Public bug reported:
[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
** Affects: ebtables (Ubuntu)
Importance: Medium
Assignee: Dan Streetman (ddstreet)
Status: In Progress
** Affects: ebtables (Ubuntu Trusty)
Importance: Undecided
Status: In Progress
** Affects: ebtables (Ubuntu Xenial)
Importance: Undecided
Status: New
** Affects: ebtables (Ubuntu Artful)
Importance: Undecided
Status: New
** Affects: ebtables (Ubuntu Bionic)
Importance: Undecided
Status: New
** Affects: ebtables (Ubuntu Cosmic)
Importance: Medium
Assignee: Dan Streetman (ddstreet)
Status: In Progress
** Changed in: ebtables (Ubuntu)
Status: New => In Progress
** Changed in: ebtables (Ubuntu)
Assignee: (unassigned) => Dan Streetman (ddstreet)
** Changed in: ebtables (Ubuntu)
Importance: Undecided => Medium
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https://bugs.launchpad.net/bugs/1772456
Title:
ebtables lock file fcntl errno value not correctly checked
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