[PATCH] unshare system call -v5: Documentation file

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.