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unshare system call:
--------------------
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:
-----------
version 0.1  Initial document, Janak Desai (janak@us.ibm.com), Jan 11, 2006

Contents:
---------
        1) Overview
        2) Benefits
        3) Cost
        4) Requirements
        5) Functional Specification
        6) High Level Design
        7) Low Level Design
        8) Test Specification
        9) Future Work

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
kernel, in a clever and simple manner, does not make distinction
between processes and "threads". The kernel allows processes to share
resources and thus they can achieve legacy "threads" behavior without
requiring additional data structures and mechanisms in the kernel. The
power of implementing threads in this manner comes not only from
its simplicity but also from allowing application programmers to work
outside the confinement of all-or-nothing shared resources of legacy
threads. On Linux, at the time of thread creation using the clone system
call, applications can selectively choose which resources to share
between threads.

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
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
usefulness of Linux threads for applications that would like to control
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
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
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.

2.1 Per-security context namespaces
-----------------------------------
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
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
of shared-tree feature in the Linux kernel, even regular Linux systems
can benefit from setting up private namespaces at login and
polyinstantiating /tmp, /var/tmp and other directories deemed
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
decide what needs to be shared at the time of creating the process
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
useful.

3) Cost
-------
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
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
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,
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
the meaning of the flags from the way they are used in clone(2).
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
new context flags without requiring a rebuild of old applications.
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

SYNOPSIS
        #include <sched.h>

        int unshare(int flags);

DESCRIPTION
        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,
        such as the virtual memory, open file descriptors, etc, may be
        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
        shared execution context without creating a new process.

        The flags argument specifies one or bitwise-or'ed of several of
        the following constants.

        CLONE_FS
                If CLONE_FS is set, file system information of the caller
                is disassociated from the shared file system information.

        CLONE_FILES
                If CLONE_FILES is set, the file descriptor table of the
                caller is disassociated from the shared file descriptor
                table.

        CLONE_NEWNS
                If CLONE_NEWNS is set, the namespace of the caller is
                disassociated from the shared namespace.

        CLONE_VM
                If CLONE_VM is set, the virtual memory of the caller is
                disassociated from the shared virtual memory.

RETURN VALUE
        On success, zero returned. On failure, -1 is returned and errno is

ERRORS
        EPERM   CLONE_NEWNS was specified by a non-root process (process
                without CAP_SYS_ADMIN).

        ENOMEM  Cannot allocate sufficient memory to copy parts of caller's
                context that need to be unshared.

        EINVAL  Invalid flag was specified as an argument.

CONFORMING TO
        The unshare() call is Linux-specific and  should  not be used
        in programs intended to be portable.

SEE ALSO
        clone(2), fork(2)

6) High Level Design
--------------------
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.
  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()
     before associating newly duplicated 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
     to backout due to an error. Consider the case of unsharing
     both virtual memory and namespace. After successfully unsharing
     vm, if the system call encounters an error while allocating
     new namespace structure, the error return code will have to
     reverse the unsharing of vm. As part of the reversal the
     system call will have to go back to older, shared, vm
     structure, which may not exist anymore.

Therefore code from copy_* functions that allocated and duplicated
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
the other hand performs the following:
  1) Check flags to force missing, but implied, flags
  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
items:
  a) Reorganization of existing copy_* functions
  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
       * 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.

  7.4) Appropriately modify architecture specific code to register the
       new system call.

8) Test Specification
---------------------
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.
  2) Missing/implied flags: Test to make sure that if unsharing
        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.
  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.

9) Future Work
--------------
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.