[Jack-Devel] segfault in libjack from incorrect regex in jack_get_ports()

Jörn Nettingsmeier nettings at stackingdwarves.net
Sat Feb 16 19:33:46 CET 2019

Hi guys,

I'm seeing segfaults caused by an incorrect regex (found while fooling 
around to see what syntax is allowed, since it's not really documented).

Turns out the correct answer to that is "regexec.c", and thus, POSIX 
regexes. Someone might want to put that in the docs somewhere.

Thread 1 "shairport-sync" hit Breakpoint 1, jack_init (argc=<optimized 
out>, argv=<optimized out>)
     at audio_jack.c:221
221	  if (config.jack_autoconnect_pattern != NULL) {
(gdb) print config.jack_autoconnect_pattern
$1 = 0x5c8a0 "effect_1:in{L|R}"

                          ^--- this is likely the problem

(gdb) step
222	    debug(1, "config.jack_autoconnect_pattern is %s.", 
(gdb) next
223	    const char** port_list = jack_get_ports(client, 
(gdb) step

Thread 1 "shairport-sync" received signal SIGSEGV, Segmentation fault.
0x76847e8c in __regexec (preg=<optimized out>, string=0x71801e34 
"system:playback_1", nmatch=0, pmatch=0x0,
     eflags=0) at regexec.c:243
243	regexec.c: No such file or directory.
(gdb) bt
#0  0x76847e8c in __regexec (preg=<optimized out>, string=0x71801e34 
"system:playback_1", nmatch=0, pmatch=0x0,
     eflags=0) at regexec.c:243
#1  0x76f1d0f4 in ?? () from /usr/lib/arm-linux-gnueabihf/libjack.so.0
#2  0x76f1d1c4 in ?? () from /usr/lib/arm-linux-gnueabihf/libjack.so.0
#3  0x0002c988 in jack_init (argc=<optimized out>, argv=<optimized out>) 
at audio_jack.c:223
#4  0x00016adc in main (argc=1, argv=0x7efffc54) at shairport.c:1474

It turns out that the return value of regcomp is not checked by libjack. 
That's the last point where a segfault can be prevented from a simple 
syntax error. The only way to catch that in the application is to set up 
an entirely redundant regex scaffold and check the regcomp return value, 
before throwing it all away and letting libjack do the work all over again.

In the case of configurable auto-connect patterns, which many JACK 
clients now offer as a sane and useful replacement of auto-connecting to 
system:playback_N, this SEGV is triggered by end-user action, not 
programmer mistake. That's a harsh user experience, and should be fixed.
A brief look at the source seems to indicate the problem also exists in 

I don't really feel qualified to tackle this myself, since it will 
require lots of error handling in a multi-threaded library, which I 
don't know too much about... glad to help testing, though.

All best,


Jörn Nettingsmeier
Tuinbouwstraat 180, 1097 ZB Amsterdam, Nederland
Tel. +49 177 7937487

Meister für Veranstaltungstechnik (Bühne/Studio), Tonmeister VDT

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