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[Launchpad Bug Tracker <1772456@xxxxxxxxxxxxxxxxxx>]

This bug was fixed in the package ebtables - 2.0.10.4-3.5ubuntu2.18.04.1

---------------
ebtables (2.0.10.4-3.5ubuntu2.18.04.1) bionic; urgency=medium

  * Fix check of fcntl errno value, to allow lockfile contention to work.
    (LP: #1772456)

 -- Dan Streetman <ddstreet@xxxxxxxxxxxxx>  Mon, 21 May 2018 13:20:15
-0400

** Changed in: ebtables (Ubuntu Bionic)
       Status: Fix Committed => Fix Released

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https://bugs.launchpad.net/bugs/1772456

Title:
  ebtables lock file fcntl errno value not correctly checked

Status in ebtables package in Ubuntu:
  Fix Released
Status in ebtables source package in Trusty:
  Fix Released
Status in ebtables source package in Xenial:
  Fix Released
Status in ebtables source package in Artful:
  Fix Released
Status in ebtables source package in Bionic:
  Fix Released
Status in ebtables source package in Cosmic:
  Fix Released

Bug description:
  [impact]

  bug 1645324 introduced code to improve the existing file-based
  locking, by using fcntl instead of exclusive file opening.

  However, when fcntl fails the new code tries to check for errno of
  EAGAIN or EACCES, but does so incorrectly:

  +		ret = fcntl(lockfd, F_SETLK, &fl);
  +		if (ret == -1 && errno != (EAGAIN || EACCES))

  "errno != (EAGAIN || EACCES)" is not correct, because it will always
  evaluate to true when errno is either EAGAIN or EACCES; the bitwise OR
  of EAGAIN (11) and EACCES (13) produces 15 (ENOTBLK) which will never
  match either, so != always is true.

  [test case]

  run ebtables in a tight loop from two separate shells, to force
  lockfile contention, for example:

  #!/bin/bash

  dev="$1"

  while test 1; do
  sleep 0
  ebtables --concurrent -t nat -A PREROUTING -i ${dev} -j ACCEPT
  if [ $? -ne 0 ]; then
  echo "odd!"
  fi
  ebtables --concurrent -t nat -D PREROUTING -i ${dev} -j ACCEPT
  if [ $? -ne 0 ]; then
  echo "odd!"
  fi
  done

  that can be run with any text param (e.g. "foo" and "bar") from 2
  shells, to produce the failure:

  Unable to create lock file /var/lib/ebtables/lock.

  [regression potential]

  like the previous patch, this change also has the potential to
  introduce errors in file locking that ebtables uses; however
  considering the file locking currently does not work at all due to the
  above logic error, this patch should only help.  The only regression
  potential is see is the possibility of breaking non-contended file
  locking, which currently does work.

  [other info]

  see previous bug 1645324
  https://launchpad.net/bugs/1645324

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