sshd blocks SIGALRM

[Loic Domaigne]

Hello, 

> > We have just found an interesting issue regarding the sshd daemon on our
> > SuSE system. For some reasons, the /usr/sbin/sshd process blocks SIGALRM
> > as shown in the /proc/pid/status:
> > 
> > $ cat /proc/`cat /var/run/sshd.init.pid`/status
> > Name:   sshd
> > State:  S (sleeping)
> > SleepAVG:       0%
> > [...]
> > SigPnd: 0000000000000000
> > ShdPnd: 0000000000000000
> > SigBlk: 0000000000002000  <-- SIGALRM is blocked
> > SigIgn: 0000000000001000
> 
> you are looking at the wrong thing. You should look at the signal masks
> of the descendant processes. We clear the ALRM handler in the parent
> after accept().

Perhaps I wasn't clear enough. We execute a command in a (remote) bash shell
started by the sshd. This command never timed out, because alarm() didn't
worked as it should. 

After some non trivial investigations, we found out that SIGALRM was blocked
in the signal set of the remote shell we were using, and hence in our
command since the signal mask is inherited accross fork()/exec(). 

I traced back the process hierarchy going from child to parent to find out
who did block SIGALRM in first place, and I found: /usr/sbin/sshd. Note that
the same command works fine in a "local" shell (that is a shell that is not
started by sshd).

This is the same problem than the one discussed in the following
thread:
http://groups.google.de/group/linux.kernel/browse_thread/thread/8d0527bf96afa479/bf08f9b0faaa8ca6%23bf08f9b0faaa8ca6?sa=X&oi=groupsr&start=0&num=3

Regards, 
Loic Domaigne. 

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