+unshare system call
+===================
-unshare system call:
---------------------
-This document describes the new system call, unshare. The document
+This document describes the new system call, unshare(). The document
provides an overview of the feature, why it is needed, how it can
be used, its interface specification, design, implementation and
how it can be tested.
-Change Log:
------------
+Change Log
+----------
version 0.1 Initial document, Janak Desai (janak@us.ibm.com), Jan 11, 2006
-Contents:
----------
+Contents
+--------
1) Overview
2) Benefits
3) Cost
1) Overview
-----------
+
Most legacy operating system kernels support an abstraction of threads
as multiple execution contexts within a process. These kernels provide
special resources and mechanisms to maintain these "threads". The Linux
call, applications can selectively choose which resources to share
between threads.
-unshare system call adds a primitive to the Linux thread model that
+unshare() system call adds a primitive to the Linux thread model that
allows threads to selectively 'unshare' any resources that were being
-shared at the time of their creation. unshare was conceptualized by
+shared at the time of their creation. unshare() was conceptualized by
Al Viro in the August of 2000, on the Linux-Kernel mailing list, as part
-of the discussion on POSIX threads on Linux. unshare augments the
+of the discussion on POSIX threads on Linux. unshare() augments the
usefulness of Linux threads for applications that would like to control
-shared resources without creating a new process. unshare is a natural
+shared resources without creating a new process. unshare() is a natural
addition to the set of available primitives on Linux that implement
the concept of process/thread as a virtual machine.
2) Benefits
-----------
-unshare would be useful to large application frameworks such as PAM
+
+unshare() would be useful to large application frameworks such as PAM
where creating a new process to control sharing/unsharing of process
resources is not possible. Since namespaces are shared by default
-when creating a new process using fork or clone, unshare can benefit
+when creating a new process using fork or clone, unshare() can benefit
even non-threaded applications if they have a need to disassociate
from default shared namespace. The following lists two use-cases
-where unshare can be used.
+where unshare() can be used.
2.1 Per-security context namespaces
------------------------------------
-unshare can be used to implement polyinstantiated directories using
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+unshare() can be used to implement polyinstantiated directories using
the kernel's per-process namespace mechanism. Polyinstantiated directories,
such as per-user and/or per-security context instance of /tmp, /var/tmp or
per-security context instance of a user's home directory, isolate user
-processes when working with these directories. Using unshare, a PAM
+processes when working with these directories. Using unshare(), a PAM
module can easily setup a private namespace for a user at login.
Polyinstantiated directories are required for Common Criteria certification
with Labeled System Protection Profile, however, with the availability
appropriate by system administrators.
2.2 unsharing of virtual memory and/or open files
--------------------------------------------------
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
Consider a client/server application where the server is processing
client requests by creating processes that share resources such as
-virtual memory and open files. Without unshare, the server has to
+virtual memory and open files. Without unshare(), the server has to
decide what needs to be shared at the time of creating the process
-which services the request. unshare allows the server an ability to
+which services the request. unshare() allows the server an ability to
disassociate parts of the context during the servicing of the
request. For large and complex middleware application frameworks, this
-ability to unshare after the process was created can be very
+ability to unshare() after the process was created can be very
useful.
3) Cost
-------
-In order to not duplicate code and to handle the fact that unshare
+
+In order to not duplicate code and to handle the fact that unshare()
works on an active task (as opposed to clone/fork working on a newly
-allocated inactive task) unshare had to make minor reorganizational
+allocated inactive task) unshare() had to make minor reorganizational
changes to copy_* functions utilized by clone/fork system call.
There is a cost associated with altering existing, well tested and
stable code to implement a new feature that may not get exercised
extensively in the beginning. However, with proper design and code
-review of the changes and creation of an unshare test for the LTP
+review of the changes and creation of an unshare() test for the LTP
the benefits of this new feature can exceed its cost.
4) Requirements
---------------
-unshare reverses sharing that was done using clone(2) system call,
-so unshare should have a similar interface as clone(2). That is,
+
+unshare() reverses sharing that was done using clone(2) system call,
+so unshare() should have a similar interface as clone(2). That is,
since flags in clone(int flags, void *stack) specifies what should
be shared, similar flags in unshare(int flags) should specify
what should be unshared. Unfortunately, this may appear to invert
However, there was no easy solution that was less confusing and that
allowed incremental context unsharing in future without an ABI change.
-unshare interface should accommodate possible future addition of
+unshare() interface should accommodate possible future addition of
new context flags without requiring a rebuild of old applications.
-If and when new context flags are added, unshare design should allow
+If and when new context flags are added, unshare() design should allow
incremental unsharing of those resources on an as needed basis.
5) Functional Specification
---------------------------
+
NAME
unshare - disassociate parts of the process execution context
int unshare(int flags);
DESCRIPTION
- unshare allows a process to disassociate parts of its execution
+ unshare() allows a process to disassociate parts of its execution
context that are currently being shared with other processes. Part
of execution context, such as the namespace, is shared by default
when a new process is created using fork(2), while other parts,
shared by explicit request to share them when creating a process
using clone(2).
- The main use of unshare is to allow a process to control its
+ The main use of unshare() is to allow a process to control its
shared execution context without creating a new process.
The flags argument specifies one or bitwise-or'ed of several of
6) High Level Design
--------------------
-Depending on the flags argument, the unshare system call allocates
+
+Depending on the flags argument, the unshare() system call allocates
appropriate process context structures, populates it with values from
the current shared version, associates newly duplicated structures
with the current task structure and releases corresponding shared
versions. Helper functions of clone (copy_*) could not be used
-directly by unshare because of the following two reasons.
+directly by unshare() because of the following two reasons.
+
1) clone operates on a newly allocated not-yet-active task
- structure, where as unshare operates on the current active
- task. Therefore unshare has to take appropriate task_lock()
+ structure, where as unshare() operates on the current active
+ task. Therefore unshare() has to take appropriate task_lock()
before associating newly duplicated context structures
- 2) unshare has to allocate and duplicate all context structures
+
+ 2) unshare() has to allocate and duplicate all context structures
that are being unshared, before associating them with the
current task and releasing older shared structures. Failure
do so will create race conditions and/or oops when trying
current context structure was moved into new dup_* functions. Now,
copy_* functions call dup_* functions to allocate and duplicate
appropriate context structures and then associate them with the
-task structure that is being constructed. unshare system call on
+task structure that is being constructed. unshare() system call on
the other hand performs the following:
+
1) Check flags to force missing, but implied, flags
- 2) For each context structure, call the corresponding unshare
+
+ 2) For each context structure, call the corresponding unshare()
helper function to allocate and duplicate a new context
structure, if the appropriate bit is set in the flags argument.
+
3) If there is no error in allocation and duplication and there
are new context structures then lock the current task structure,
associate new context structures with the current task structure,
and release the lock on the current task structure.
+
4) Appropriately release older, shared, context structures.
7) Low Level Design
-------------------
-Implementation of unshare can be grouped in the following 4 different
+
+Implementation of unshare() can be grouped in the following 4 different
items:
+
a) Reorganization of existing copy_* functions
- b) unshare system call service function
- c) unshare helper functions for each different process context
+
+ b) unshare() system call service function
+
+ c) unshare() helper functions for each different process context
+
d) Registration of system call number for different architectures
- 7.1) Reorganization of copy_* functions
- Each copy function such as copy_mm, copy_namespace, copy_files,
- etc, had roughly two components. The first component allocated
- and duplicated the appropriate structure and the second component
- linked it to the task structure passed in as an argument to the copy
- function. The first component was split into its own function.
- These dup_* functions allocated and duplicated the appropriate
- context structure. The reorganized copy_* functions invoked
- their corresponding dup_* functions and then linked the newly
- duplicated structures to the task structure with which the
- copy function was called.
-
- 7.2) unshare system call service function
+7.1) Reorganization of copy_* functions
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Each copy function such as copy_mm, copy_namespace, copy_files,
+etc, had roughly two components. The first component allocated
+and duplicated the appropriate structure and the second component
+linked it to the task structure passed in as an argument to the copy
+function. The first component was split into its own function.
+These dup_* functions allocated and duplicated the appropriate
+context structure. The reorganized copy_* functions invoked
+their corresponding dup_* functions and then linked the newly
+duplicated structures to the task structure with which the
+copy function was called.
+
+7.2) unshare() system call service function
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
* Check flags
Force implied flags. If CLONE_THREAD is set force CLONE_VM.
If CLONE_VM is set, force CLONE_SIGHAND. If CLONE_SIGHAND is
set and signals are also being shared, force CLONE_THREAD. If
CLONE_NEWNS is set, force CLONE_FS.
+
* For each context flag, invoke the corresponding unshare_*
helper routine with flags passed into the system call and a
reference to pointer pointing the new unshared structure
+
* If any new structures are created by unshare_* helper
functions, take the task_lock() on the current task,
modify appropriate context pointers, and release the
task lock.
+
* For all newly unshared structures, release the corresponding
older, shared, structures.
- 7.3) unshare_* helper functions
- For unshare_* helpers corresponding to CLONE_SYSVSEM, CLONE_SIGHAND,
- and CLONE_THREAD, return -EINVAL since they are not implemented yet.
- For others, check the flag value to see if the unsharing is
- required for that structure. If it is, invoke the corresponding
- dup_* function to allocate and duplicate the structure and return
- a pointer to it.
+7.3) unshare_* helper functions
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
- 7.4) Appropriately modify architecture specific code to register the
- new system call.
+For unshare_* helpers corresponding to CLONE_SYSVSEM, CLONE_SIGHAND,
+and CLONE_THREAD, return -EINVAL since they are not implemented yet.
+For others, check the flag value to see if the unsharing is
+required for that structure. If it is, invoke the corresponding
+dup_* function to allocate and duplicate the structure and return
+a pointer to it.
+
+7.4) Finally
+~~~~~~~~~~~~
+
+Appropriately modify architecture specific code to register the
+new system call.
8) Test Specification
---------------------
-The test for unshare should test the following:
+
+The test for unshare() should test the following:
+
1) Valid flags: Test to check that clone flags for signal and
- signal handlers, for which unsharing is not implemented
- yet, return -EINVAL.
+ signal handlers, for which unsharing is not implemented
+ yet, return -EINVAL.
+
2) Missing/implied flags: Test to make sure that if unsharing
- namespace without specifying unsharing of filesystem, correctly
- unshares both namespace and filesystem information.
+ namespace without specifying unsharing of filesystem, correctly
+ unshares both namespace and filesystem information.
+
3) For each of the four (namespace, filesystem, files and vm)
- supported unsharing, verify that the system call correctly
- unshares the appropriate structure. Verify that unsharing
- them individually as well as in combination with each
- other works as expected.
+ supported unsharing, verify that the system call correctly
+ unshares the appropriate structure. Verify that unsharing
+ them individually as well as in combination with each
+ other works as expected.
+
4) Concurrent execution: Use shared memory segments and futex on
- an address in the shm segment to synchronize execution of
- about 10 threads. Have a couple of threads execute execve,
- a couple _exit and the rest unshare with different combination
- of flags. Verify that unsharing is performed as expected and
- that there are no oops or hangs.
+ an address in the shm segment to synchronize execution of
+ about 10 threads. Have a couple of threads execute execve,
+ a couple _exit and the rest unshare with different combination
+ of flags. Verify that unsharing is performed as expected and
+ that there are no oops or hangs.
9) Future Work
--------------
-The current implementation of unshare does not allow unsharing of
+
+The current implementation of unshare() does not allow unsharing of
signals and signal handlers. Signals are complex to begin with and
to unshare signals and/or signal handlers of a currently running
process is even more complex. If in the future there is a specific
need to allow unsharing of signals and/or signal handlers, it can
-be incrementally added to unshare without affecting legacy
-applications using unshare.
+be incrementally added to unshare() without affecting legacy
+applications using unshare().
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