A bug in commit: Use flock() for --concurrent option

A bug in commit: Use flock() for --concurrent option

[Tangchen (UVP)]

Hello Phil Sutter

I found a problem with this commit, 

Use flock() for --concurrent option
The previous locking mechanism was not atomic, hence it was possible
that a killed ebtables process would leave the lock file in place which
in turn made future ebtables processes wait indefinitely for the lock to
become free.

Fix this by using flock(). This also simplifies code quite a bit because
there is no need for a custom signal handler or an __exit routine
anymore.

Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

if /var/lib/ebtables/ does not exist, ebtables cannot create the lock file.

# ls /var/lib/ebtables
ls: cannot access /var/lib/ebtables: No such file or directory
# ebtables --concurrent -L
Unable to create lock file /var/lib/ebtables/lock.

Thanks

Re: A bug in commit: Use flock() for --concurrent option

[Phil Sutter]

Hi Chen Tang,

On Fri, Jan 12, 2018 at 08:12:01AM +0000, Tangchen (UVP) wrote:
> I found a problem with this commit, 
> 
> Use flock() for --concurrent option
> The previous locking mechanism was not atomic, hence it was possible
> that a killed ebtables process would leave the lock file in place which
> in turn made future ebtables processes wait indefinitely for the lock to
> become free.
> 
> Fix this by using flock(). This also simplifies code quite a bit because
> there is no need for a custom signal handler or an __exit routine
> anymore.
> 
> Signed-off-by: Phil Sutter <phil@nwl.cc>
> Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
> 
> if /var/lib/ebtables/ does not exist, ebtables cannot create the lock file.
> 
> # ls /var/lib/ebtables
> ls: cannot access /var/lib/ebtables: No such file or directory
> # ebtables --concurrent -L
> Unable to create lock file /var/lib/ebtables/lock.

Thanks for reporting this! I missed that the old code calls mkdir() in
one of the error paths. Will send a follow-up patch in a moment.

Cheers, Phil