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From Igor Seletskiy <isele...@cloudlinux.com>
Subject mod_fcgid can kill all the services on the server via kill -15 -1
Date Sun, 17 Apr 2011 14:40:26 GMT
Hello,

There is a very interesting, and quite a rare bug in mod_fcgid. It is easy
to reproduce if you can cause fork to fail (which can be done with
CloudLinux -- if anyone wants to replicate it).

*Here is how it works: *
mod_fcgid tries to spawn a new process (proc_spawn_process in
fcgid_proc_unix.c), but fork returns -1.
More exactly fcgid_create_privileged_process function call returns error,
and fills in tmpproc.pid with -1 & tmpproc is assiged to procnode->proc_id).

Now, if at the same time service httpd restart is executed, function
kill_all_subprocess in fcgid_pm_main.c will execute, and it will try to go
through all procnodes, sending SIGTERM via proc_kill_gracefully, (and then
SIGKILL via proc_kill_force) to procnode->proc_id.pid
Yet, one procnode will be pointing to procnode->proc_id.pid, causing kill
-15 -1 (kill all).
The end results all services on the server failing, including SSH, apache,
syslogd, etc..

I guess the problem is really rare for most people. Also it is quite hard to
diagnose, as it is completely not clear where the signal came from, and it
took us some time to correlate them with apache restarts.. Yet due to our OS
being used by shared hosts (where httpd restart is common thing), and our
ability to limit memory available to processes on per virtual host bases
(which causes fork to fail once that virtual host reaches memory limit), we
see the issue quite often.

The solution is quite simple (not sure if it is the best / right solution),
in file: fcgid_proc_unix.c, in methods proc_kill_gracefully, line:

    rv = apr_proc_kill(&(procnode->proc_id), SIGTERM);

should be changed to:
   if (procnode->proc_id.pid != -1) {
          rv = apr_proc_kill(&(procnode->proc_id), SIGTERM);
   } else {
          rv = APR_SUCCESS;
   }

Similarly in proc_kill_force
    rv = apr_proc_kill(&(procnode->proc_id), SIGKILL);
should be changed to:
   if (procnode->proc_id.pid != -1) {
          rv = apr_proc_kill(&(procnode->proc_id), SIGKILL);
   } else {
          rv = APR_SUCCESS;
   }

Regards,
Igor Seletskiy
CEO @ Cloud Linux Inc

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