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  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.
  9 Change Log:
 10 -----------
 11 version 0.1  Initial document, Janak Desai (, Jan 11, 2006
 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
 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.
 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.
 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.
 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.
 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.
 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.
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.
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.
116 5) Functional Specification
117 ---------------------------
118 NAME
119         unshare - disassociate parts of the process execution context
122         #include <sched.h>
124         int unshare(int flags);
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).
135         The main use of unshare is to allow a process to control its
136         shared execution context without creating a new process.
138         The flags argument specifies one or bitwise-or'ed of several of
139         the following constants.
141         CLONE_FS
142                 If CLONE_FS is set, file system information of the caller
143                 is disassociated from the shared file system information.
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.
150         CLONE_NEWNS
151                 If CLONE_NEWNS is set, the namespace of the caller is
152                 disassociated from the shared namespace.
154         CLONE_VM
155                 If CLONE_VM is set, the virtual memory of the caller is
156                 disassociated from the shared virtual memory.
159         On success, zero returned. On failure, -1 is returned and errno is
162         EPERM   CLONE_NEWNS was specified by a non-root process (process
163                 without CAP_SYS_ADMIN).
165         ENOMEM  Cannot allocate sufficient memory to copy parts of caller's
166                 context that need to be unshared.
168         EINVAL  Invalid flag was specified as an argument.
171         The unshare() call is Linux-specific and  should  not be used
172         in programs intended to be portable.
175         clone(2), fork(2)
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.
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.
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
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.
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.
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.
262   7.4) Appropriately modify architecture specific code to register the
263        new system call.
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.
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.

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