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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 (janak@us.ibm.com), 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
121 SYNOPSIS
122 #include <sched.h>
124 int unshare(int flags);
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).
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.
158 RETURN VALUE
159 On success, zero returned. On failure, -1 is returned and errno is
161 ERRORS
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.
170 CONFORMING TO
171 The unshare() call is Linux-specific and should not be used
172 in programs intended to be portable.
174 SEE ALSO
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.