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[Wolfgang <1390223@xxxxxxxxxxxxxxxxxx>]

Ah those "upstream" kernels weren't pure... Just tested manually compiled kernel master branch and tag 4.2, no issues. Copied over a packaged one: problem reappears.
It's not a container problem though. Running this on a host has the same effect:
# aa-exec -p $pick_your_favorite_profile -- socat UNIX:/var/spool/postfix/public/showq -
also gets an EPERM.
Also not 32/64 bit dependent, however we do have some containers where it always works, and some where it always fails, and that was their only obvious distinction, which now seems unrelated.

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https://bugs.launchpad.net/bugs/1390223

Title:
  Apparmor related regression on access to unix sockets on a candidate
  3.16 backport kernel

Status in linux package in Ubuntu:
  Fix Committed
Status in linux source package in Utopic:
  Confirmed
Status in linux source package in Vivid:
  Fix Committed

Bug description:
  I recently noticed a bunch of containers failing in a rather odd way
  when running postfix.

  The most visible example is when running mailq on an empty queue.
  Without apparmor (unconfined container) I see that the queue is empty,
  with apparmor, I get Permission denied.

  That's all running as root so the permission denied looks a tiny bit
  odd. Also, running the 3.13 kernel, I don't get any of that weirdness.

  My guess is that it has to do with the work that went into the 3.16
  kernel for socket mediation. In theory only systems that run the
  utopic apparmor (which I DO NOT) should be seeing that kind of
  behavior, but it looks like some code path isn't checking things
  properly :)

  == strace in unconfined container ==
  chdir("/var/spool/postfix")             = 0
  rt_sigaction(SIGPIPE, {SIG_IGN, [PIPE], SA_RESTORER|SA_RESTART, 0x7f8963a62c30}, {SIG_IGN, [], 0}, 8) = 0
  getuid()                                = 0
  socket(PF_LOCAL, SOCK_STREAM, 0)        = 4
  fcntl(4, F_GETFL)                       = 0x2 (flags O_RDWR)
  fcntl(4, F_SETFL, O_RDWR)               = 0
  connect(4, {sa_family=AF_LOCAL, sun_path="public/showq"}, 110) = 0
  poll([{fd=4, events=POLLIN}], 1, 3600000) = 1 ([{fd=4, revents=POLLIN|POLLHUP}])
  read(4, "Mail queue is empty\n", 4096)  = 20
  poll([{fd=4, events=POLLIN}], 1, 3600000) = 1 ([{fd=4, revents=POLLIN|POLLHUP}])
  read(4, "", 4096)                       = 0
  write(1, "Mail queue is empty\n", 20Mail queue is empty
  )   = 20
  close(4)                                = 0
  exit_group(0)                           = ?
  +++ exited with 0 +++

  == strace in confined container ==
  chdir("/var/spool/postfix")             = 0
  rt_sigaction(SIGPIPE, {SIG_IGN, [PIPE], SA_RESTORER|SA_RESTART, 0x7ffe62de4c30}, {SIG_IGN, [], 0}, 8) = 0
  getuid()                                = 0
  socket(PF_LOCAL, SOCK_STREAM, 0)        = 4
  fcntl(4, F_GETFL)                       = 0x2 (flags O_RDWR)
  fcntl(4, F_SETFL, O_RDWR)               = 0
  connect(4, {sa_family=AF_LOCAL, sun_path="public/showq"}, 110) = 0
  poll([{fd=4, events=POLLIN}], 1, 3600000) = 1 ([{fd=4, revents=POLLIN|POLLHUP}])
  read(4, 0x7ffe65b35c00, 4096)           = -1 EACCES (Permission denied)
  close(4)                                = 0
  write(2, "postqueue: warning: close: Permi"..., 45postqueue: warning: close: Permission denied
  ) = 45
  sendto(3, "<20>Nov  6 20:40:42 postfix/post"..., 78, MSG_NOSIGNAL, NULL, 0) = 78
  exit_group(0)                           = ?

  Kernel is a slightly outdated version of the kernel from the kernel team PPA:
  Linux shell01 3.16.0-23-generic #31-Ubuntu SMP Thu Oct 23 20:13:35 UTC 2014 x86_64 x86_64 x86_64 GNU/Linux

  If you think the latest build will improve this, I can test it, but
  seeing how this is a production server, I can't just flip kernels
  every 5 minutes (I'm running 3.16 to avoid a nasty btrfs bug on 3.13).

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