+++ /dev/null
-=======================
-Kernel Probes (Kprobes)
-=======================
-
-:Author: Jim Keniston <jkenisto@us.ibm.com>
-:Author: Prasanna S Panchamukhi <prasanna.panchamukhi@gmail.com>
-:Author: Masami Hiramatsu <mhiramat@redhat.com>
-
-.. CONTENTS
-
- 1. Concepts: Kprobes, and Return Probes
- 2. Architectures Supported
- 3. Configuring Kprobes
- 4. API Reference
- 5. Kprobes Features and Limitations
- 6. Probe Overhead
- 7. TODO
- 8. Kprobes Example
- 9. Kretprobes Example
- 10. Deprecated Features
- Appendix A: The kprobes debugfs interface
- Appendix B: The kprobes sysctl interface
- Appendix C: References
-
-Concepts: Kprobes and Return Probes
-=========================================
-
-Kprobes enables you to dynamically break into any kernel routine and
-collect debugging and performance information non-disruptively. You
-can trap at almost any kernel code address [1]_, specifying a handler
-routine to be invoked when the breakpoint is hit.
-
-.. [1] some parts of the kernel code can not be trapped, see
- :ref:`kprobes_blacklist`)
-
-There are currently two types of probes: kprobes, and kretprobes
-(also called return probes). A kprobe can be inserted on virtually
-any instruction in the kernel. A return probe fires when a specified
-function returns.
-
-In the typical case, Kprobes-based instrumentation is packaged as
-a kernel module. The module's init function installs ("registers")
-one or more probes, and the exit function unregisters them. A
-registration function such as register_kprobe() specifies where
-the probe is to be inserted and what handler is to be called when
-the probe is hit.
-
-There are also ``register_/unregister_*probes()`` functions for batch
-registration/unregistration of a group of ``*probes``. These functions
-can speed up unregistration process when you have to unregister
-a lot of probes at once.
-
-The next four subsections explain how the different types of
-probes work and how jump optimization works. They explain certain
-things that you'll need to know in order to make the best use of
-Kprobes -- e.g., the difference between a pre_handler and
-a post_handler, and how to use the maxactive and nmissed fields of
-a kretprobe. But if you're in a hurry to start using Kprobes, you
-can skip ahead to :ref:`kprobes_archs_supported`.
-
-How Does a Kprobe Work?
------------------------
-
-When a kprobe is registered, Kprobes makes a copy of the probed
-instruction and replaces the first byte(s) of the probed instruction
-with a breakpoint instruction (e.g., int3 on i386 and x86_64).
-
-When a CPU hits the breakpoint instruction, a trap occurs, the CPU's
-registers are saved, and control passes to Kprobes via the
-notifier_call_chain mechanism. Kprobes executes the "pre_handler"
-associated with the kprobe, passing the handler the addresses of the
-kprobe struct and the saved registers.
-
-Next, Kprobes single-steps its copy of the probed instruction.
-(It would be simpler to single-step the actual instruction in place,
-but then Kprobes would have to temporarily remove the breakpoint
-instruction. This would open a small time window when another CPU
-could sail right past the probepoint.)
-
-After the instruction is single-stepped, Kprobes executes the
-"post_handler," if any, that is associated with the kprobe.
-Execution then continues with the instruction following the probepoint.
-
-Changing Execution Path
------------------------
-
-Since kprobes can probe into a running kernel code, it can change the
-register set, including instruction pointer. This operation requires
-maximum care, such as keeping the stack frame, recovering the execution
-path etc. Since it operates on a running kernel and needs deep knowledge
-of computer architecture and concurrent computing, you can easily shoot
-your foot.
-
-If you change the instruction pointer (and set up other related
-registers) in pre_handler, you must return !0 so that kprobes stops
-single stepping and just returns to the given address.
-This also means post_handler should not be called anymore.
-
-Note that this operation may be harder on some architectures which use
-TOC (Table of Contents) for function call, since you have to setup a new
-TOC for your function in your module, and recover the old one after
-returning from it.
-
-Return Probes
--------------
-
-How Does a Return Probe Work?
-^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-
-When you call register_kretprobe(), Kprobes establishes a kprobe at
-the entry to the function. When the probed function is called and this
-probe is hit, Kprobes saves a copy of the return address, and replaces
-the return address with the address of a "trampoline." The trampoline
-is an arbitrary piece of code -- typically just a nop instruction.
-At boot time, Kprobes registers a kprobe at the trampoline.
-
-When the probed function executes its return instruction, control
-passes to the trampoline and that probe is hit. Kprobes' trampoline
-handler calls the user-specified return handler associated with the
-kretprobe, then sets the saved instruction pointer to the saved return
-address, and that's where execution resumes upon return from the trap.
-
-While the probed function is executing, its return address is
-stored in an object of type kretprobe_instance. Before calling
-register_kretprobe(), the user sets the maxactive field of the
-kretprobe struct to specify how many instances of the specified
-function can be probed simultaneously. register_kretprobe()
-pre-allocates the indicated number of kretprobe_instance objects.
-
-For example, if the function is non-recursive and is called with a
-spinlock held, maxactive = 1 should be enough. If the function is
-non-recursive and can never relinquish the CPU (e.g., via a semaphore
-or preemption), NR_CPUS should be enough. If maxactive <= 0, it is
-set to a default value. If CONFIG_PREEMPT is enabled, the default
-is max(10, 2*NR_CPUS). Otherwise, the default is NR_CPUS.
-
-It's not a disaster if you set maxactive too low; you'll just miss
-some probes. In the kretprobe struct, the nmissed field is set to
-zero when the return probe is registered, and is incremented every
-time the probed function is entered but there is no kretprobe_instance
-object available for establishing the return probe.
-
-Kretprobe entry-handler
-^^^^^^^^^^^^^^^^^^^^^^^
-
-Kretprobes also provides an optional user-specified handler which runs
-on function entry. This handler is specified by setting the entry_handler
-field of the kretprobe struct. Whenever the kprobe placed by kretprobe at the
-function entry is hit, the user-defined entry_handler, if any, is invoked.
-If the entry_handler returns 0 (success) then a corresponding return handler
-is guaranteed to be called upon function return. If the entry_handler
-returns a non-zero error then Kprobes leaves the return address as is, and
-the kretprobe has no further effect for that particular function instance.
-
-Multiple entry and return handler invocations are matched using the unique
-kretprobe_instance object associated with them. Additionally, a user
-may also specify per return-instance private data to be part of each
-kretprobe_instance object. This is especially useful when sharing private
-data between corresponding user entry and return handlers. The size of each
-private data object can be specified at kretprobe registration time by
-setting the data_size field of the kretprobe struct. This data can be
-accessed through the data field of each kretprobe_instance object.
-
-In case probed function is entered but there is no kretprobe_instance
-object available, then in addition to incrementing the nmissed count,
-the user entry_handler invocation is also skipped.
-
-.. _kprobes_jump_optimization:
-
-How Does Jump Optimization Work?
---------------------------------
-
-If your kernel is built with CONFIG_OPTPROBES=y (currently this flag
-is automatically set 'y' on x86/x86-64, non-preemptive kernel) and
-the "debug.kprobes_optimization" kernel parameter is set to 1 (see
-sysctl(8)), Kprobes tries to reduce probe-hit overhead by using a jump
-instruction instead of a breakpoint instruction at each probepoint.
-
-Init a Kprobe
-^^^^^^^^^^^^^
-
-When a probe is registered, before attempting this optimization,
-Kprobes inserts an ordinary, breakpoint-based kprobe at the specified
-address. So, even if it's not possible to optimize this particular
-probepoint, there'll be a probe there.
-
-Safety Check
-^^^^^^^^^^^^
-
-Before optimizing a probe, Kprobes performs the following safety checks:
-
-- Kprobes verifies that the region that will be replaced by the jump
- instruction (the "optimized region") lies entirely within one function.
- (A jump instruction is multiple bytes, and so may overlay multiple
- instructions.)
-
-- Kprobes analyzes the entire function and verifies that there is no
- jump into the optimized region. Specifically:
-
- - the function contains no indirect jump;
- - the function contains no instruction that causes an exception (since
- the fixup code triggered by the exception could jump back into the
- optimized region -- Kprobes checks the exception tables to verify this);
- - there is no near jump to the optimized region (other than to the first
- byte).
-
-- For each instruction in the optimized region, Kprobes verifies that
- the instruction can be executed out of line.
-
-Preparing Detour Buffer
-^^^^^^^^^^^^^^^^^^^^^^^
-
-Next, Kprobes prepares a "detour" buffer, which contains the following
-instruction sequence:
-
-- code to push the CPU's registers (emulating a breakpoint trap)
-- a call to the trampoline code which calls user's probe handlers.
-- code to restore registers
-- the instructions from the optimized region
-- a jump back to the original execution path.
-
-Pre-optimization
-^^^^^^^^^^^^^^^^
-
-After preparing the detour buffer, Kprobes verifies that none of the
-following situations exist:
-
-- The probe has a post_handler.
-- Other instructions in the optimized region are probed.
-- The probe is disabled.
-
-In any of the above cases, Kprobes won't start optimizing the probe.
-Since these are temporary situations, Kprobes tries to start
-optimizing it again if the situation is changed.
-
-If the kprobe can be optimized, Kprobes enqueues the kprobe to an
-optimizing list, and kicks the kprobe-optimizer workqueue to optimize
-it. If the to-be-optimized probepoint is hit before being optimized,
-Kprobes returns control to the original instruction path by setting
-the CPU's instruction pointer to the copied code in the detour buffer
--- thus at least avoiding the single-step.
-
-Optimization
-^^^^^^^^^^^^
-
-The Kprobe-optimizer doesn't insert the jump instruction immediately;
-rather, it calls synchronize_rcu() for safety first, because it's
-possible for a CPU to be interrupted in the middle of executing the
-optimized region [3]_. As you know, synchronize_rcu() can ensure
-that all interruptions that were active when synchronize_rcu()
-was called are done, but only if CONFIG_PREEMPT=n. So, this version
-of kprobe optimization supports only kernels with CONFIG_PREEMPT=n [4]_.
-
-After that, the Kprobe-optimizer calls stop_machine() to replace
-the optimized region with a jump instruction to the detour buffer,
-using text_poke_smp().
-
-Unoptimization
-^^^^^^^^^^^^^^
-
-When an optimized kprobe is unregistered, disabled, or blocked by
-another kprobe, it will be unoptimized. If this happens before
-the optimization is complete, the kprobe is just dequeued from the
-optimized list. If the optimization has been done, the jump is
-replaced with the original code (except for an int3 breakpoint in
-the first byte) by using text_poke_smp().
-
-.. [3] Please imagine that the 2nd instruction is interrupted and then
- the optimizer replaces the 2nd instruction with the jump *address*
- while the interrupt handler is running. When the interrupt
- returns to original address, there is no valid instruction,
- and it causes an unexpected result.
-
-.. [4] This optimization-safety checking may be replaced with the
- stop-machine method that ksplice uses for supporting a CONFIG_PREEMPT=y
- kernel.
-
-NOTE for geeks:
-The jump optimization changes the kprobe's pre_handler behavior.
-Without optimization, the pre_handler can change the kernel's execution
-path by changing regs->ip and returning 1. However, when the probe
-is optimized, that modification is ignored. Thus, if you want to
-tweak the kernel's execution path, you need to suppress optimization,
-using one of the following techniques:
-
-- Specify an empty function for the kprobe's post_handler.
-
-or
-
-- Execute 'sysctl -w debug.kprobes_optimization=n'
-
-.. _kprobes_blacklist:
-
-Blacklist
----------
-
-Kprobes can probe most of the kernel except itself. This means
-that there are some functions where kprobes cannot probe. Probing
-(trapping) such functions can cause a recursive trap (e.g. double
-fault) or the nested probe handler may never be called.
-Kprobes manages such functions as a blacklist.
-If you want to add a function into the blacklist, you just need
-to (1) include linux/kprobes.h and (2) use NOKPROBE_SYMBOL() macro
-to specify a blacklisted function.
-Kprobes checks the given probe address against the blacklist and
-rejects registering it, if the given address is in the blacklist.
-
-.. _kprobes_archs_supported:
-
-Architectures Supported
-=======================
-
-Kprobes and return probes are implemented on the following
-architectures:
-
-- i386 (Supports jump optimization)
-- x86_64 (AMD-64, EM64T) (Supports jump optimization)
-- ppc64
-- ia64 (Does not support probes on instruction slot1.)
-- sparc64 (Return probes not yet implemented.)
-- arm
-- ppc
-- mips
-- s390
-- parisc
-
-Configuring Kprobes
-===================
-
-When configuring the kernel using make menuconfig/xconfig/oldconfig,
-ensure that CONFIG_KPROBES is set to "y". Under "General setup", look
-for "Kprobes".
-
-So that you can load and unload Kprobes-based instrumentation modules,
-make sure "Loadable module support" (CONFIG_MODULES) and "Module
-unloading" (CONFIG_MODULE_UNLOAD) are set to "y".
-
-Also make sure that CONFIG_KALLSYMS and perhaps even CONFIG_KALLSYMS_ALL
-are set to "y", since kallsyms_lookup_name() is used by the in-kernel
-kprobe address resolution code.
-
-If you need to insert a probe in the middle of a function, you may find
-it useful to "Compile the kernel with debug info" (CONFIG_DEBUG_INFO),
-so you can use "objdump -d -l vmlinux" to see the source-to-object
-code mapping.
-
-API Reference
-=============
-
-The Kprobes API includes a "register" function and an "unregister"
-function for each type of probe. The API also includes "register_*probes"
-and "unregister_*probes" functions for (un)registering arrays of probes.
-Here are terse, mini-man-page specifications for these functions and
-the associated probe handlers that you'll write. See the files in the
-samples/kprobes/ sub-directory for examples.
-
-register_kprobe
----------------
-
-::
-
- #include <linux/kprobes.h>
- int register_kprobe(struct kprobe *kp);
-
-Sets a breakpoint at the address kp->addr. When the breakpoint is
-hit, Kprobes calls kp->pre_handler. After the probed instruction
-is single-stepped, Kprobe calls kp->post_handler. If a fault
-occurs during execution of kp->pre_handler or kp->post_handler,
-or during single-stepping of the probed instruction, Kprobes calls
-kp->fault_handler. Any or all handlers can be NULL. If kp->flags
-is set KPROBE_FLAG_DISABLED, that kp will be registered but disabled,
-so, its handlers aren't hit until calling enable_kprobe(kp).
-
-.. note::
-
- 1. With the introduction of the "symbol_name" field to struct kprobe,
- the probepoint address resolution will now be taken care of by the kernel.
- The following will now work::
-
- kp.symbol_name = "symbol_name";
-
- (64-bit powerpc intricacies such as function descriptors are handled
- transparently)
-
- 2. Use the "offset" field of struct kprobe if the offset into the symbol
- to install a probepoint is known. This field is used to calculate the
- probepoint.
-
- 3. Specify either the kprobe "symbol_name" OR the "addr". If both are
- specified, kprobe registration will fail with -EINVAL.
-
- 4. With CISC architectures (such as i386 and x86_64), the kprobes code
- does not validate if the kprobe.addr is at an instruction boundary.
- Use "offset" with caution.
-
-register_kprobe() returns 0 on success, or a negative errno otherwise.
-
-User's pre-handler (kp->pre_handler)::
-
- #include <linux/kprobes.h>
- #include <linux/ptrace.h>
- int pre_handler(struct kprobe *p, struct pt_regs *regs);
-
-Called with p pointing to the kprobe associated with the breakpoint,
-and regs pointing to the struct containing the registers saved when
-the breakpoint was hit. Return 0 here unless you're a Kprobes geek.
-
-User's post-handler (kp->post_handler)::
-
- #include <linux/kprobes.h>
- #include <linux/ptrace.h>
- void post_handler(struct kprobe *p, struct pt_regs *regs,
- unsigned long flags);
-
-p and regs are as described for the pre_handler. flags always seems
-to be zero.
-
-User's fault-handler (kp->fault_handler)::
-
- #include <linux/kprobes.h>
- #include <linux/ptrace.h>
- int fault_handler(struct kprobe *p, struct pt_regs *regs, int trapnr);
-
-p and regs are as described for the pre_handler. trapnr is the
-architecture-specific trap number associated with the fault (e.g.,
-on i386, 13 for a general protection fault or 14 for a page fault).
-Returns 1 if it successfully handled the exception.
-
-register_kretprobe
-------------------
-
-::
-
- #include <linux/kprobes.h>
- int register_kretprobe(struct kretprobe *rp);
-
-Establishes a return probe for the function whose address is
-rp->kp.addr. When that function returns, Kprobes calls rp->handler.
-You must set rp->maxactive appropriately before you call
-register_kretprobe(); see "How Does a Return Probe Work?" for details.
-
-register_kretprobe() returns 0 on success, or a negative errno
-otherwise.
-
-User's return-probe handler (rp->handler)::
-
- #include <linux/kprobes.h>
- #include <linux/ptrace.h>
- int kretprobe_handler(struct kretprobe_instance *ri,
- struct pt_regs *regs);
-
-regs is as described for kprobe.pre_handler. ri points to the
-kretprobe_instance object, of which the following fields may be
-of interest:
-
-- ret_addr: the return address
-- rp: points to the corresponding kretprobe object
-- task: points to the corresponding task struct
-- data: points to per return-instance private data; see "Kretprobe
- entry-handler" for details.
-
-The regs_return_value(regs) macro provides a simple abstraction to
-extract the return value from the appropriate register as defined by
-the architecture's ABI.
-
-The handler's return value is currently ignored.
-
-unregister_*probe
-------------------
-
-::
-
- #include <linux/kprobes.h>
- void unregister_kprobe(struct kprobe *kp);
- void unregister_kretprobe(struct kretprobe *rp);
-
-Removes the specified probe. The unregister function can be called
-at any time after the probe has been registered.
-
-.. note::
-
- If the functions find an incorrect probe (ex. an unregistered probe),
- they clear the addr field of the probe.
-
-register_*probes
-----------------
-
-::
-
- #include <linux/kprobes.h>
- int register_kprobes(struct kprobe **kps, int num);
- int register_kretprobes(struct kretprobe **rps, int num);
-
-Registers each of the num probes in the specified array. If any
-error occurs during registration, all probes in the array, up to
-the bad probe, are safely unregistered before the register_*probes
-function returns.
-
-- kps/rps: an array of pointers to ``*probe`` data structures
-- num: the number of the array entries.
-
-.. note::
-
- You have to allocate(or define) an array of pointers and set all
- of the array entries before using these functions.
-
-unregister_*probes
-------------------
-
-::
-
- #include <linux/kprobes.h>
- void unregister_kprobes(struct kprobe **kps, int num);
- void unregister_kretprobes(struct kretprobe **rps, int num);
-
-Removes each of the num probes in the specified array at once.
-
-.. note::
-
- If the functions find some incorrect probes (ex. unregistered
- probes) in the specified array, they clear the addr field of those
- incorrect probes. However, other probes in the array are
- unregistered correctly.
-
-disable_*probe
---------------
-
-::
-
- #include <linux/kprobes.h>
- int disable_kprobe(struct kprobe *kp);
- int disable_kretprobe(struct kretprobe *rp);
-
-Temporarily disables the specified ``*probe``. You can enable it again by using
-enable_*probe(). You must specify the probe which has been registered.
-
-enable_*probe
--------------
-
-::
-
- #include <linux/kprobes.h>
- int enable_kprobe(struct kprobe *kp);
- int enable_kretprobe(struct kretprobe *rp);
-
-Enables ``*probe`` which has been disabled by disable_*probe(). You must specify
-the probe which has been registered.
-
-Kprobes Features and Limitations
-================================
-
-Kprobes allows multiple probes at the same address. Also,
-a probepoint for which there is a post_handler cannot be optimized.
-So if you install a kprobe with a post_handler, at an optimized
-probepoint, the probepoint will be unoptimized automatically.
-
-In general, you can install a probe anywhere in the kernel.
-In particular, you can probe interrupt handlers. Known exceptions
-are discussed in this section.
-
-The register_*probe functions will return -EINVAL if you attempt
-to install a probe in the code that implements Kprobes (mostly
-kernel/kprobes.c and ``arch/*/kernel/kprobes.c``, but also functions such
-as do_page_fault and notifier_call_chain).
-
-If you install a probe in an inline-able function, Kprobes makes
-no attempt to chase down all inline instances of the function and
-install probes there. gcc may inline a function without being asked,
-so keep this in mind if you're not seeing the probe hits you expect.
-
-A probe handler can modify the environment of the probed function
--- e.g., by modifying kernel data structures, or by modifying the
-contents of the pt_regs struct (which are restored to the registers
-upon return from the breakpoint). So Kprobes can be used, for example,
-to install a bug fix or to inject faults for testing. Kprobes, of
-course, has no way to distinguish the deliberately injected faults
-from the accidental ones. Don't drink and probe.
-
-Kprobes makes no attempt to prevent probe handlers from stepping on
-each other -- e.g., probing printk() and then calling printk() from a
-probe handler. If a probe handler hits a probe, that second probe's
-handlers won't be run in that instance, and the kprobe.nmissed member
-of the second probe will be incremented.
-
-As of Linux v2.6.15-rc1, multiple handlers (or multiple instances of
-the same handler) may run concurrently on different CPUs.
-
-Kprobes does not use mutexes or allocate memory except during
-registration and unregistration.
-
-Probe handlers are run with preemption disabled or interrupt disabled,
-which depends on the architecture and optimization state. (e.g.,
-kretprobe handlers and optimized kprobe handlers run without interrupt
-disabled on x86/x86-64). In any case, your handler should not yield
-the CPU (e.g., by attempting to acquire a semaphore, or waiting I/O).
-
-Since a return probe is implemented by replacing the return
-address with the trampoline's address, stack backtraces and calls
-to __builtin_return_address() will typically yield the trampoline's
-address instead of the real return address for kretprobed functions.
-(As far as we can tell, __builtin_return_address() is used only
-for instrumentation and error reporting.)
-
-If the number of times a function is called does not match the number
-of times it returns, registering a return probe on that function may
-produce undesirable results. In such a case, a line:
-kretprobe BUG!: Processing kretprobe d000000000041aa8 @ c00000000004f48c
-gets printed. With this information, one will be able to correlate the
-exact instance of the kretprobe that caused the problem. We have the
-do_exit() case covered. do_execve() and do_fork() are not an issue.
-We're unaware of other specific cases where this could be a problem.
-
-If, upon entry to or exit from a function, the CPU is running on
-a stack other than that of the current task, registering a return
-probe on that function may produce undesirable results. For this
-reason, Kprobes doesn't support return probes (or kprobes)
-on the x86_64 version of __switch_to(); the registration functions
-return -EINVAL.
-
-On x86/x86-64, since the Jump Optimization of Kprobes modifies
-instructions widely, there are some limitations to optimization. To
-explain it, we introduce some terminology. Imagine a 3-instruction
-sequence consisting of a two 2-byte instructions and one 3-byte
-instruction.
-
-::
-
- IA
- |
- [-2][-1][0][1][2][3][4][5][6][7]
- [ins1][ins2][ ins3 ]
- [<- DCR ->]
- [<- JTPR ->]
-
- ins1: 1st Instruction
- ins2: 2nd Instruction
- ins3: 3rd Instruction
- IA: Insertion Address
- JTPR: Jump Target Prohibition Region
- DCR: Detoured Code Region
-
-The instructions in DCR are copied to the out-of-line buffer
-of the kprobe, because the bytes in DCR are replaced by
-a 5-byte jump instruction. So there are several limitations.
-
-a) The instructions in DCR must be relocatable.
-b) The instructions in DCR must not include a call instruction.
-c) JTPR must not be targeted by any jump or call instruction.
-d) DCR must not straddle the border between functions.
-
-Anyway, these limitations are checked by the in-kernel instruction
-decoder, so you don't need to worry about that.
-
-Probe Overhead
-==============
-
-On a typical CPU in use in 2005, a kprobe hit takes 0.5 to 1.0
-microseconds to process. Specifically, a benchmark that hits the same
-probepoint repeatedly, firing a simple handler each time, reports 1-2
-million hits per second, depending on the architecture. A return-probe
-hit typically takes 50-75% longer than a kprobe hit.
-When you have a return probe set on a function, adding a kprobe at
-the entry to that function adds essentially no overhead.
-
-Here are sample overhead figures (in usec) for different architectures::
-
- k = kprobe; r = return probe; kr = kprobe + return probe
- on same function
-
- i386: Intel Pentium M, 1495 MHz, 2957.31 bogomips
- k = 0.57 usec; r = 0.92; kr = 0.99
-
- x86_64: AMD Opteron 246, 1994 MHz, 3971.48 bogomips
- k = 0.49 usec; r = 0.80; kr = 0.82
-
- ppc64: POWER5 (gr), 1656 MHz (SMT disabled, 1 virtual CPU per physical CPU)
- k = 0.77 usec; r = 1.26; kr = 1.45
-
-Optimized Probe Overhead
-------------------------
-
-Typically, an optimized kprobe hit takes 0.07 to 0.1 microseconds to
-process. Here are sample overhead figures (in usec) for x86 architectures::
-
- k = unoptimized kprobe, b = boosted (single-step skipped), o = optimized kprobe,
- r = unoptimized kretprobe, rb = boosted kretprobe, ro = optimized kretprobe.
-
- i386: Intel(R) Xeon(R) E5410, 2.33GHz, 4656.90 bogomips
- k = 0.80 usec; b = 0.33; o = 0.05; r = 1.10; rb = 0.61; ro = 0.33
-
- x86-64: Intel(R) Xeon(R) E5410, 2.33GHz, 4656.90 bogomips
- k = 0.99 usec; b = 0.43; o = 0.06; r = 1.24; rb = 0.68; ro = 0.30
-
-TODO
-====
-
-a. SystemTap (http://sourceware.org/systemtap): Provides a simplified
- programming interface for probe-based instrumentation. Try it out.
-b. Kernel return probes for sparc64.
-c. Support for other architectures.
-d. User-space probes.
-e. Watchpoint probes (which fire on data references).
-
-Kprobes Example
-===============
-
-See samples/kprobes/kprobe_example.c
-
-Kretprobes Example
-==================
-
-See samples/kprobes/kretprobe_example.c
-
-Deprecated Features
-===================
-
-Jprobes is now a deprecated feature. People who are depending on it should
-migrate to other tracing features or use older kernels. Please consider to
-migrate your tool to one of the following options:
-
-- Use trace-event to trace target function with arguments.
-
- trace-event is a low-overhead (and almost no visible overhead if it
- is off) statically defined event interface. You can define new events
- and trace it via ftrace or any other tracing tools.
-
- See the following urls:
-
- - https://lwn.net/Articles/379903/
- - https://lwn.net/Articles/381064/
- - https://lwn.net/Articles/383362/
-
-- Use ftrace dynamic events (kprobe event) with perf-probe.
-
- If you build your kernel with debug info (CONFIG_DEBUG_INFO=y), you can
- find which register/stack is assigned to which local variable or arguments
- by using perf-probe and set up new event to trace it.
-
- See following documents:
-
- - Documentation/trace/kprobetrace.rst
- - Documentation/trace/events.rst
- - tools/perf/Documentation/perf-probe.txt
-
-
-The kprobes debugfs interface
-=============================
-
-
-With recent kernels (> 2.6.20) the list of registered kprobes is visible
-under the /sys/kernel/debug/kprobes/ directory (assuming debugfs is mounted at //sys/kernel/debug).
-
-/sys/kernel/debug/kprobes/list: Lists all registered probes on the system::
-
- c015d71a k vfs_read+0x0
- c03dedc5 r tcp_v4_rcv+0x0
-
-The first column provides the kernel address where the probe is inserted.
-The second column identifies the type of probe (k - kprobe and r - kretprobe)
-while the third column specifies the symbol+offset of the probe.
-If the probed function belongs to a module, the module name is also
-specified. Following columns show probe status. If the probe is on
-a virtual address that is no longer valid (module init sections, module
-virtual addresses that correspond to modules that've been unloaded),
-such probes are marked with [GONE]. If the probe is temporarily disabled,
-such probes are marked with [DISABLED]. If the probe is optimized, it is
-marked with [OPTIMIZED]. If the probe is ftrace-based, it is marked with
-[FTRACE].
-
-/sys/kernel/debug/kprobes/enabled: Turn kprobes ON/OFF forcibly.
-
-Provides a knob to globally and forcibly turn registered kprobes ON or OFF.
-By default, all kprobes are enabled. By echoing "0" to this file, all
-registered probes will be disarmed, till such time a "1" is echoed to this
-file. Note that this knob just disarms and arms all kprobes and doesn't
-change each probe's disabling state. This means that disabled kprobes (marked
-[DISABLED]) will be not enabled if you turn ON all kprobes by this knob.
-
-
-The kprobes sysctl interface
-============================
-
-/proc/sys/debug/kprobes-optimization: Turn kprobes optimization ON/OFF.
-
-When CONFIG_OPTPROBES=y, this sysctl interface appears and it provides
-a knob to globally and forcibly turn jump optimization (see section
-:ref:`kprobes_jump_optimization`) ON or OFF. By default, jump optimization
-is allowed (ON). If you echo "0" to this file or set
-"debug.kprobes_optimization" to 0 via sysctl, all optimized probes will be
-unoptimized, and any new probes registered after that will not be optimized.
-
-Note that this knob *changes* the optimized state. This means that optimized
-probes (marked [OPTIMIZED]) will be unoptimized ([OPTIMIZED] tag will be
-removed). If the knob is turned on, they will be optimized again.
-
-References
-==========
-
-For additional information on Kprobes, refer to the following URLs:
-
-- https://www.ibm.com/developerworks/library/l-kprobes/index.html
-- https://www.kernel.org/doc/ols/2006/ols2006v2-pages-109-124.pdf
-
--- /dev/null
+=======================
+Kernel Probes (Kprobes)
+=======================
+
+:Author: Jim Keniston <jkenisto@us.ibm.com>
+:Author: Prasanna S Panchamukhi <prasanna.panchamukhi@gmail.com>
+:Author: Masami Hiramatsu <mhiramat@redhat.com>
+
+.. CONTENTS
+
+ 1. Concepts: Kprobes, and Return Probes
+ 2. Architectures Supported
+ 3. Configuring Kprobes
+ 4. API Reference
+ 5. Kprobes Features and Limitations
+ 6. Probe Overhead
+ 7. TODO
+ 8. Kprobes Example
+ 9. Kretprobes Example
+ 10. Deprecated Features
+ Appendix A: The kprobes debugfs interface
+ Appendix B: The kprobes sysctl interface
+ Appendix C: References
+
+Concepts: Kprobes and Return Probes
+=========================================
+
+Kprobes enables you to dynamically break into any kernel routine and
+collect debugging and performance information non-disruptively. You
+can trap at almost any kernel code address [1]_, specifying a handler
+routine to be invoked when the breakpoint is hit.
+
+.. [1] some parts of the kernel code can not be trapped, see
+ :ref:`kprobes_blacklist`)
+
+There are currently two types of probes: kprobes, and kretprobes
+(also called return probes). A kprobe can be inserted on virtually
+any instruction in the kernel. A return probe fires when a specified
+function returns.
+
+In the typical case, Kprobes-based instrumentation is packaged as
+a kernel module. The module's init function installs ("registers")
+one or more probes, and the exit function unregisters them. A
+registration function such as register_kprobe() specifies where
+the probe is to be inserted and what handler is to be called when
+the probe is hit.
+
+There are also ``register_/unregister_*probes()`` functions for batch
+registration/unregistration of a group of ``*probes``. These functions
+can speed up unregistration process when you have to unregister
+a lot of probes at once.
+
+The next four subsections explain how the different types of
+probes work and how jump optimization works. They explain certain
+things that you'll need to know in order to make the best use of
+Kprobes -- e.g., the difference between a pre_handler and
+a post_handler, and how to use the maxactive and nmissed fields of
+a kretprobe. But if you're in a hurry to start using Kprobes, you
+can skip ahead to :ref:`kprobes_archs_supported`.
+
+How Does a Kprobe Work?
+-----------------------
+
+When a kprobe is registered, Kprobes makes a copy of the probed
+instruction and replaces the first byte(s) of the probed instruction
+with a breakpoint instruction (e.g., int3 on i386 and x86_64).
+
+When a CPU hits the breakpoint instruction, a trap occurs, the CPU's
+registers are saved, and control passes to Kprobes via the
+notifier_call_chain mechanism. Kprobes executes the "pre_handler"
+associated with the kprobe, passing the handler the addresses of the
+kprobe struct and the saved registers.
+
+Next, Kprobes single-steps its copy of the probed instruction.
+(It would be simpler to single-step the actual instruction in place,
+but then Kprobes would have to temporarily remove the breakpoint
+instruction. This would open a small time window when another CPU
+could sail right past the probepoint.)
+
+After the instruction is single-stepped, Kprobes executes the
+"post_handler," if any, that is associated with the kprobe.
+Execution then continues with the instruction following the probepoint.
+
+Changing Execution Path
+-----------------------
+
+Since kprobes can probe into a running kernel code, it can change the
+register set, including instruction pointer. This operation requires
+maximum care, such as keeping the stack frame, recovering the execution
+path etc. Since it operates on a running kernel and needs deep knowledge
+of computer architecture and concurrent computing, you can easily shoot
+your foot.
+
+If you change the instruction pointer (and set up other related
+registers) in pre_handler, you must return !0 so that kprobes stops
+single stepping and just returns to the given address.
+This also means post_handler should not be called anymore.
+
+Note that this operation may be harder on some architectures which use
+TOC (Table of Contents) for function call, since you have to setup a new
+TOC for your function in your module, and recover the old one after
+returning from it.
+
+Return Probes
+-------------
+
+How Does a Return Probe Work?
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+When you call register_kretprobe(), Kprobes establishes a kprobe at
+the entry to the function. When the probed function is called and this
+probe is hit, Kprobes saves a copy of the return address, and replaces
+the return address with the address of a "trampoline." The trampoline
+is an arbitrary piece of code -- typically just a nop instruction.
+At boot time, Kprobes registers a kprobe at the trampoline.
+
+When the probed function executes its return instruction, control
+passes to the trampoline and that probe is hit. Kprobes' trampoline
+handler calls the user-specified return handler associated with the
+kretprobe, then sets the saved instruction pointer to the saved return
+address, and that's where execution resumes upon return from the trap.
+
+While the probed function is executing, its return address is
+stored in an object of type kretprobe_instance. Before calling
+register_kretprobe(), the user sets the maxactive field of the
+kretprobe struct to specify how many instances of the specified
+function can be probed simultaneously. register_kretprobe()
+pre-allocates the indicated number of kretprobe_instance objects.
+
+For example, if the function is non-recursive and is called with a
+spinlock held, maxactive = 1 should be enough. If the function is
+non-recursive and can never relinquish the CPU (e.g., via a semaphore
+or preemption), NR_CPUS should be enough. If maxactive <= 0, it is
+set to a default value. If CONFIG_PREEMPT is enabled, the default
+is max(10, 2*NR_CPUS). Otherwise, the default is NR_CPUS.
+
+It's not a disaster if you set maxactive too low; you'll just miss
+some probes. In the kretprobe struct, the nmissed field is set to
+zero when the return probe is registered, and is incremented every
+time the probed function is entered but there is no kretprobe_instance
+object available for establishing the return probe.
+
+Kretprobe entry-handler
+^^^^^^^^^^^^^^^^^^^^^^^
+
+Kretprobes also provides an optional user-specified handler which runs
+on function entry. This handler is specified by setting the entry_handler
+field of the kretprobe struct. Whenever the kprobe placed by kretprobe at the
+function entry is hit, the user-defined entry_handler, if any, is invoked.
+If the entry_handler returns 0 (success) then a corresponding return handler
+is guaranteed to be called upon function return. If the entry_handler
+returns a non-zero error then Kprobes leaves the return address as is, and
+the kretprobe has no further effect for that particular function instance.
+
+Multiple entry and return handler invocations are matched using the unique
+kretprobe_instance object associated with them. Additionally, a user
+may also specify per return-instance private data to be part of each
+kretprobe_instance object. This is especially useful when sharing private
+data between corresponding user entry and return handlers. The size of each
+private data object can be specified at kretprobe registration time by
+setting the data_size field of the kretprobe struct. This data can be
+accessed through the data field of each kretprobe_instance object.
+
+In case probed function is entered but there is no kretprobe_instance
+object available, then in addition to incrementing the nmissed count,
+the user entry_handler invocation is also skipped.
+
+.. _kprobes_jump_optimization:
+
+How Does Jump Optimization Work?
+--------------------------------
+
+If your kernel is built with CONFIG_OPTPROBES=y (currently this flag
+is automatically set 'y' on x86/x86-64, non-preemptive kernel) and
+the "debug.kprobes_optimization" kernel parameter is set to 1 (see
+sysctl(8)), Kprobes tries to reduce probe-hit overhead by using a jump
+instruction instead of a breakpoint instruction at each probepoint.
+
+Init a Kprobe
+^^^^^^^^^^^^^
+
+When a probe is registered, before attempting this optimization,
+Kprobes inserts an ordinary, breakpoint-based kprobe at the specified
+address. So, even if it's not possible to optimize this particular
+probepoint, there'll be a probe there.
+
+Safety Check
+^^^^^^^^^^^^
+
+Before optimizing a probe, Kprobes performs the following safety checks:
+
+- Kprobes verifies that the region that will be replaced by the jump
+ instruction (the "optimized region") lies entirely within one function.
+ (A jump instruction is multiple bytes, and so may overlay multiple
+ instructions.)
+
+- Kprobes analyzes the entire function and verifies that there is no
+ jump into the optimized region. Specifically:
+
+ - the function contains no indirect jump;
+ - the function contains no instruction that causes an exception (since
+ the fixup code triggered by the exception could jump back into the
+ optimized region -- Kprobes checks the exception tables to verify this);
+ - there is no near jump to the optimized region (other than to the first
+ byte).
+
+- For each instruction in the optimized region, Kprobes verifies that
+ the instruction can be executed out of line.
+
+Preparing Detour Buffer
+^^^^^^^^^^^^^^^^^^^^^^^
+
+Next, Kprobes prepares a "detour" buffer, which contains the following
+instruction sequence:
+
+- code to push the CPU's registers (emulating a breakpoint trap)
+- a call to the trampoline code which calls user's probe handlers.
+- code to restore registers
+- the instructions from the optimized region
+- a jump back to the original execution path.
+
+Pre-optimization
+^^^^^^^^^^^^^^^^
+
+After preparing the detour buffer, Kprobes verifies that none of the
+following situations exist:
+
+- The probe has a post_handler.
+- Other instructions in the optimized region are probed.
+- The probe is disabled.
+
+In any of the above cases, Kprobes won't start optimizing the probe.
+Since these are temporary situations, Kprobes tries to start
+optimizing it again if the situation is changed.
+
+If the kprobe can be optimized, Kprobes enqueues the kprobe to an
+optimizing list, and kicks the kprobe-optimizer workqueue to optimize
+it. If the to-be-optimized probepoint is hit before being optimized,
+Kprobes returns control to the original instruction path by setting
+the CPU's instruction pointer to the copied code in the detour buffer
+-- thus at least avoiding the single-step.
+
+Optimization
+^^^^^^^^^^^^
+
+The Kprobe-optimizer doesn't insert the jump instruction immediately;
+rather, it calls synchronize_rcu() for safety first, because it's
+possible for a CPU to be interrupted in the middle of executing the
+optimized region [3]_. As you know, synchronize_rcu() can ensure
+that all interruptions that were active when synchronize_rcu()
+was called are done, but only if CONFIG_PREEMPT=n. So, this version
+of kprobe optimization supports only kernels with CONFIG_PREEMPT=n [4]_.
+
+After that, the Kprobe-optimizer calls stop_machine() to replace
+the optimized region with a jump instruction to the detour buffer,
+using text_poke_smp().
+
+Unoptimization
+^^^^^^^^^^^^^^
+
+When an optimized kprobe is unregistered, disabled, or blocked by
+another kprobe, it will be unoptimized. If this happens before
+the optimization is complete, the kprobe is just dequeued from the
+optimized list. If the optimization has been done, the jump is
+replaced with the original code (except for an int3 breakpoint in
+the first byte) by using text_poke_smp().
+
+.. [3] Please imagine that the 2nd instruction is interrupted and then
+ the optimizer replaces the 2nd instruction with the jump *address*
+ while the interrupt handler is running. When the interrupt
+ returns to original address, there is no valid instruction,
+ and it causes an unexpected result.
+
+.. [4] This optimization-safety checking may be replaced with the
+ stop-machine method that ksplice uses for supporting a CONFIG_PREEMPT=y
+ kernel.
+
+NOTE for geeks:
+The jump optimization changes the kprobe's pre_handler behavior.
+Without optimization, the pre_handler can change the kernel's execution
+path by changing regs->ip and returning 1. However, when the probe
+is optimized, that modification is ignored. Thus, if you want to
+tweak the kernel's execution path, you need to suppress optimization,
+using one of the following techniques:
+
+- Specify an empty function for the kprobe's post_handler.
+
+or
+
+- Execute 'sysctl -w debug.kprobes_optimization=n'
+
+.. _kprobes_blacklist:
+
+Blacklist
+---------
+
+Kprobes can probe most of the kernel except itself. This means
+that there are some functions where kprobes cannot probe. Probing
+(trapping) such functions can cause a recursive trap (e.g. double
+fault) or the nested probe handler may never be called.
+Kprobes manages such functions as a blacklist.
+If you want to add a function into the blacklist, you just need
+to (1) include linux/kprobes.h and (2) use NOKPROBE_SYMBOL() macro
+to specify a blacklisted function.
+Kprobes checks the given probe address against the blacklist and
+rejects registering it, if the given address is in the blacklist.
+
+.. _kprobes_archs_supported:
+
+Architectures Supported
+=======================
+
+Kprobes and return probes are implemented on the following
+architectures:
+
+- i386 (Supports jump optimization)
+- x86_64 (AMD-64, EM64T) (Supports jump optimization)
+- ppc64
+- ia64 (Does not support probes on instruction slot1.)
+- sparc64 (Return probes not yet implemented.)
+- arm
+- ppc
+- mips
+- s390
+- parisc
+
+Configuring Kprobes
+===================
+
+When configuring the kernel using make menuconfig/xconfig/oldconfig,
+ensure that CONFIG_KPROBES is set to "y". Under "General setup", look
+for "Kprobes".
+
+So that you can load and unload Kprobes-based instrumentation modules,
+make sure "Loadable module support" (CONFIG_MODULES) and "Module
+unloading" (CONFIG_MODULE_UNLOAD) are set to "y".
+
+Also make sure that CONFIG_KALLSYMS and perhaps even CONFIG_KALLSYMS_ALL
+are set to "y", since kallsyms_lookup_name() is used by the in-kernel
+kprobe address resolution code.
+
+If you need to insert a probe in the middle of a function, you may find
+it useful to "Compile the kernel with debug info" (CONFIG_DEBUG_INFO),
+so you can use "objdump -d -l vmlinux" to see the source-to-object
+code mapping.
+
+API Reference
+=============
+
+The Kprobes API includes a "register" function and an "unregister"
+function for each type of probe. The API also includes "register_*probes"
+and "unregister_*probes" functions for (un)registering arrays of probes.
+Here are terse, mini-man-page specifications for these functions and
+the associated probe handlers that you'll write. See the files in the
+samples/kprobes/ sub-directory for examples.
+
+register_kprobe
+---------------
+
+::
+
+ #include <linux/kprobes.h>
+ int register_kprobe(struct kprobe *kp);
+
+Sets a breakpoint at the address kp->addr. When the breakpoint is
+hit, Kprobes calls kp->pre_handler. After the probed instruction
+is single-stepped, Kprobe calls kp->post_handler. If a fault
+occurs during execution of kp->pre_handler or kp->post_handler,
+or during single-stepping of the probed instruction, Kprobes calls
+kp->fault_handler. Any or all handlers can be NULL. If kp->flags
+is set KPROBE_FLAG_DISABLED, that kp will be registered but disabled,
+so, its handlers aren't hit until calling enable_kprobe(kp).
+
+.. note::
+
+ 1. With the introduction of the "symbol_name" field to struct kprobe,
+ the probepoint address resolution will now be taken care of by the kernel.
+ The following will now work::
+
+ kp.symbol_name = "symbol_name";
+
+ (64-bit powerpc intricacies such as function descriptors are handled
+ transparently)
+
+ 2. Use the "offset" field of struct kprobe if the offset into the symbol
+ to install a probepoint is known. This field is used to calculate the
+ probepoint.
+
+ 3. Specify either the kprobe "symbol_name" OR the "addr". If both are
+ specified, kprobe registration will fail with -EINVAL.
+
+ 4. With CISC architectures (such as i386 and x86_64), the kprobes code
+ does not validate if the kprobe.addr is at an instruction boundary.
+ Use "offset" with caution.
+
+register_kprobe() returns 0 on success, or a negative errno otherwise.
+
+User's pre-handler (kp->pre_handler)::
+
+ #include <linux/kprobes.h>
+ #include <linux/ptrace.h>
+ int pre_handler(struct kprobe *p, struct pt_regs *regs);
+
+Called with p pointing to the kprobe associated with the breakpoint,
+and regs pointing to the struct containing the registers saved when
+the breakpoint was hit. Return 0 here unless you're a Kprobes geek.
+
+User's post-handler (kp->post_handler)::
+
+ #include <linux/kprobes.h>
+ #include <linux/ptrace.h>
+ void post_handler(struct kprobe *p, struct pt_regs *regs,
+ unsigned long flags);
+
+p and regs are as described for the pre_handler. flags always seems
+to be zero.
+
+User's fault-handler (kp->fault_handler)::
+
+ #include <linux/kprobes.h>
+ #include <linux/ptrace.h>
+ int fault_handler(struct kprobe *p, struct pt_regs *regs, int trapnr);
+
+p and regs are as described for the pre_handler. trapnr is the
+architecture-specific trap number associated with the fault (e.g.,
+on i386, 13 for a general protection fault or 14 for a page fault).
+Returns 1 if it successfully handled the exception.
+
+register_kretprobe
+------------------
+
+::
+
+ #include <linux/kprobes.h>
+ int register_kretprobe(struct kretprobe *rp);
+
+Establishes a return probe for the function whose address is
+rp->kp.addr. When that function returns, Kprobes calls rp->handler.
+You must set rp->maxactive appropriately before you call
+register_kretprobe(); see "How Does a Return Probe Work?" for details.
+
+register_kretprobe() returns 0 on success, or a negative errno
+otherwise.
+
+User's return-probe handler (rp->handler)::
+
+ #include <linux/kprobes.h>
+ #include <linux/ptrace.h>
+ int kretprobe_handler(struct kretprobe_instance *ri,
+ struct pt_regs *regs);
+
+regs is as described for kprobe.pre_handler. ri points to the
+kretprobe_instance object, of which the following fields may be
+of interest:
+
+- ret_addr: the return address
+- rp: points to the corresponding kretprobe object
+- task: points to the corresponding task struct
+- data: points to per return-instance private data; see "Kretprobe
+ entry-handler" for details.
+
+The regs_return_value(regs) macro provides a simple abstraction to
+extract the return value from the appropriate register as defined by
+the architecture's ABI.
+
+The handler's return value is currently ignored.
+
+unregister_*probe
+------------------
+
+::
+
+ #include <linux/kprobes.h>
+ void unregister_kprobe(struct kprobe *kp);
+ void unregister_kretprobe(struct kretprobe *rp);
+
+Removes the specified probe. The unregister function can be called
+at any time after the probe has been registered.
+
+.. note::
+
+ If the functions find an incorrect probe (ex. an unregistered probe),
+ they clear the addr field of the probe.
+
+register_*probes
+----------------
+
+::
+
+ #include <linux/kprobes.h>
+ int register_kprobes(struct kprobe **kps, int num);
+ int register_kretprobes(struct kretprobe **rps, int num);
+
+Registers each of the num probes in the specified array. If any
+error occurs during registration, all probes in the array, up to
+the bad probe, are safely unregistered before the register_*probes
+function returns.
+
+- kps/rps: an array of pointers to ``*probe`` data structures
+- num: the number of the array entries.
+
+.. note::
+
+ You have to allocate(or define) an array of pointers and set all
+ of the array entries before using these functions.
+
+unregister_*probes
+------------------
+
+::
+
+ #include <linux/kprobes.h>
+ void unregister_kprobes(struct kprobe **kps, int num);
+ void unregister_kretprobes(struct kretprobe **rps, int num);
+
+Removes each of the num probes in the specified array at once.
+
+.. note::
+
+ If the functions find some incorrect probes (ex. unregistered
+ probes) in the specified array, they clear the addr field of those
+ incorrect probes. However, other probes in the array are
+ unregistered correctly.
+
+disable_*probe
+--------------
+
+::
+
+ #include <linux/kprobes.h>
+ int disable_kprobe(struct kprobe *kp);
+ int disable_kretprobe(struct kretprobe *rp);
+
+Temporarily disables the specified ``*probe``. You can enable it again by using
+enable_*probe(). You must specify the probe which has been registered.
+
+enable_*probe
+-------------
+
+::
+
+ #include <linux/kprobes.h>
+ int enable_kprobe(struct kprobe *kp);
+ int enable_kretprobe(struct kretprobe *rp);
+
+Enables ``*probe`` which has been disabled by disable_*probe(). You must specify
+the probe which has been registered.
+
+Kprobes Features and Limitations
+================================
+
+Kprobes allows multiple probes at the same address. Also,
+a probepoint for which there is a post_handler cannot be optimized.
+So if you install a kprobe with a post_handler, at an optimized
+probepoint, the probepoint will be unoptimized automatically.
+
+In general, you can install a probe anywhere in the kernel.
+In particular, you can probe interrupt handlers. Known exceptions
+are discussed in this section.
+
+The register_*probe functions will return -EINVAL if you attempt
+to install a probe in the code that implements Kprobes (mostly
+kernel/kprobes.c and ``arch/*/kernel/kprobes.c``, but also functions such
+as do_page_fault and notifier_call_chain).
+
+If you install a probe in an inline-able function, Kprobes makes
+no attempt to chase down all inline instances of the function and
+install probes there. gcc may inline a function without being asked,
+so keep this in mind if you're not seeing the probe hits you expect.
+
+A probe handler can modify the environment of the probed function
+-- e.g., by modifying kernel data structures, or by modifying the
+contents of the pt_regs struct (which are restored to the registers
+upon return from the breakpoint). So Kprobes can be used, for example,
+to install a bug fix or to inject faults for testing. Kprobes, of
+course, has no way to distinguish the deliberately injected faults
+from the accidental ones. Don't drink and probe.
+
+Kprobes makes no attempt to prevent probe handlers from stepping on
+each other -- e.g., probing printk() and then calling printk() from a
+probe handler. If a probe handler hits a probe, that second probe's
+handlers won't be run in that instance, and the kprobe.nmissed member
+of the second probe will be incremented.
+
+As of Linux v2.6.15-rc1, multiple handlers (or multiple instances of
+the same handler) may run concurrently on different CPUs.
+
+Kprobes does not use mutexes or allocate memory except during
+registration and unregistration.
+
+Probe handlers are run with preemption disabled or interrupt disabled,
+which depends on the architecture and optimization state. (e.g.,
+kretprobe handlers and optimized kprobe handlers run without interrupt
+disabled on x86/x86-64). In any case, your handler should not yield
+the CPU (e.g., by attempting to acquire a semaphore, or waiting I/O).
+
+Since a return probe is implemented by replacing the return
+address with the trampoline's address, stack backtraces and calls
+to __builtin_return_address() will typically yield the trampoline's
+address instead of the real return address for kretprobed functions.
+(As far as we can tell, __builtin_return_address() is used only
+for instrumentation and error reporting.)
+
+If the number of times a function is called does not match the number
+of times it returns, registering a return probe on that function may
+produce undesirable results. In such a case, a line:
+kretprobe BUG!: Processing kretprobe d000000000041aa8 @ c00000000004f48c
+gets printed. With this information, one will be able to correlate the
+exact instance of the kretprobe that caused the problem. We have the
+do_exit() case covered. do_execve() and do_fork() are not an issue.
+We're unaware of other specific cases where this could be a problem.
+
+If, upon entry to or exit from a function, the CPU is running on
+a stack other than that of the current task, registering a return
+probe on that function may produce undesirable results. For this
+reason, Kprobes doesn't support return probes (or kprobes)
+on the x86_64 version of __switch_to(); the registration functions
+return -EINVAL.
+
+On x86/x86-64, since the Jump Optimization of Kprobes modifies
+instructions widely, there are some limitations to optimization. To
+explain it, we introduce some terminology. Imagine a 3-instruction
+sequence consisting of a two 2-byte instructions and one 3-byte
+instruction.
+
+::
+
+ IA
+ |
+ [-2][-1][0][1][2][3][4][5][6][7]
+ [ins1][ins2][ ins3 ]
+ [<- DCR ->]
+ [<- JTPR ->]
+
+ ins1: 1st Instruction
+ ins2: 2nd Instruction
+ ins3: 3rd Instruction
+ IA: Insertion Address
+ JTPR: Jump Target Prohibition Region
+ DCR: Detoured Code Region
+
+The instructions in DCR are copied to the out-of-line buffer
+of the kprobe, because the bytes in DCR are replaced by
+a 5-byte jump instruction. So there are several limitations.
+
+a) The instructions in DCR must be relocatable.
+b) The instructions in DCR must not include a call instruction.
+c) JTPR must not be targeted by any jump or call instruction.
+d) DCR must not straddle the border between functions.
+
+Anyway, these limitations are checked by the in-kernel instruction
+decoder, so you don't need to worry about that.
+
+Probe Overhead
+==============
+
+On a typical CPU in use in 2005, a kprobe hit takes 0.5 to 1.0
+microseconds to process. Specifically, a benchmark that hits the same
+probepoint repeatedly, firing a simple handler each time, reports 1-2
+million hits per second, depending on the architecture. A return-probe
+hit typically takes 50-75% longer than a kprobe hit.
+When you have a return probe set on a function, adding a kprobe at
+the entry to that function adds essentially no overhead.
+
+Here are sample overhead figures (in usec) for different architectures::
+
+ k = kprobe; r = return probe; kr = kprobe + return probe
+ on same function
+
+ i386: Intel Pentium M, 1495 MHz, 2957.31 bogomips
+ k = 0.57 usec; r = 0.92; kr = 0.99
+
+ x86_64: AMD Opteron 246, 1994 MHz, 3971.48 bogomips
+ k = 0.49 usec; r = 0.80; kr = 0.82
+
+ ppc64: POWER5 (gr), 1656 MHz (SMT disabled, 1 virtual CPU per physical CPU)
+ k = 0.77 usec; r = 1.26; kr = 1.45
+
+Optimized Probe Overhead
+------------------------
+
+Typically, an optimized kprobe hit takes 0.07 to 0.1 microseconds to
+process. Here are sample overhead figures (in usec) for x86 architectures::
+
+ k = unoptimized kprobe, b = boosted (single-step skipped), o = optimized kprobe,
+ r = unoptimized kretprobe, rb = boosted kretprobe, ro = optimized kretprobe.
+
+ i386: Intel(R) Xeon(R) E5410, 2.33GHz, 4656.90 bogomips
+ k = 0.80 usec; b = 0.33; o = 0.05; r = 1.10; rb = 0.61; ro = 0.33
+
+ x86-64: Intel(R) Xeon(R) E5410, 2.33GHz, 4656.90 bogomips
+ k = 0.99 usec; b = 0.43; o = 0.06; r = 1.24; rb = 0.68; ro = 0.30
+
+TODO
+====
+
+a. SystemTap (http://sourceware.org/systemtap): Provides a simplified
+ programming interface for probe-based instrumentation. Try it out.
+b. Kernel return probes for sparc64.
+c. Support for other architectures.
+d. User-space probes.
+e. Watchpoint probes (which fire on data references).
+
+Kprobes Example
+===============
+
+See samples/kprobes/kprobe_example.c
+
+Kretprobes Example
+==================
+
+See samples/kprobes/kretprobe_example.c
+
+Deprecated Features
+===================
+
+Jprobes is now a deprecated feature. People who are depending on it should
+migrate to other tracing features or use older kernels. Please consider to
+migrate your tool to one of the following options:
+
+- Use trace-event to trace target function with arguments.
+
+ trace-event is a low-overhead (and almost no visible overhead if it
+ is off) statically defined event interface. You can define new events
+ and trace it via ftrace or any other tracing tools.
+
+ See the following urls:
+
+ - https://lwn.net/Articles/379903/
+ - https://lwn.net/Articles/381064/
+ - https://lwn.net/Articles/383362/
+
+- Use ftrace dynamic events (kprobe event) with perf-probe.
+
+ If you build your kernel with debug info (CONFIG_DEBUG_INFO=y), you can
+ find which register/stack is assigned to which local variable or arguments
+ by using perf-probe and set up new event to trace it.
+
+ See following documents:
+
+ - Documentation/trace/kprobetrace.rst
+ - Documentation/trace/events.rst
+ - tools/perf/Documentation/perf-probe.txt
+
+
+The kprobes debugfs interface
+=============================
+
+
+With recent kernels (> 2.6.20) the list of registered kprobes is visible
+under the /sys/kernel/debug/kprobes/ directory (assuming debugfs is mounted at //sys/kernel/debug).
+
+/sys/kernel/debug/kprobes/list: Lists all registered probes on the system::
+
+ c015d71a k vfs_read+0x0
+ c03dedc5 r tcp_v4_rcv+0x0
+
+The first column provides the kernel address where the probe is inserted.
+The second column identifies the type of probe (k - kprobe and r - kretprobe)
+while the third column specifies the symbol+offset of the probe.
+If the probed function belongs to a module, the module name is also
+specified. Following columns show probe status. If the probe is on
+a virtual address that is no longer valid (module init sections, module
+virtual addresses that correspond to modules that've been unloaded),
+such probes are marked with [GONE]. If the probe is temporarily disabled,
+such probes are marked with [DISABLED]. If the probe is optimized, it is
+marked with [OPTIMIZED]. If the probe is ftrace-based, it is marked with
+[FTRACE].
+
+/sys/kernel/debug/kprobes/enabled: Turn kprobes ON/OFF forcibly.
+
+Provides a knob to globally and forcibly turn registered kprobes ON or OFF.
+By default, all kprobes are enabled. By echoing "0" to this file, all
+registered probes will be disarmed, till such time a "1" is echoed to this
+file. Note that this knob just disarms and arms all kprobes and doesn't
+change each probe's disabling state. This means that disabled kprobes (marked
+[DISABLED]) will be not enabled if you turn ON all kprobes by this knob.
+
+
+The kprobes sysctl interface
+============================
+
+/proc/sys/debug/kprobes-optimization: Turn kprobes optimization ON/OFF.
+
+When CONFIG_OPTPROBES=y, this sysctl interface appears and it provides
+a knob to globally and forcibly turn jump optimization (see section
+:ref:`kprobes_jump_optimization`) ON or OFF. By default, jump optimization
+is allowed (ON). If you echo "0" to this file or set
+"debug.kprobes_optimization" to 0 via sysctl, all optimized probes will be
+unoptimized, and any new probes registered after that will not be optimized.
+
+Note that this knob *changes* the optimized state. This means that optimized
+probes (marked [OPTIMIZED]) will be unoptimized ([OPTIMIZED] tag will be
+removed). If the knob is turned on, they will be optimized again.
+
+References
+==========
+
+For additional information on Kprobes, refer to the following URLs:
+
+- https://www.ibm.com/developerworks/library/l-kprobes/index.html
+- https://www.kernel.org/doc/ols/2006/ols2006v2-pages-109-124.pdf
+
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