Documentation/unshare.txt

   1
   2 unshare system call:
   3 --------------------
   4 This document describes the new system call, unshare. The document
   5 provides an overview of the feature, why it is needed, how it can
   6 be used, its interface specification, design, implementation and
   7 how it can be tested.
   8
   9 Change Log:
  10 -----------
  11 version 0.1  Initial document, Janak Desai (janak@us.ibm.com), Jan 11, 2006
  12
  13 Contents:
  14 ---------
  15         1) Overview
  16         2) Benefits
  17         3) Cost
  18         4) Requirements
  19         5) Functional Specification
  20         6) High Level Design
  21         7) Low Level Design
  22         8) Test Specification
  23         9) Future Work
  24
  25 1) Overview
  26 -----------
  27 Most legacy operating system kernels support an abstraction of threads
  28 as multiple execution contexts within a process. These kernels provide
  29 special resources and mechanisms to maintain these "threads". The Linux
  30 kernel, in a clever and simple manner, does not make distinction
  31 between processes and "threads". The kernel allows processes to share
  32 resources and thus they can achieve legacy "threads" behavior without
  33 requiring additional data structures and mechanisms in the kernel. The
  34 power of implementing threads in this manner comes not only from
  35 its simplicity but also from allowing application programmers to work
  36 outside the confinement of all-or-nothing shared resources of legacy
  37 threads. On Linux, at the time of thread creation using the clone system
  38 call, applications can selectively choose which resources to share
  39 between threads.
  40
  41 unshare system call adds a primitive to the Linux thread model that
  42 allows threads to selectively 'unshare' any resources that were being
  43 shared at the time of their creation. unshare was conceptualized by
  44 Al Viro in the August of 2000, on the Linux-Kernel mailing list, as part
  45 of the discussion on POSIX threads on Linux.  unshare augments the
  46 usefulness of Linux threads for applications that would like to control
  47 shared resources without creating a new process. unshare is a natural
  48 addition to the set of available primitives on Linux that implement
  49 the concept of process/thread as a virtual machine.
  50
  51 2) Benefits
  52 -----------
  53 unshare would be useful to large application frameworks such as PAM
  54 where creating a new process to control sharing/unsharing of process
  55 resources is not possible. Since namespaces are shared by default
  56 when creating a new process using fork or clone, unshare can benefit
  57 even non-threaded applications if they have a need to disassociate
  58 from default shared namespace. The following lists two use-cases
  59 where unshare can be used.
  60
  61 2.1 Per-security context namespaces
  62 -----------------------------------
  63 unshare can be used to implement polyinstantiated directories using
  64 the kernel's per-process namespace mechanism. Polyinstantiated directories,
  65 such as per-user and/or per-security context instance of /tmp, /var/tmp or
  66 per-security context instance of a user's home directory, isolate user
  67 processes when working with these directories. Using unshare, a PAM
  68 module can easily setup a private namespace for a user at login.
  69 Polyinstantiated directories are required for Common Criteria certification
  70 with Labeled System Protection Profile, however, with the availability
  71 of shared-tree feature in the Linux kernel, even regular Linux systems
  72 can benefit from setting up private namespaces at login and
  73 polyinstantiating /tmp, /var/tmp and other directories deemed
  74 appropriate by system administrators.
  75
  76 2.2 unsharing of virtual memory and/or open files
  77 -------------------------------------------------
  78 Consider a client/server application where the server is processing
  79 client requests by creating processes that share resources such as
  80 virtual memory and open files. Without unshare, the server has to
  81 decide what needs to be shared at the time of creating the process
  82 which services the request. unshare allows the server an ability to
  83 disassociate parts of the context during the servicing of the
  84 request. For large and complex middleware application frameworks, this
  85 ability to unshare after the process was created can be very
  86 useful.
  87
  88 3) Cost
  89 -------
  90 In order to not duplicate code and to handle the fact that unshare
  91 works on an active task (as opposed to clone/fork working on a newly
  92 allocated inactive task) unshare had to make minor reorganizational
  93 changes to copy_* functions utilized by clone/fork system call.
  94 There is a cost associated with altering existing, well tested and
  95 stable code to implement a new feature that may not get exercised
  96 extensively in the beginning. However, with proper design and code
  97 review of the changes and creation of an unshare test for the LTP
  98 the benefits of this new feature can exceed its cost.
  99
 100 4) Requirements
 101 ---------------
 102 unshare reverses sharing that was done using clone(2) system call,
 103 so unshare should have a similar interface as clone(2). That is,
 104 since flags in clone(int flags, void *stack) specifies what should
 105 be shared, similar flags in unshare(int flags) should specify
 106 what should be unshared. Unfortunately, this may appear to invert
 107 the meaning of the flags from the way they are used in clone(2).
 108 However, there was no easy solution that was less confusing and that
 109 allowed incremental context unsharing in future without an ABI change.
 110
 111 unshare interface should accommodate possible future addition of
 112 new context flags without requiring a rebuild of old applications.
 113 If and when new context flags are added, unshare design should allow
 114 incremental unsharing of those resources on an as needed basis.
 115
 116 5) Functional Specification
 117 ---------------------------
 118 NAME
 119         unshare - disassociate parts of the process execution context
 120
 121 SYNOPSIS
 122         #include <sched.h>
 123
 124         int unshare(int flags);
 125
 126 DESCRIPTION
 127         unshare allows a process to disassociate parts of its execution
 128         context that are currently being shared with other processes. Part
 129         of execution context, such as the namespace, is shared by default
 130         when a new process is created using fork(2), while other parts,
 131         such as the virtual memory, open file descriptors, etc, may be
 132         shared by explicit request to share them when creating a process
 133         using clone(2).
 134
 135         The main use of unshare is to allow a process to control its
 136         shared execution context without creating a new process.
 137
 138         The flags argument specifies one or bitwise-or'ed of several of
 139         the following constants.
 140
 141         CLONE_FS
 142                 If CLONE_FS is set, file system information of the caller
 143                 is disassociated from the shared file system information.
 144
 145         CLONE_FILES
 146                 If CLONE_FILES is set, the file descriptor table of the
 147                 caller is disassociated from the shared file descriptor
 148                 table.
 149
 150         CLONE_NEWNS
 151                 If CLONE_NEWNS is set, the namespace of the caller is
 152                 disassociated from the shared namespace.
 153
 154         CLONE_VM
 155                 If CLONE_VM is set, the virtual memory of the caller is
 156                 disassociated from the shared virtual memory.
 157
 158 RETURN VALUE
 159         On success, zero returned. On failure, -1 is returned and errno is
 160
 161 ERRORS
 162         EPERM   CLONE_NEWNS was specified by a non-root process (process
 163                 without CAP_SYS_ADMIN).
 164
 165         ENOMEM  Cannot allocate sufficient memory to copy parts of caller's
 166                 context that need to be unshared.
 167
 168         EINVAL  Invalid flag was specified as an argument.
 169
 170 CONFORMING TO
 171         The unshare() call is Linux-specific and  should  not be used
 172         in programs intended to be portable.
 173
 174 SEE ALSO
 175         clone(2), fork(2)
 176
 177 6) High Level Design
 178 --------------------
 179 Depending on the flags argument, the unshare system call allocates
 180 appropriate process context structures, populates it with values from
 181 the current shared version, associates newly duplicated structures
 182 with the current task structure and releases corresponding shared
 183 versions. Helper functions of clone (copy_*) could not be used
 184 directly by unshare because of the following two reasons.
 185   1) clone operates on a newly allocated not-yet-active task
 186      structure, where as unshare operates on the current active
 187      task. Therefore unshare has to take appropriate task_lock()
 188      before associating newly duplicated context structures
 189   2) unshare has to allocate and duplicate all context structures
 190      that are being unshared, before associating them with the
 191      current task and releasing older shared structures. Failure
 192      do so will create race conditions and/or oops when trying
 193      to backout due to an error. Consider the case of unsharing
 194      both virtual memory and namespace. After successfully unsharing
 195      vm, if the system call encounters an error while allocating
 196      new namespace structure, the error return code will have to
 197      reverse the unsharing of vm. As part of the reversal the
 198      system call will have to go back to older, shared, vm
 199      structure, which may not exist anymore.
 200
 201 Therefore code from copy_* functions that allocated and duplicated
 202 current context structure was moved into new dup_* functions. Now,
 203 copy_* functions call dup_* functions to allocate and duplicate
 204 appropriate context structures and then associate them with the
 205 task structure that is being constructed. unshare system call on
 206 the other hand performs the following:
 207   1) Check flags to force missing, but implied, flags
 208   2) For each context structure, call the corresponding unshare
 209      helper function to allocate and duplicate a new context
 210      structure, if the appropriate bit is set in the flags argument.
 211   3) If there is no error in allocation and duplication and there
 212      are new context structures then lock the current task structure,
 213      associate new context structures with the current task structure,
 214      and release the lock on the current task structure.
 215   4) Appropriately release older, shared, context structures.
 216
 217 7) Low Level Design
 218 -------------------
 219 Implementation of unshare can be grouped in the following 4 different
 220 items:
 221   a) Reorganization of existing copy_* functions
 222   b) unshare system call service function
 223   c) unshare helper functions for each different process context
 224   d) Registration of system call number for different architectures
 225
 226   7.1) Reorganization of copy_* functions
 227        Each copy function such as copy_mm, copy_namespace, copy_files,
 228        etc, had roughly two components. The first component allocated
 229        and duplicated the appropriate structure and the second component
 230        linked it to the task structure passed in as an argument to the copy
 231        function. The first component was split into its own function.
 232        These dup_* functions allocated and duplicated the appropriate
 233        context structure. The reorganized copy_* functions invoked
 234        their corresponding dup_* functions and then linked the newly
 235        duplicated structures to the task structure with which the
 236        copy function was called.
 237
 238   7.2) unshare system call service function
 239        * Check flags
 240          Force implied flags. If CLONE_THREAD is set force CLONE_VM.
 241          If CLONE_VM is set, force CLONE_SIGHAND. If CLONE_SIGHAND is
 242          set and signals are also being shared, force CLONE_THREAD. If
 243          CLONE_NEWNS is set, force CLONE_FS.
 244        * For each context flag, invoke the corresponding unshare_*
 245          helper routine with flags passed into the system call and a
 246          reference to pointer pointing the new unshared structure
 247        * If any new structures are created by unshare_* helper
 248          functions, take the task_lock() on the current task,
 249          modify appropriate context pointers, and release the
 250          task lock.
 251        * For all newly unshared structures, release the corresponding
 252          older, shared, structures.
 253
 254   7.3) unshare_* helper functions
 255        For unshare_* helpers corresponding to CLONE_SYSVSEM, CLONE_SIGHAND,
 256        and CLONE_THREAD, return -EINVAL since they are not implemented yet.
 257        For others, check the flag value to see if the unsharing is
 258        required for that structure. If it is, invoke the corresponding
 259        dup_* function to allocate and duplicate the structure and return
 260        a pointer to it.
 261
 262   7.4) Appropriately modify architecture specific code to register the
 263        new system call.
 264
 265 8) Test Specification
 266 ---------------------
 267 The test for unshare should test the following:
 268   1) Valid flags: Test to check that clone flags for signal and
 269         signal handlers, for which unsharing is not implemented
 270         yet, return -EINVAL.
 271   2) Missing/implied flags: Test to make sure that if unsharing
 272         namespace without specifying unsharing of filesystem, correctly
 273         unshares both namespace and filesystem information.
 274   3) For each of the four (namespace, filesystem, files and vm)
 275         supported unsharing, verify that the system call correctly
 276         unshares the appropriate structure. Verify that unsharing
 277         them individually as well as in combination with each
 278         other works as expected.
 279   4) Concurrent execution: Use shared memory segments and futex on
 280         an address in the shm segment to synchronize execution of
 281         about 10 threads. Have a couple of threads execute execve,
 282         a couple _exit and the rest unshare with different combination
 283         of flags. Verify that unsharing is performed as expected and
 284         that there are no oops or hangs.
 285
 286 9) Future Work
 287 --------------
 288 The current implementation of unshare does not allow unsharing of
 289 signals and signal handlers. Signals are complex to begin with and
 290 to unshare signals and/or signal handlers of a currently running
 291 process is even more complex. If in the future there is a specific
 292 need to allow unsharing of signals and/or signal handlers, it can
 293 be incrementally added to unshare without affecting legacy
 294 applications using unshare.
 295