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diff --git a/Documentation/userspace-api/unshare.rst b/Documentation/userspace-api/unshare.rst new file mode 100644 index 0000000000..877e90a352 --- /dev/null +++ b/Documentation/userspace-api/unshare.rst @@ -0,0 +1,332 @@ +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) Finally +~~~~~~~~~~~~ + +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(). + |