Re: core dump analysis, was Re: stack smashing detected

Michael Schmitz
2023-04-04T23:22:27Z

Hi Finn,

On 4/04/23 12:13, Finn Thain wrote:
> It looks like I messed up. waitproc() appears to have been invoked
> twice, which is why wait3 was invoked twice...
>
> GNU gdb (Debian 13.1-2) 13.1
> ...
> (gdb) set osabi GNU/Linux
> (gdb) file /bin/dash
> Reading symbols from /bin/dash...
> Reading symbols from /usr/lib/debug/.build-id/aa/4160f84f3eeee809c554cb9f3e1ef0686b8dcc.debug...
> (gdb) b waitproc
> Breakpoint 1 at 0xc346: file jobs.c, line 1168.
> (gdb) b jobs.c:1180
> Breakpoint 2 at 0xc390: file jobs.c, line 1180.
> (gdb) run
> Starting program: /usr/bin/dash
> [Thread debugging using libthread_db enabled]
> Using host libthread_db library "/lib/m68k-linux-gnu/libthread_db.so.1".
> # x=$(:)
> [Detaching after fork from child process 570]
>
> Breakpoint 1, waitproc (status=0xeffff86a, block=1) at jobs.c:1168
> 1168 jobs.c: No such file or directory.
> (gdb) c
> Continuing.
>
> Breakpoint 2, waitproc (status=0xeffff86a, block=1) at jobs.c:1180
> 1180 in jobs.c
> (gdb) info locals
> oldmask = {__val = {1997799424, 49154, 396623872, 184321, 3223896090, 53249,
> 3836788738, 1049411610, 867225601, 3094609920, 0, 1048580, 2857693183,
> 4184129547, 3435708442, 863764480, 184321, 3844141055, 4190425089,
> 4127248385, 3094659084, 597610497, 4135112705, 3844079616, 131072,
> 37355520, 184320, 3878473729, 3844132865, 3094663168, 3549089793,
> 3844132865}}
> flags = 2
> err = 570
> oldmask = <optimized out>
> flags = <optimized out>
> err = <optimized out>
> (gdb) c
> Continuing.
>
> Breakpoint 1, waitproc (status=0xeffff86a, block=0) at jobs.c:1168
> 1168 in jobs.c
> (gdb) c
> Continuing.
>
> Breakpoint 2, waitproc (status=0xeffff86a, block=0) at jobs.c:1180
> 1180 in jobs.c
> (gdb) info locals
> oldmask = {__val = {1997799424, 49154, 396623872, 184321, 3223896090, 53249,
> 3836788738, 1049411610, 867225601, 3094609920, 0, 1048580, 2857693183,
> 4184129547, 3435708442, 863764480, 184321, 3844141055, 4190425089,
> 4127248385, 3094659084, 597610497, 4135112705, 3844079616, 131072,
> 37355520, 184320, 3878473729, 3844132865, 3094663168, 3549089793,
> 3844132865}}
> flags = 3
> err = -1
> oldmask = <optimized out>
> flags = <optimized out>
> err = <optimized out>
> (gdb) c
> Continuing.
> #
>
That means we may well see both signals delivered at the same time if
the parent shell wasn't scheduled to run until the second subshell
terminated (answering the question I was about to ask on your other
mail, the one about the crashy script with multiple subshells).

Now does waitproc() handle that case correctly? The first signal
delivered results in err == child PID so the break is taken, causing
exit from waitproc(). Does waitproc() get called repeatedly until an
error is returned?

Cheers,

Michael