https://bugs.exim.org/show_bug.cgi?id=1715
Bug ID: 1715
Summary: Race condition in exicyclog
Product: Exim
Version: 4.86
Hardware: x86
OS: All
Status: NEW
Severity: bug
Priority: medium
Component: Logging
Assignee: nigel@???
Reporter: graeme@???
CC: exim-dev@???
Created attachment 843
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https://bugs.exim.org/attachment.cgi?id=843&action=edit
Fix race condition in exicyclog
There's a race condition in exicyclog which can cause Exim to panic and exit.
It appears the process flow in exicyclog is similar to that of logrotate,
being:
* If file exists, then
* Rename older versions (do the numeric shuffle)
* Move existing log to previous version (in our case, mainlog to mainlog.01)
* Touch new file
* chmod & chown new file
The race exists between the 'touch' and 'chown/chmod' steps, resulting in odd
behaviour like this which I've never seen before the weekend when one of my
MTAs decided to stop accepting mail:
2015-11-07 00:01:02 [19442] Cannot open main log file "/var/log/exim/mainlog":
Permission denied: euid=93 egid=93
The chances of this happening are magnified on high-volume systems but are
still vanishingly small - in many years of running high-volume Exim instances
this is the very first time I've come across this.
Patch attached using pid of running process as suffix for intermediate file
before moving it to be the new one.
Immediate thoughts are that some Exim instances may have more log files than
others, resulting in the pid clashing with an existing file; also that usage of
$$ might not be 100% portable. Trying to avoid usage of a fixed suffix, though,
which may cause other conflicts if sites have already used it for some reason.
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