296 lines
13 KiB
Plaintext
296 lines
13 KiB
Plaintext
|
|
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.
|
|
|