FreeBSD/Linux Kernel Cross Reference
sys/Documentation/unshare.txt

     
     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.
   

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