[syslog-ng] Critical hang while opening tty

[Balazs Scheidler]

On Fri, 2009-06-05 at 13:22 +0200, Jean-Baptiste Quenot wrote:
> Hi there,
> 
> I'm using syslog-ng 2.0.9-4.1 on Ubuntu 9.04.  In the configuration
> file, emergency messages are configured to be logged to all ttys.
> Sometimes syslog-ng blocks the whole system while opening /dev/ttyS0:
> 
> # pgrep -f syslog-ng
> # strace -f -p 3172
> Process 3172 attached - interrupt to quit
> open("/dev/ttyS0", O_WRONLY|O_NOCTTY|O_APPEND
> 
> I'm surprised because I saw the same bug report, and this was supposed
> to be fixed in Ubuntu's 2.0.9-1 version as stated in the changelog at
> See http://changelogs.ubuntu.com/changelogs/pool/universe/s/syslog-ng/syslog-ng_2.0.9-4.1/changelog
> 
> The changelog says:
> 
> syslog-ng (2.0.9-1) unstable; urgency=low
> 
>   * New upstream version. (Closes: #397650, #447105)
> 
> This corresponds to this bug:
> 
> syslog-ng hangs writing to /dev/ttyN
> http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=397650
> 
> I notice the bug is fixed in syslog-ng:
> http://git.balabit.hu/?p=bazsi/syslog-ng-2.0.git;a=commit;h=c5cc92d54d2e33b9b55d01af0104d536e6d8b797
> 
> Is there maybe another call to tty open() that was not covered by alarm()?
> 
> The issue happens on several machines, but I don't know what triggers
> this.  Reloading syslog-ng fixes the problem.

That fix was applied to the usertty() driver, are you using that? But
yes, as you guessed right, the open() call was not expected to block,
thus it is not protected by the alarm() call. I didn't know that even
open might block. 

The patch below makes the open() call nonblocking, which means that if
the tty is blocked, the message will be dropped. But this is probably
the only sane thing to do.

diff --git a/src/afuser.c b/src/afuser.c
index e3aa50a..245cdcf 100644
--- a/src/afuser.c
+++ b/src/afuser.c
@@ -95,7 +95,7 @@ afuser_dd_queue(LogPipe *s, LogMessage *msg, const LogPathOptions *path_options)
           else
             line[0] = 0;
           strncat(line, ut->ut_line, sizeof(line));
-          fd = open(line, O_NOCTTY | O_APPEND | O_WRONLY);
+          fd = open(line, O_NOCTTY | O_APPEND | O_WRONLY | O_NONBLOCK);
           if (fd != -1) 
             {
               alarm_set(10);

I've also committed this to syslog-ng 3.0 tree as this patch:

commit 75c5000a4d6ea6ab04e4f2158e6008dec0a1cdcd
Author: Balazs Scheidler <bazsi at balabit.hu>
Date:   Sun Jun 7 11:47:51 2009 +0200

    [afuser] open the tty in nonblocking mode to avoid blocking on serial lines
    
    Reported-By: Jean-Baptiste Quenot



-- 
Bazsi