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docs: Convert unshare.txt to RST and add to the user-space API manual

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This is a straightforward conversion, without any real textual changes.
Since this document has seen no substantive changes since its addition in
2006, some such changes are probably warranted.

Signed-off-by: Jonathan Corbet <corbet@lwn.net>
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Jonathan Corbet 2017-04-02 15:18:32 -06:00
parent 1d596dee38
commit f504d47be5
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2
Documentation/userspace-api/index.rst
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@ -16,6 +16,8 @@ place where this information is gathered.
.. toctree::
:maxdepth: 2
unshare
.. only:: subproject and html
Indices

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Documentation/unshare.txt → Documentation/userspace-api/unshare.rst
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@ -1,17 +1,17 @@
unshare system call
===================
unshare system call:
--------------------
This document describes the new system call, unshare. The document
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:
-----------
Change Log
----------
version 0.1 Initial document, Janak Desai (janak@us.ibm.com), Jan 11, 2006
Contents:
---------
Contents
--------
1) Overview
2) Benefits
3) Cost
@ -24,6 +24,7 @@ Contents:
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
@ -38,33 +39,35 @@ 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
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
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
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
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
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
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.
where unshare() can be used.
2.1 Per-security context namespaces
-----------------------------------
unshare can be used to implement polyinstantiated directories using
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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
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
@ -74,33 +77,36 @@ 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
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
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
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
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
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
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,
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
@ -108,13 +114,14 @@ 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
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
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
@ -124,7 +131,7 @@ SYNOPSIS
int unshare(int flags);
DESCRIPTION
unshare allows a process to disassociate parts of its execution
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,
@ -132,7 +139,7 @@ DESCRIPTION
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
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
@ -176,17 +183,20 @@ SEE ALSO
6) High Level Design
--------------------
Depending on the flags argument, the unshare system call allocates
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.
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()
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
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
@ -202,94 +212,121 @@ 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
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
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
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
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.
7.1) 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
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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.
7.3) unshare_* helper functions
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
7.4) Appropriately modify architecture specific code to register the
new system call.
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:
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.
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.
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.
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.
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
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.
be incrementally added to unshare() without affecting legacy
applications using unshare().