From: Manfred Spraul <manfred@colorfullife.com>

Actual implementation of the posix message queues, written by Krzysztof
Benedyczak and Michal Wronski.  The complete implementation is dependant on
CONFIG_POSIX_MQUEUE.

It passed the openposix test suite with two exceptions: one mq_unlink test
was bad and tested undefined behavior.  And Linux succeeds
mq_close(open(,,,)).  The spec mandates EBADF, but we have decided to ignore
that: we would have to add a new syscall just for the right error code.

The patch intentionally doesn't use all helpers from fs/libfs for kernel-only
filesystems: step 5 allows user space mounts of the file system.



Signal changes:

The patch redefines SI_MESGQ using __SI_CODE: The generic Linux ABI uses
a negative value (i.e.  from user) for SI_MESGQ, but the kernel internal
value must be posive to pass check_kill_value.  Additionally, the patch
adds support into copy_siginfo_to_user to copy the "new" signal type to
user space.



Changes in signal code caused by POSIX message queues patch:

General & rationale:

  mqueues generated signals (only upon notification) must have si_code
  == SI_MESGQ.  In fact such a signal is send from one process which
  caused notification (== sent message to empty message queue) to
  another which requested it.  Both processes can be of course unrelated
  in terms of uids/euids.  So SI_MESGQ signals must be classified as
  SI_FROMKERNEL to pass check_kill_permissions (not need to say that
  this signals ARE from kernel).

  Signals generated by message queues notification need the same
  fields in siginfo struct's union _sifields as POSIX.1b signals and we
  can reuse its union entry.

  SI_MESGQ was previously defined to -3 in kernel and also in glibc. 
  So in userspace SI_MESGQ must be still visible as -3.

Solution:

  SI_MESGQ is defined in the same style as SI_TIMER using __SI_CODE macro.

  Details:

    Fortunately copy_siginfo_to_user copies si_code as short.  So we
    can use remaining part of int value freely.  __SI_CODE does the
    work.  SI_MESGQ is in kernel:

 		6<<16 | (-3 & 0xffff) what is > 0

    but to userspace is copied

 		(short) SI_MESGQ == -3

Actual changes:

  Changes in include/asm-generic/siginfo.h

  __SI_MESGQ added in signal.h to represent inside-kernel prefix of
  SI_MESGQ.  SI_MESGQ is redefined from -3 to __SI_CODE(__SI_MESGQ, -3)

  Except mips architecture those changes should be arch independent
  (asm-generic/siginfo.h is included in arch versions).  On mips
  SI_MESGQ is redefined to -4 in order to be compatible with IRIX.  But
  the same schema can be used.

  Change in copy_siginfo_to_user: We only add one line to order the
  same copy semantics as for _SI_RT.

  This change isn't very portable - some arch have its own
  copy_siginfo_to_user.  All those should have similar change (but
  possibly not one-line as _SI_RT case was sometimes ignored because i
  wasn't used yet, e.g.  see ia64 signal.c).

Update:
mq: only fail with invalid timespec if mq_timed{send,receive} needs to block
From: Jakub Jelinek <jakub@redhat.com>

POSIX requires EINVAL to be set if:
"The process or thread would have blocked, and the abs_timeout parameter
specified a nanoseconds field value less than zero or greater than or equal
to 1000 million."
but 2.6.5-mm3 returns -EINVAL even if the process or thread would not block
(if the queue is not empty for timedreceive or not full for timedsend).