[stunnel-users] Possible signal handling bug in fork mode

[Philippe Anctil]

I am proposing the following patch.

Signal blocking effectively prevents signals from being processed by the
child before it has the time to change disposition.

I re-ran my test script and the result is now correct. Signals sent to
child are no longer caught by parent.

With a small program I also tested what happens with pended signals if
their disposition is changed before they're delivered. They're simply
delivered according to the new disposition. Example. If a SIGTERM is sent
to the child right after the fork, the signal will be pended. The child
will then change the signal disposition to SIG_DFL (terminate). When the
child unblocks the signals, SIGTERM is delivered to the child and the child
terminates.

Additional notes from http://man7.org/linux/man-pages/man7/signal.7.html:
- A child created via fork(2) inherits a copy of its parent's signal mask;
the signal mask is preserved across execve(2).
- A child created via fork(2) initially has an empty pending signal set;
the pending signal set is preserved across an execve(2).

All this is on RHEL5.11.

Regards,



2016-02-17 18:36 GMT-05:00 Philippe Anctil <philippe.anctil at gmail.com>:

> Hi,
>
> I have looked further into this issue. Part of the solution would be to
> reset signal disposition to SIG_DFL, either in the child after the fork, or
> around the call to create_client in accept connection for the child to
> inherit appropriate disposition from the start. Temporarily blocking
> signals could help. I have to verify exactly how it behaves.
>
> By the way, why is SIG_CHLD set to null_handler in create_client? The
> default disposition for this signal is SIG_IGN.
>
> Regards,
>
>
> 2016-02-16 11:45 GMT-05:00 Philippe Anctil <philippe.anctil at gmail.com>:
>
>> Hi,
>>
>> In fork mode, sending a sigterm signal to a child is caught by the
>> parent. I suspect it has something to do with signal_pipe being shared by
>> the child and parent after the fork.
>>
>> Here I sent a signal to the parent while a child process was also
>> running. The parent shut down as expected. After sending sigterm, the child
>> remains.
>>
>> Processes before sigterm:
>>
>> UID        PID  PPID  C STIME TTY      STAT   TIME CMD
>> user      1423     1  0 10:32 ?        Ss     0:00
>> /usr/local/stunnel/bin/stunnel.bin /usr/local/stunnel/etc/server.conf
>> user      1921  1423  0 10:37 ?        S      0:00  \_
>> /usr/local/stunnel/bin/stunnel.bin /usr/local/stunnel/etc/server.conf
>>
>> selected pid 1423
>>
>> Processes after:
>>
>> UID        PID  PPID  C STIME TTY      STAT   TIME CMD
>> user      1921     1  0 10:37 ?        S      0:00
>> /usr/local/stunnel/bin/stunnel.bin /usr/local/stunnel/etc/server.conf
>>
>> Here I sent the signal to the child. The parent shut down and the child
>> completed normally.
>>
>> Processes before:
>>
>> UID        PID  PPID  C STIME TTY      STAT   TIME CMD
>> user      1276     1  0 10:31 ?        Ss     0:00
>> /usr/local/stunnel/bin/stunnel.bin /usr/local/stunnel/etc/server.conf
>> user      1309  1276  0 10:31 ?        S      0:00  \_
>> /usr/local/stunnel/bin/stunnel.bin /usr/local/stunnel/etc/server.conf
>>
>> selected pid 1309
>>
>> Processes after:
>>
>> UID        PID  PPID  C STIME TTY      STAT   TIME CMD
>> user      1309     1  0 10:31 ?        S      0:00
>> /usr/local/stunnel/bin/stunnel.bin /usr/local/stunnel/etc/server.conf
>>
>> I know I should be trying threads. I will eventually. I can only switch
>> once I validate it can sustain our level of traffic. Fork has done that
>> flawlessly for many years now.
>>
>> Best regards,
>>
>>
>> --
>> Philippe Anctil
>>
>
>
>
> --
> Philippe Anctil
>



-- 
Philippe Anctil
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://www.stunnel.org/pipermail/stunnel-users/attachments/20160218/6a74b6a0/attachment.html>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: fork_signal_disposition.patch
Type: application/octet-stream
Size: 1127 bytes
Desc: not available
URL: <http://www.stunnel.org/pipermail/stunnel-users/attachments/20160218/6a74b6a0/attachment.obj>