejected when the handler is already in use by the other.
-
-
- + Kprobes in the original function are ignored when the code is
- redirected to the new implementation.
-
- There is a work in progress to add warnings about this situation.
--- /dev/null
+===========================
+Livepatch module Elf format
+===========================
+
+This document outlines the Elf format requirements that livepatch modules must follow.
+
+
+.. Table of Contents
+
+ 0. Background and motivation
+ 1. Livepatch modinfo field
+ 2. Livepatch relocation sections
+ 2.1 What are livepatch relocation sections?
+ 2.2 Livepatch relocation section format
+ 2.2.1 Required flags
+ 2.2.2 Required name format
+ 2.2.3 Example livepatch relocation section names
+ 2.2.4 Example `readelf --sections` output
+ 2.2.5 Example `readelf --relocs` output
+ 3. Livepatch symbols
+ 3.1 What are livepatch symbols?
+ 3.2 A livepatch module's symbol table
+ 3.3 Livepatch symbol format
+ 3.3.1 Required flags
+ 3.3.2 Required name format
+ 3.3.3 Example livepatch symbol names
+ 3.3.4 Example `readelf --symbols` output
+ 4. Architecture-specific sections
+ 5. Symbol table and Elf section access
+
+----------------------------
+0. Background and motivation
+----------------------------
+
+Formerly, livepatch required separate architecture-specific code to write
+relocations. However, arch-specific code to write relocations already
+exists in the module loader, so this former approach produced redundant
+code. So, instead of duplicating code and re-implementing what the module
+loader can already do, livepatch leverages existing code in the module
+loader to perform the all the arch-specific relocation work. Specifically,
+livepatch reuses the apply_relocate_add() function in the module loader to
+write relocations. The patch module Elf format described in this document
+enables livepatch to be able to do this. The hope is that this will make
+livepatch more easily portable to other architectures and reduce the amount
+of arch-specific code required to port livepatch to a particular
+architecture.
+
+Since apply_relocate_add() requires access to a module's section header
+table, symbol table, and relocation section indices, Elf information is
+preserved for livepatch modules (see section 5). Livepatch manages its own
+relocation sections and symbols, which are described in this document. The
+Elf constants used to mark livepatch symbols and relocation sections were
+selected from OS-specific ranges according to the definitions from glibc.
+
+0.1 Why does livepatch need to write its own relocations?
+---------------------------------------------------------
+A typical livepatch module contains patched versions of functions that can
+reference non-exported global symbols and non-included local symbols.
+Relocations referencing these types of symbols cannot be left in as-is
+since the kernel module loader cannot resolve them and will therefore
+reject the livepatch module. Furthermore, we cannot apply relocations that
+affect modules not yet loaded at patch module load time (e.g. a patch to a
+driver that is not loaded). Formerly, livepatch solved this problem by
+embedding special "dynrela" (dynamic rela) sections in the resulting patch
+module Elf output. Using these dynrela sections, livepatch could resolve
+symbols while taking into account its scope and what module the symbol
+belongs to, and then manually apply the dynamic relocations. However this
+approach required livepatch to supply arch-specific code in order to write
+these relocations. In the new format, livepatch manages its own SHT_RELA
+relocation sections in place of dynrela sections, and the symbols that the
+relas reference are special livepatch symbols (see section 2 and 3). The
+arch-specific livepatch relocation code is replaced by a call to
+apply_relocate_add().
+
+================================
+PATCH MODULE FORMAT REQUIREMENTS
+================================
+
+--------------------------
+1. Livepatch modinfo field
+--------------------------
+
+Livepatch modules are required to have the "livepatch" modinfo attribute.
+See the sample livepatch module in samples/livepatch/ for how this is done.
+
+Livepatch modules can be identified by users by using the 'modinfo' command
+and looking for the presence of the "livepatch" field. This field is also
+used by the kernel module loader to identify livepatch modules.
+
+Example modinfo output:
+-----------------------
+
+::
+
+ % modinfo livepatch-meminfo.ko
+ filename: livepatch-meminfo.ko
+ livepatch: Y
+ license: GPL
+ depends:
+ vermagic: 4.3.0+ SMP mod_unload
+
+--------------------------------
+2. Livepatch relocation sections
+--------------------------------
+
+-------------------------------------------
+2.1 What are livepatch relocation sections?
+-------------------------------------------
+A livepatch module manages its own Elf relocation sections to apply
+relocations to modules as well as to the kernel (vmlinux) at the
+appropriate time. For example, if a patch module patches a driver that is
+not currently loaded, livepatch will apply the corresponding livepatch
+relocation section(s) to the driver once it loads.
+
+Each "object" (e.g. vmlinux, or a module) within a patch module may have
+multiple livepatch relocation sections associated with it (e.g. patches to
+multiple functions within the same object). There is a 1-1 correspondence
+between a livepatch relocation section and the target section (usually the
+text section of a function) to which the relocation(s) apply. It is
+also possible for a livepatch module to have no livepatch relocation
+sections, as in the case of the sample livepatch module (see
+samples/livepatch).
+
+Since Elf information is preserved for livepatch modules (see Section 5), a
+livepatch relocation section can be applied simply by passing in the
+appropriate section index to apply_relocate_add(), which then uses it to
+access the relocation section and apply the relocations.
+
+Every symbol referenced by a rela in a livepatch relocation section is a
+livepatch symbol. These must be resolved before livepatch can call
+apply_relocate_add(). See Section 3 for more information.
+
+---------------------------------------
+2.2 Livepatch relocation section format
+---------------------------------------
+
+2.2.1 Required flags
+--------------------
+Livepatch relocation sections must be marked with the SHF_RELA_LIVEPATCH
+section flag. See include/uapi/linux/elf.h for the definition. The module
+loader recognizes this flag and will avoid applying those relocation sections
+at patch module load time. These sections must also be marked with SHF_ALLOC,
+so that the module loader doesn't discard them on module load (i.e. they will
+be copied into memory along with the other SHF_ALLOC sections).
+
+2.2.2 Required name format
+--------------------------
+The name of a livepatch relocation section must conform to the following
+format::
+
+ .klp.rela.objname.section_name
+ ^ ^^ ^ ^ ^
+ |________||_____| |__________|
+ [A] [B] [C]
+
+ [A] The relocation section name is prefixed with the string ".klp.rela."
+ [B] The name of the object (i.e. "vmlinux" or name of module) to
+ which the relocation section belongs follows immediately after the prefix.
+ [C] The actual name of the section to which this relocation section applies.
+
+2.2.3 Example livepatch relocation section names:
+-------------------------------------------------
+.klp.rela.ext4.text.ext4_attr_store
+.klp.rela.vmlinux.text.cmdline_proc_show
+
+2.2.4 Example `readelf --sections` output for a patch
+module that patches vmlinux and modules 9p, btrfs, ext4:
+--------------------------------------------------------
+
+::
+
+ Section Headers:
+ [Nr] Name Type Address Off Size ES Flg Lk Inf Al
+ [ snip ]
+ [29] .klp.rela.9p.text.caches.show RELA 0000000000000000 002d58 0000c0 18 AIo 64 9 8
+ [30] .klp.rela.btrfs.text.btrfs.feature.attr.show RELA 0000000000000000 002e18 000060 18 AIo 64 11 8
+ [ snip ]
+ [34] .klp.rela.ext4.text.ext4.attr.store RELA 0000000000000000 002fd8 0000d8 18 AIo 64 13 8
+ [35] .klp.rela.ext4.text.ext4.attr.show RELA 0000000000000000 0030b0 000150 18 AIo 64 15 8
+ [36] .klp.rela.vmlinux.text.cmdline.proc.show RELA 0000000000000000 003200 000018 18 AIo 64 17 8
+ [37] .klp.rela.vmlinux.text.meminfo.proc.show RELA 0000000000000000 003218 0000f0 18 AIo 64 19 8
+ [ snip ] ^ ^
+ | |
+ [*] [*]
+ [*] Livepatch relocation sections are SHT_RELA sections but with a few special
+ characteristics. Notice that they are marked SHF_ALLOC ("A") so that they will
+ not be discarded when the module is loaded into memory, as well as with the
+ SHF_RELA_LIVEPATCH flag ("o" - for OS-specific).
+
+2.2.5 Example `readelf --relocs` output for a patch module:
+-----------------------------------------------------------
+
+::
+
+ Relocation section '.klp.rela.btrfs.text.btrfs_feature_attr_show' at offset 0x2ba0 contains 4 entries:
+ Offset Info Type Symbol's Value Symbol's Name + Addend
+ 000000000000001f 0000005e00000002 R_X86_64_PC32 0000000000000000 .klp.sym.vmlinux.printk,0 - 4
+ 0000000000000028 0000003d0000000b R_X86_64_32S 0000000000000000 .klp.sym.btrfs.btrfs_ktype,0 + 0
+ 0000000000000036 0000003b00000002 R_X86_64_PC32 0000000000000000 .klp.sym.btrfs.can_modify_feature.isra.3,0 - 4
+ 000000000000004c 0000004900000002 R_X86_64_PC32 0000000000000000 .klp.sym.vmlinux.snprintf,0 - 4
+ [ snip ] ^
+ |
+ [*]
+ [*] Every symbol referenced by a relocation is a livepatch symbol.
+
+--------------------
+3. Livepatch symbols
+--------------------
+
+-------------------------------
+3.1 What are livepatch symbols?
+-------------------------------
+Livepatch symbols are symbols referred to by livepatch relocation sections.
+These are symbols accessed from new versions of functions for patched
+objects, whose addresses cannot be resolved by the module loader (because
+they are local or unexported global syms). Since the module loader only
+resolves exported syms, and not every symbol referenced by the new patched
+functions is exported, livepatch symbols were introduced. They are used
+also in cases where we cannot immediately know the address of a symbol when
+a patch module loads. For example, this is the case when livepatch patches
+a module that is not loaded yet. In this case, the relevant livepatch
+symbols are resolved simply when the target module loads. In any case, for
+any livepatch relocation section, all livepatch symbols referenced by that
+section must be resolved before livepatch can call apply_relocate_add() for
+that reloc section.
+
+Livepatch symbols must be marked with SHN_LIVEPATCH so that the module
+loader can identify and ignore them. Livepatch modules keep these symbols
+in their symbol tables, and the symbol table is made accessible through
+module->symtab.
+
+-------------------------------------
+3.2 A livepatch module's symbol table
+-------------------------------------
+Normally, a stripped down copy of a module's symbol table (containing only
+"core" symbols) is made available through module->symtab (See layout_symtab()
+in kernel/module.c). For livepatch modules, the symbol table copied into memory
+on module load must be exactly the same as the symbol table produced when the
+patch module was compiled. This is because the relocations in each livepatch
+relocation section refer to their respective symbols with their symbol indices,
+and the original symbol indices (and thus the symtab ordering) must be
+preserved in order for apply_relocate_add() to find the right symbol.
+
+For example, take this particular rela from a livepatch module:::
+
+ Relocation section '.klp.rela.btrfs.text.btrfs_feature_attr_show' at offset 0x2ba0 contains 4 entries:
+ Offset Info Type Symbol's Value Symbol's Name + Addend
+ 000000000000001f 0000005e00000002 R_X86_64_PC32 0000000000000000 .klp.sym.vmlinux.printk,0 - 4
+
+ This rela refers to the symbol '.klp.sym.vmlinux.printk,0', and the symbol index is encoded
+ in 'Info'. Here its symbol index is 0x5e, which is 94 in decimal, which refers to the
+ symbol index 94.
+ And in this patch module's corresponding symbol table, symbol index 94 refers to that very symbol:
+ [ snip ]
+ 94: 0000000000000000 0 NOTYPE GLOBAL DEFAULT OS [0xff20] .klp.sym.vmlinux.printk,0
+ [ snip ]
+
+---------------------------
+3.3 Livepatch symbol format
+---------------------------
+
+3.3.1 Required flags
+--------------------
+Livepatch symbols must have their section index marked as SHN_LIVEPATCH, so
+that the module loader can identify them and not attempt to resolve them.
+See include/uapi/linux/elf.h for the actual definitions.
+
+3.3.2 Required name format
+--------------------------
+Livepatch symbol names must conform to the following format::
+
+ .klp.sym.objname.symbol_name,sympos
+ ^ ^^ ^ ^ ^ ^
+ |_______||_____| |_________| |
+ [A] [B] [C] [D]
+
+ [A] The symbol name is prefixed with the string ".klp.sym."
+ [B] The name of the object (i.e. "vmlinux" or name of module) to
+ which the symbol belongs follows immediately after the prefix.
+ [C] The actual name of the symbol.
+ [D] The position of the symbol in the object (as according to kallsyms)
+ This is used to differentiate duplicate symbols within the same
+ object. The symbol position is expressed numerically (0, 1, 2...).
+ The symbol position of a unique symbol is 0.
+
+3.3.3 Example livepatch symbol names:
+-------------------------------------
+
+::
+
+ .klp.sym.vmlinux.snprintf,0
+ .klp.sym.vmlinux.printk,0
+ .klp.sym.btrfs.btrfs_ktype,0
+
+3.3.4 Example `readelf --symbols` output for a patch module:
+------------------------------------------------------------
+
+::
+
+ Symbol table '.symtab' contains 127 entries:
+ Num: Value Size Type Bind Vis Ndx Name
+ [ snip ]
+ 73: 0000000000000000 0 NOTYPE GLOBAL DEFAULT OS [0xff20] .klp.sym.vmlinux.snprintf,0
+ 74: 0000000000000000 0 NOTYPE GLOBAL DEFAULT OS [0xff20] .klp.sym.vmlinux.capable,0
+ 75: 0000000000000000 0 NOTYPE GLOBAL DEFAULT OS [0xff20] .klp.sym.vmlinux.find_next_bit,0
+ 76: 0000000000000000 0 NOTYPE GLOBAL DEFAULT OS [0xff20] .klp.sym.vmlinux.si_swapinfo,0
+ [ snip ] ^
+ |
+ [*]
+ [*] Note that the 'Ndx' (Section index) for these symbols is SHN_LIVEPATCH (0xff20).
+ "OS" means OS-specific.
+
+---------------------------------
+4. Architecture-specific sections
+---------------------------------
+Architectures may override arch_klp_init_object_loaded() to perform
+additional arch-specific tasks when a target module loads, such as applying
+arch-specific sections. On x86 for example, we must apply per-object
+.altinstructions and .parainstructions sections when a target module loads.
+These sections must be prefixed with ".klp.arch.$objname." so that they can
+be easily identified when iterating through a patch module's Elf sections
+(See arch/x86/kernel/livepatch.c for a complete example).
+
+--------------------------------------
+5. Symbol table and Elf section access
+--------------------------------------
+A livepatch module's symbol table is accessible through module->symtab.
+
+Since apply_relocate_add() requires access to a module's section headers,
+symbol table, and relocation section indices, Elf information is preserved for
+livepatch modules and is made accessible by the module loader through
+module->klp_info, which is a klp_modinfo struct. When a livepatch module loads,
+this struct is filled in by the module loader. Its fields are documented below::
+
+ struct klp_modinfo {
+ Elf_Ehdr hdr; /* Elf header */
+ Elf_Shdr *sechdrs; /* Section header table */
+ char *secstrings; /* String table for the section headers */
+ unsigned int symndx; /* The symbol table section index */
+ };
+++ /dev/null
-===========================
-Livepatch module Elf format
-===========================
-
-This document outlines the Elf format requirements that livepatch modules must follow.
-
------------------
-Table of Contents
------------------
-0. Background and motivation
-1. Livepatch modinfo field
-2. Livepatch relocation sections
- 2.1 What are livepatch relocation sections?
- 2.2 Livepatch relocation section format
- 2.2.1 Required flags
- 2.2.2 Required name format
- 2.2.3 Example livepatch relocation section names
- 2.2.4 Example `readelf --sections` output
- 2.2.5 Example `readelf --relocs` output
-3. Livepatch symbols
- 3.1 What are livepatch symbols?
- 3.2 A livepatch module's symbol table
- 3.3 Livepatch symbol format
- 3.3.1 Required flags
- 3.3.2 Required name format
- 3.3.3 Example livepatch symbol names
- 3.3.4 Example `readelf --symbols` output
-4. Architecture-specific sections
-5. Symbol table and Elf section access
-
-----------------------------
-0. Background and motivation
-----------------------------
-
-Formerly, livepatch required separate architecture-specific code to write
-relocations. However, arch-specific code to write relocations already
-exists in the module loader, so this former approach produced redundant
-code. So, instead of duplicating code and re-implementing what the module
-loader can already do, livepatch leverages existing code in the module
-loader to perform the all the arch-specific relocation work. Specifically,
-livepatch reuses the apply_relocate_add() function in the module loader to
-write relocations. The patch module Elf format described in this document
-enables livepatch to be able to do this. The hope is that this will make
-livepatch more easily portable to other architectures and reduce the amount
-of arch-specific code required to port livepatch to a particular
-architecture.
-
-Since apply_relocate_add() requires access to a module's section header
-table, symbol table, and relocation section indices, Elf information is
-preserved for livepatch modules (see section 5). Livepatch manages its own
-relocation sections and symbols, which are described in this document. The
-Elf constants used to mark livepatch symbols and relocation sections were
-selected from OS-specific ranges according to the definitions from glibc.
-
-0.1 Why does livepatch need to write its own relocations?
----------------------------------------------------------
-A typical livepatch module contains patched versions of functions that can
-reference non-exported global symbols and non-included local symbols.
-Relocations referencing these types of symbols cannot be left in as-is
-since the kernel module loader cannot resolve them and will therefore
-reject the livepatch module. Furthermore, we cannot apply relocations that
-affect modules not yet loaded at patch module load time (e.g. a patch to a
-driver that is not loaded). Formerly, livepatch solved this problem by
-embedding special "dynrela" (dynamic rela) sections in the resulting patch
-module Elf output. Using these dynrela sections, livepatch could resolve
-symbols while taking into account its scope and what module the symbol
-belongs to, and then manually apply the dynamic relocations. However this
-approach required livepatch to supply arch-specific code in order to write
-these relocations. In the new format, livepatch manages its own SHT_RELA
-relocation sections in place of dynrela sections, and the symbols that the
-relas reference are special livepatch symbols (see section 2 and 3). The
-arch-specific livepatch relocation code is replaced by a call to
-apply_relocate_add().
-
-================================
-PATCH MODULE FORMAT REQUIREMENTS
-================================
-
---------------------------
-1. Livepatch modinfo field
---------------------------
-
-Livepatch modules are required to have the "livepatch" modinfo attribute.
-See the sample livepatch module in samples/livepatch/ for how this is done.
-
-Livepatch modules can be identified by users by using the 'modinfo' command
-and looking for the presence of the "livepatch" field. This field is also
-used by the kernel module loader to identify livepatch modules.
-
-Example modinfo output:
------------------------
-% modinfo livepatch-meminfo.ko
-filename: livepatch-meminfo.ko
-livepatch: Y
-license: GPL
-depends:
-vermagic: 4.3.0+ SMP mod_unload
-
---------------------------------
-2. Livepatch relocation sections
---------------------------------
-
--------------------------------------------
-2.1 What are livepatch relocation sections?
--------------------------------------------
-A livepatch module manages its own Elf relocation sections to apply
-relocations to modules as well as to the kernel (vmlinux) at the
-appropriate time. For example, if a patch module patches a driver that is
-not currently loaded, livepatch will apply the corresponding livepatch
-relocation section(s) to the driver once it loads.
-
-Each "object" (e.g. vmlinux, or a module) within a patch module may have
-multiple livepatch relocation sections associated with it (e.g. patches to
-multiple functions within the same object). There is a 1-1 correspondence
-between a livepatch relocation section and the target section (usually the
-text section of a function) to which the relocation(s) apply. It is
-also possible for a livepatch module to have no livepatch relocation
-sections, as in the case of the sample livepatch module (see
-samples/livepatch).
-
-Since Elf information is preserved for livepatch modules (see Section 5), a
-livepatch relocation section can be applied simply by passing in the
-appropriate section index to apply_relocate_add(), which then uses it to
-access the relocation section and apply the relocations.
-
-Every symbol referenced by a rela in a livepatch relocation section is a
-livepatch symbol. These must be resolved before livepatch can call
-apply_relocate_add(). See Section 3 for more information.
-
----------------------------------------
-2.2 Livepatch relocation section format
----------------------------------------
-
-2.2.1 Required flags
---------------------
-Livepatch relocation sections must be marked with the SHF_RELA_LIVEPATCH
-section flag. See include/uapi/linux/elf.h for the definition. The module
-loader recognizes this flag and will avoid applying those relocation sections
-at patch module load time. These sections must also be marked with SHF_ALLOC,
-so that the module loader doesn't discard them on module load (i.e. they will
-be copied into memory along with the other SHF_ALLOC sections).
-
-2.2.2 Required name format
---------------------------
-The name of a livepatch relocation section must conform to the following format:
-
-.klp.rela.objname.section_name
-^ ^^ ^ ^ ^
-|________||_____| |__________|
- [A] [B] [C]
-
-[A] The relocation section name is prefixed with the string ".klp.rela."
-[B] The name of the object (i.e. "vmlinux" or name of module) to
- which the relocation section belongs follows immediately after the prefix.
-[C] The actual name of the section to which this relocation section applies.
-
-2.2.3 Example livepatch relocation section names:
--------------------------------------------------
-.klp.rela.ext4.text.ext4_attr_store
-.klp.rela.vmlinux.text.cmdline_proc_show
-
-2.2.4 Example `readelf --sections` output for a patch
-module that patches vmlinux and modules 9p, btrfs, ext4:
---------------------------------------------------------
- Section Headers:
- [Nr] Name Type Address Off Size ES Flg Lk Inf Al
- [ snip ]
- [29] .klp.rela.9p.text.caches.show RELA 0000000000000000 002d58 0000c0 18 AIo 64 9 8
- [30] .klp.rela.btrfs.text.btrfs.feature.attr.show RELA 0000000000000000 002e18 000060 18 AIo 64 11 8
- [ snip ]
- [34] .klp.rela.ext4.text.ext4.attr.store RELA 0000000000000000 002fd8 0000d8 18 AIo 64 13 8
- [35] .klp.rela.ext4.text.ext4.attr.show RELA 0000000000000000 0030b0 000150 18 AIo 64 15 8
- [36] .klp.rela.vmlinux.text.cmdline.proc.show RELA 0000000000000000 003200 000018 18 AIo 64 17 8
- [37] .klp.rela.vmlinux.text.meminfo.proc.show RELA 0000000000000000 003218 0000f0 18 AIo 64 19 8
- [ snip ] ^ ^
- | |
- [*] [*]
-[*] Livepatch relocation sections are SHT_RELA sections but with a few special
-characteristics. Notice that they are marked SHF_ALLOC ("A") so that they will
-not be discarded when the module is loaded into memory, as well as with the
-SHF_RELA_LIVEPATCH flag ("o" - for OS-specific).
-
-2.2.5 Example `readelf --relocs` output for a patch module:
------------------------------------------------------------
-Relocation section '.klp.rela.btrfs.text.btrfs_feature_attr_show' at offset 0x2ba0 contains 4 entries:
- Offset Info Type Symbol's Value Symbol's Name + Addend
-000000000000001f 0000005e00000002 R_X86_64_PC32 0000000000000000 .klp.sym.vmlinux.printk,0 - 4
-0000000000000028 0000003d0000000b R_X86_64_32S 0000000000000000 .klp.sym.btrfs.btrfs_ktype,0 + 0
-0000000000000036 0000003b00000002 R_X86_64_PC32 0000000000000000 .klp.sym.btrfs.can_modify_feature.isra.3,0 - 4
-000000000000004c 0000004900000002 R_X86_64_PC32 0000000000000000 .klp.sym.vmlinux.snprintf,0 - 4
-[ snip ] ^
- |
- [*]
-[*] Every symbol referenced by a relocation is a livepatch symbol.
-
---------------------
-3. Livepatch symbols
---------------------
-
--------------------------------
-3.1 What are livepatch symbols?
--------------------------------
-Livepatch symbols are symbols referred to by livepatch relocation sections.
-These are symbols accessed from new versions of functions for patched
-objects, whose addresses cannot be resolved by the module loader (because
-they are local or unexported global syms). Since the module loader only
-resolves exported syms, and not every symbol referenced by the new patched
-functions is exported, livepatch symbols were introduced. They are used
-also in cases where we cannot immediately know the address of a symbol when
-a patch module loads. For example, this is the case when livepatch patches
-a module that is not loaded yet. In this case, the relevant livepatch
-symbols are resolved simply when the target module loads. In any case, for
-any livepatch relocation section, all livepatch symbols referenced by that
-section must be resolved before livepatch can call apply_relocate_add() for
-that reloc section.
-
-Livepatch symbols must be marked with SHN_LIVEPATCH so that the module
-loader can identify and ignore them. Livepatch modules keep these symbols
-in their symbol tables, and the symbol table is made accessible through
-module->symtab.
-
--------------------------------------
-3.2 A livepatch module's symbol table
--------------------------------------
-Normally, a stripped down copy of a module's symbol table (containing only
-"core" symbols) is made available through module->symtab (See layout_symtab()
-in kernel/module.c). For livepatch modules, the symbol table copied into memory
-on module load must be exactly the same as the symbol table produced when the
-patch module was compiled. This is because the relocations in each livepatch
-relocation section refer to their respective symbols with their symbol indices,
-and the original symbol indices (and thus the symtab ordering) must be
-preserved in order for apply_relocate_add() to find the right symbol.
-
-For example, take this particular rela from a livepatch module:
-Relocation section '.klp.rela.btrfs.text.btrfs_feature_attr_show' at offset 0x2ba0 contains 4 entries:
- Offset Info Type Symbol's Value Symbol's Name + Addend
-000000000000001f 0000005e00000002 R_X86_64_PC32 0000000000000000 .klp.sym.vmlinux.printk,0 - 4
-
-This rela refers to the symbol '.klp.sym.vmlinux.printk,0', and the symbol index is encoded
-in 'Info'. Here its symbol index is 0x5e, which is 94 in decimal, which refers to the
-symbol index 94.
-And in this patch module's corresponding symbol table, symbol index 94 refers to that very symbol:
-[ snip ]
-94: 0000000000000000 0 NOTYPE GLOBAL DEFAULT OS [0xff20] .klp.sym.vmlinux.printk,0
-[ snip ]
-
----------------------------
-3.3 Livepatch symbol format
----------------------------
-
-3.3.1 Required flags
---------------------
-Livepatch symbols must have their section index marked as SHN_LIVEPATCH, so
-that the module loader can identify them and not attempt to resolve them.
-See include/uapi/linux/elf.h for the actual definitions.
-
-3.3.2 Required name format
---------------------------
-Livepatch symbol names must conform to the following format:
-
-.klp.sym.objname.symbol_name,sympos
-^ ^^ ^ ^ ^ ^
-|_______||_____| |_________| |
- [A] [B] [C] [D]
-
-[A] The symbol name is prefixed with the string ".klp.sym."
-[B] The name of the object (i.e. "vmlinux" or name of module) to
- which the symbol belongs follows immediately after the prefix.
-[C] The actual name of the symbol.
-[D] The position of the symbol in the object (as according to kallsyms)
- This is used to differentiate duplicate symbols within the same
- object. The symbol position is expressed numerically (0, 1, 2...).
- The symbol position of a unique symbol is 0.
-
-3.3.3 Example livepatch symbol names:
--------------------------------------
-.klp.sym.vmlinux.snprintf,0
-.klp.sym.vmlinux.printk,0
-.klp.sym.btrfs.btrfs_ktype,0
-
-3.3.4 Example `readelf --symbols` output for a patch module:
-------------------------------------------------------------
-Symbol table '.symtab' contains 127 entries:
- Num: Value Size Type Bind Vis Ndx Name
- [ snip ]
- 73: 0000000000000000 0 NOTYPE GLOBAL DEFAULT OS [0xff20] .klp.sym.vmlinux.snprintf,0
- 74: 0000000000000000 0 NOTYPE GLOBAL DEFAULT OS [0xff20] .klp.sym.vmlinux.capable,0
- 75: 0000000000000000 0 NOTYPE GLOBAL DEFAULT OS [0xff20] .klp.sym.vmlinux.find_next_bit,0
- 76: 0000000000000000 0 NOTYPE GLOBAL DEFAULT OS [0xff20] .klp.sym.vmlinux.si_swapinfo,0
- [ snip ] ^
- |
- [*]
-[*] Note that the 'Ndx' (Section index) for these symbols is SHN_LIVEPATCH (0xff20).
- "OS" means OS-specific.
-
----------------------------------
-4. Architecture-specific sections
----------------------------------
-Architectures may override arch_klp_init_object_loaded() to perform
-additional arch-specific tasks when a target module loads, such as applying
-arch-specific sections. On x86 for example, we must apply per-object
-.altinstructions and .parainstructions sections when a target module loads.
-These sections must be prefixed with ".klp.arch.$objname." so that they can
-be easily identified when iterating through a patch module's Elf sections
-(See arch/x86/kernel/livepatch.c for a complete example).
-
---------------------------------------
-5. Symbol table and Elf section access
---------------------------------------
-A livepatch module's symbol table is accessible through module->symtab.
-
-Since apply_relocate_add() requires access to a module's section headers,
-symbol table, and relocation section indices, Elf information is preserved for
-livepatch modules and is made accessible by the module loader through
-module->klp_info, which is a klp_modinfo struct. When a livepatch module loads,
-this struct is filled in by the module loader. Its fields are documented below:
-
-struct klp_modinfo {
- Elf_Ehdr hdr; /* Elf header */
- Elf_Shdr *sechdrs; /* Section header table */
- char *secstrings; /* String table for the section headers */
- unsigned int symndx; /* The symbol table section index */
-};
--- /dev/null
+================
+Shadow Variables
+================
+
+Shadow variables are a simple way for livepatch modules to associate
+additional "shadow" data with existing data structures. Shadow data is
+allocated separately from parent data structures, which are left
+unmodified. The shadow variable API described in this document is used
+to allocate/add and remove/free shadow variables to/from their parents.
+
+The implementation introduces a global, in-kernel hashtable that
+associates pointers to parent objects and a numeric identifier of the
+shadow data. The numeric identifier is a simple enumeration that may be
+used to describe shadow variable version, class or type, etc. More
+specifically, the parent pointer serves as the hashtable key while the
+numeric id subsequently filters hashtable queries. Multiple shadow
+variables may attach to the same parent object, but their numeric
+identifier distinguishes between them.
+
+
+1. Brief API summary
+====================
+
+(See the full API usage docbook notes in livepatch/shadow.c.)
+
+A hashtable references all shadow variables. These references are
+stored and retrieved through a <obj, id> pair.
+
+* The klp_shadow variable data structure encapsulates both tracking
+ meta-data and shadow-data:
+
+ - meta-data
+
+ - obj - pointer to parent object
+ - id - data identifier
+
+ - data[] - storage for shadow data
+
+It is important to note that the klp_shadow_alloc() and
+klp_shadow_get_or_alloc() are zeroing the variable by default.
+They also allow to call a custom constructor function when a non-zero
+value is needed. Callers should provide whatever mutual exclusion
+is required.
+
+Note that the constructor is called under klp_shadow_lock spinlock. It allows
+to do actions that can be done only once when a new variable is allocated.
+
+* klp_shadow_get() - retrieve a shadow variable data pointer
+ - search hashtable for <obj, id> pair
+
+* klp_shadow_alloc() - allocate and add a new shadow variable
+ - search hashtable for <obj, id> pair
+
+ - if exists
+
+ - WARN and return NULL
+
+ - if <obj, id> doesn't already exist
+
+ - allocate a new shadow variable
+ - initialize the variable using a custom constructor and data when provided
+ - add <obj, id> to the global hashtable
+
+* klp_shadow_get_or_alloc() - get existing or alloc a new shadow variable
+ - search hashtable for <obj, id> pair
+
+ - if exists
+
+ - return existing shadow variable
+
+ - if <obj, id> doesn't already exist
+
+ - allocate a new shadow variable
+ - initialize the variable using a custom constructor and data when provided
+ - add <obj, id> pair to the global hashtable
+
+* klp_shadow_free() - detach and free a <obj, id> shadow variable
+ - find and remove a <obj, id> reference from global hashtable
+
+ - if found
+
+ - call destructor function if defined
+ - free shadow variable
+
+* klp_shadow_free_all() - detach and free all <*, id> shadow variables
+ - find and remove any <*, id> references from global hashtable
+
+ - if found
+
+ - call destructor function if defined
+ - free shadow variable
+
+
+2. Use cases
+============
+
+(See the example shadow variable livepatch modules in samples/livepatch/
+for full working demonstrations.)
+
+For the following use-case examples, consider commit 1d147bfa6429
+("mac80211: fix AP powersave TX vs. wakeup race"), which added a
+spinlock to net/mac80211/sta_info.h :: struct sta_info. Each use-case
+example can be considered a stand-alone livepatch implementation of this
+fix.
+
+
+Matching parent's lifecycle
+---------------------------
+
+If parent data structures are frequently created and destroyed, it may
+be easiest to align their shadow variables lifetimes to the same
+allocation and release functions. In this case, the parent data
+structure is typically allocated, initialized, then registered in some
+manner. Shadow variable allocation and setup can then be considered
+part of the parent's initialization and should be completed before the
+parent "goes live" (ie, any shadow variable get-API requests are made
+for this <obj, id> pair.)
+
+For commit 1d147bfa6429, when a parent sta_info structure is allocated,
+allocate a shadow copy of the ps_lock pointer, then initialize it::
+
+ #define PS_LOCK 1
+ struct sta_info *sta_info_alloc(struct ieee80211_sub_if_data *sdata,
+ const u8 *addr, gfp_t gfp)
+ {
+ struct sta_info *sta;
+ spinlock_t *ps_lock;
+
+ /* Parent structure is created */
+ sta = kzalloc(sizeof(*sta) + hw->sta_data_size, gfp);
+
+ /* Attach a corresponding shadow variable, then initialize it */
+ ps_lock = klp_shadow_alloc(sta, PS_LOCK, sizeof(*ps_lock), gfp,
+ NULL, NULL);
+ if (!ps_lock)
+ goto shadow_fail;
+ spin_lock_init(ps_lock);
+ ...
+
+When requiring a ps_lock, query the shadow variable API to retrieve one
+for a specific struct sta_info:::
+
+ void ieee80211_sta_ps_deliver_wakeup(struct sta_info *sta)
+ {
+ spinlock_t *ps_lock;
+
+ /* sync with ieee80211_tx_h_unicast_ps_buf */
+ ps_lock = klp_shadow_get(sta, PS_LOCK);
+ if (ps_lock)
+ spin_lock(ps_lock);
+ ...
+
+When the parent sta_info structure is freed, first free the shadow
+variable::
+
+ void sta_info_free(struct ieee80211_local *local, struct sta_info *sta)
+ {
+ klp_shadow_free(sta, PS_LOCK, NULL);
+ kfree(sta);
+ ...
+
+
+In-flight parent objects
+------------------------
+
+Sometimes it may not be convenient or possible to allocate shadow
+variables alongside their parent objects. Or a livepatch fix may
+require shadow varibles to only a subset of parent object instances. In
+these cases, the klp_shadow_get_or_alloc() call can be used to attach
+shadow variables to parents already in-flight.
+
+For commit 1d147bfa6429, a good spot to allocate a shadow spinlock is
+inside ieee80211_sta_ps_deliver_wakeup()::
+
+ int ps_lock_shadow_ctor(void *obj, void *shadow_data, void *ctor_data)
+ {
+ spinlock_t *lock = shadow_data;
+
+ spin_lock_init(lock);
+ return 0;
+ }
+
+ #define PS_LOCK 1
+ void ieee80211_sta_ps_deliver_wakeup(struct sta_info *sta)
+ {
+ spinlock_t *ps_lock;
+
+ /* sync with ieee80211_tx_h_unicast_ps_buf */
+ ps_lock = klp_shadow_get_or_alloc(sta, PS_LOCK,
+ sizeof(*ps_lock), GFP_ATOMIC,
+ ps_lock_shadow_ctor, NULL);
+
+ if (ps_lock)
+ spin_lock(ps_lock);
+ ...
+
+This usage will create a shadow variable, only if needed, otherwise it
+will use one that was already created for this <obj, id> pair.
+
+Like the previous use-case, the shadow spinlock needs to be cleaned up.
+A shadow variable can be freed just before its parent object is freed,
+or even when the shadow variable itself is no longer required.
+
+
+Other use-cases
+---------------
+
+Shadow variables can also be used as a flag indicating that a data
+structure was allocated by new, livepatched code. In this case, it
+doesn't matter what data value the shadow variable holds, its existence
+suggests how to handle the parent object.
+
+
+3. References
+=============
+
+* https://github.com/dynup/kpatch
+
+ The livepatch implementation is based on the kpatch version of shadow
+ variables.
+
+* http://files.mkgnu.net/files/dynamos/doc/papers/dynamos_eurosys_07.pdf
+
+ Dynamic and Adaptive Updates of Non-Quiescent Subsystems in Commodity
+ Operating System Kernels (Kritis Makris, Kyung Dong Ryu 2007) presented
+ a datatype update technique called "shadow data structures".
+++ /dev/null
-================
-Shadow Variables
-================
-
-Shadow variables are a simple way for livepatch modules to associate
-additional "shadow" data with existing data structures. Shadow data is
-allocated separately from parent data structures, which are left
-unmodified. The shadow variable API described in this document is used
-to allocate/add and remove/free shadow variables to/from their parents.
-
-The implementation introduces a global, in-kernel hashtable that
-associates pointers to parent objects and a numeric identifier of the
-shadow data. The numeric identifier is a simple enumeration that may be
-used to describe shadow variable version, class or type, etc. More
-specifically, the parent pointer serves as the hashtable key while the
-numeric id subsequently filters hashtable queries. Multiple shadow
-variables may attach to the same parent object, but their numeric
-identifier distinguishes between them.
-
-
-1. Brief API summary
-====================
-
-(See the full API usage docbook notes in livepatch/shadow.c.)
-
-A hashtable references all shadow variables. These references are
-stored and retrieved through a <obj, id> pair.
-
-* The klp_shadow variable data structure encapsulates both tracking
-meta-data and shadow-data:
- - meta-data
- - obj - pointer to parent object
- - id - data identifier
- - data[] - storage for shadow data
-
-It is important to note that the klp_shadow_alloc() and
-klp_shadow_get_or_alloc() are zeroing the variable by default.
-They also allow to call a custom constructor function when a non-zero
-value is needed. Callers should provide whatever mutual exclusion
-is required.
-
-Note that the constructor is called under klp_shadow_lock spinlock. It allows
-to do actions that can be done only once when a new variable is allocated.
-
-* klp_shadow_get() - retrieve a shadow variable data pointer
- - search hashtable for <obj, id> pair
-
-* klp_shadow_alloc() - allocate and add a new shadow variable
- - search hashtable for <obj, id> pair
- - if exists
- - WARN and return NULL
- - if <obj, id> doesn't already exist
- - allocate a new shadow variable
- - initialize the variable using a custom constructor and data when provided
- - add <obj, id> to the global hashtable
-
-* klp_shadow_get_or_alloc() - get existing or alloc a new shadow variable
- - search hashtable for <obj, id> pair
- - if exists
- - return existing shadow variable
- - if <obj, id> doesn't already exist
- - allocate a new shadow variable
- - initialize the variable using a custom constructor and data when provided
- - add <obj, id> pair to the global hashtable
-
-* klp_shadow_free() - detach and free a <obj, id> shadow variable
- - find and remove a <obj, id> reference from global hashtable
- - if found
- - call destructor function if defined
- - free shadow variable
-
-* klp_shadow_free_all() - detach and free all <*, id> shadow variables
- - find and remove any <*, id> references from global hashtable
- - if found
- - call destructor function if defined
- - free shadow variable
-
-
-2. Use cases
-============
-
-(See the example shadow variable livepatch modules in samples/livepatch/
-for full working demonstrations.)
-
-For the following use-case examples, consider commit 1d147bfa6429
-("mac80211: fix AP powersave TX vs. wakeup race"), which added a
-spinlock to net/mac80211/sta_info.h :: struct sta_info. Each use-case
-example can be considered a stand-alone livepatch implementation of this
-fix.
-
-
-Matching parent's lifecycle
----------------------------
-
-If parent data structures are frequently created and destroyed, it may
-be easiest to align their shadow variables lifetimes to the same
-allocation and release functions. In this case, the parent data
-structure is typically allocated, initialized, then registered in some
-manner. Shadow variable allocation and setup can then be considered
-part of the parent's initialization and should be completed before the
-parent "goes live" (ie, any shadow variable get-API requests are made
-for this <obj, id> pair.)
-
-For commit 1d147bfa6429, when a parent sta_info structure is allocated,
-allocate a shadow copy of the ps_lock pointer, then initialize it:
-
-#define PS_LOCK 1
-struct sta_info *sta_info_alloc(struct ieee80211_sub_if_data *sdata,
- const u8 *addr, gfp_t gfp)
-{
- struct sta_info *sta;
- spinlock_t *ps_lock;
-
- /* Parent structure is created */
- sta = kzalloc(sizeof(*sta) + hw->sta_data_size, gfp);
-
- /* Attach a corresponding shadow variable, then initialize it */
- ps_lock = klp_shadow_alloc(sta, PS_LOCK, sizeof(*ps_lock), gfp,
- NULL, NULL);
- if (!ps_lock)
- goto shadow_fail;
- spin_lock_init(ps_lock);
- ...
-
-When requiring a ps_lock, query the shadow variable API to retrieve one
-for a specific struct sta_info:
-
-void ieee80211_sta_ps_deliver_wakeup(struct sta_info *sta)
-{
- spinlock_t *ps_lock;
-
- /* sync with ieee80211_tx_h_unicast_ps_buf */
- ps_lock = klp_shadow_get(sta, PS_LOCK);
- if (ps_lock)
- spin_lock(ps_lock);
- ...
-
-When the parent sta_info structure is freed, first free the shadow
-variable:
-
-void sta_info_free(struct ieee80211_local *local, struct sta_info *sta)
-{
- klp_shadow_free(sta, PS_LOCK, NULL);
- kfree(sta);
- ...
-
-
-In-flight parent objects
-------------------------
-
-Sometimes it may not be convenient or possible to allocate shadow
-variables alongside their parent objects. Or a livepatch fix may
-require shadow varibles to only a subset of parent object instances. In
-these cases, the klp_shadow_get_or_alloc() call can be used to attach
-shadow variables to parents already in-flight.
-
-For commit 1d147bfa6429, a good spot to allocate a shadow spinlock is
-inside ieee80211_sta_ps_deliver_wakeup():
-
-int ps_lock_shadow_ctor(void *obj, void *shadow_data, void *ctor_data)
-{
- spinlock_t *lock = shadow_data;
-
- spin_lock_init(lock);
- return 0;
-}
-
-#define PS_LOCK 1
-void ieee80211_sta_ps_deliver_wakeup(struct sta_info *sta)
-{
- spinlock_t *ps_lock;
-
- /* sync with ieee80211_tx_h_unicast_ps_buf */
- ps_lock = klp_shadow_get_or_alloc(sta, PS_LOCK,
- sizeof(*ps_lock), GFP_ATOMIC,
- ps_lock_shadow_ctor, NULL);
-
- if (ps_lock)
- spin_lock(ps_lock);
- ...
-
-This usage will create a shadow variable, only if needed, otherwise it
-will use one that was already created for this <obj, id> pair.
-
-Like the previous use-case, the shadow spinlock needs to be cleaned up.
-A shadow variable can be freed just before its parent object is freed,
-or even when the shadow variable itself is no longer required.
-
-
-Other use-cases
----------------
-
-Shadow variables can also be used as a flag indicating that a data
-structure was allocated by new, livepatched code. In this case, it
-doesn't matter what data value the shadow variable holds, its existence
-suggests how to handle the parent object.
-
-
-3. References
-=============
-
-* https://github.com/dynup/kpatch
-The livepatch implementation is based on the kpatch version of shadow
-variables.
-
-* http://files.mkgnu.net/files/dynamos/doc/papers/dynamos_eurosys_07.pdf
-Dynamic and Adaptive Updates of Non-Quiescent Subsystems in Commodity
-Operating System Kernels (Kritis Makris, Kyung Dong Ryu 2007) presented
-a datatype update technique called "shadow data structures".
be detectable). Objtool makes that possible.
For more details, see the livepatch documentation in the Linux kernel
- source tree at Documentation/livepatch/livepatch.txt.
+ source tree at Documentation/livepatch/livepatch.rst.
Rules
-----
projects / mirror_ubuntu-focal-kernel.git / commitdiff