[Bug 1155695] Re: mq_overview(7) contains outdated, inaccurate fs.mqueue.queues_max limit description

[Michael Kerrisk]

Arto, it's the simple fix, but before I push it further, does the
attached patch fix your problem?

At this point (ii.e., since LInux 3.8), I suspect that the limit that
the kernel code  currently tries to enforce on queues_max is anyway
meaningless. Unprivileged users can nowadays use
clone(CLONE_NEWUSER|CLONE_NEWIPC) to create an arbitrary number of new
IPC namespaces, each of which can be populated with queues_max MQs. So
the hard limit check doesn't really limit users at all (though, as
you've discovered, it does inconvenience them).

** Patch added: "mqueue_limit.patch"
   https://bugs.launchpad.net/ubuntu/+source/manpages/+bug/1155695/+attachment/3581440/+files/mqueue_limit.patch

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https://bugs.launchpad.net/bugs/1155695

Title:
  mq_overview(7) contains outdated, inaccurate fs.mqueue.queues_max
  limit description

Status in “manpages” package in Ubuntu:
  Confirmed

Bug description:
  The manpages 3.40-0.1ubuntu3 package in Ubuntu 12.10 contains an
  mq_overview(7) man page with outdated, severely inaccurate information
  regarding the /proc/sys/fs/mqueue/queues_max kernel limit, as follows:

  "The default value for queues_max is 256; it can be changed to any
  value in the range 0 to INT_MAX."

  This used to be true in Linux 3.2.x, as shipped in Ubuntu 12.04.
  However, it is no longer true with Linux 3.5.x, as shipped in 12.10.

  For some inexplicable reason, upstream lowered the hard upper limit
  for POSIX message queues (fs.mqueue.queues_max) from INT_MAX to 1024
  in the 3.5 kernel:

  # sysctl -w fs.mqueue.queues_max=1025
  sysctl: setting key "fs.mqueue.queues_max": Invalid argument
  fs.mqueue.queues_max = 1025

  You can verify the new, post-3.5 limit at:

  http://lxr.free-electrons.com/source/ipc/mq_sysctl.c?v=3.5#L40
  http://lxr.free-electrons.com/source/include/linux/ipc_namespace.h?v=3.5#L121

  Please amend the man page so that the new limit is correctly
  documented. The undocumented change from INT_MAX queues to a mere 1024
  queues is rather harsh, and it took quite some investigation to pin
  this down as an upstream kernel change.

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