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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
+       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.
+