Re: dash behaviour, was Re: core dump analysis

Finn Thain
2023-04-09T07:28:54Z

On Sun, 9 Apr 2023, Michael Schmitz wrote:

> Am 09.04.2023 um 16:42 schrieb Finn Thain:
> > On Sun, 9 Apr 2023, Michael Schmitz wrote:
> >
> >>>
> >>> The only way I have found to alter dash's inclination to crash is to
> >>> reboot. (I said previously I was unable to reproduce this in a
> >>> single user mode shell but it turned out to be more subtle.)
> >>
> >> I wonder what could change from one boot to another - can you have
> >> dash (and its subshells) dump /proc/self/maps and see whether there's
> >> any variation in that? But what we really need is the physical
> >> mappings. How can we find those?
> >>
> >> With the kernel RNG disabled, I would expect neither of these
> >> mappings to change between boots?
> >>
> >
> > It looks like the stack area still changes across invocations:
>
> Yep, but running the same commands in the same order across different
> boots, does it still change?
>
> (I'm making a huge assumption here - that timing of the boot process and
> hence evolution of the kernel RNG is sufficiently deterministic. And
> this might apply only to the shells run from sysvinit, since that does
> require no keyboard input ...)
>
> Looks like cat < /proc/self/maps | grep stack would give us enough
> information without overwhelming the serial console?
>
> OTOH - if you can show the error is gone without stack address
> randomization, that would be a hint maybe?
>

The results below were produced with 'norandmaps' added to the kernel
parameters to avoid ASLR.

root@debian:~# sh /etc/init.d/mountdevsubfs.sh start
root@debian:~# sh /etc/init.d/mountdevsubfs.sh start
root@debian:~# sh /etc/init.d/mountdevsubfs.sh start
root@debian:~# sh /etc/init.d/mountdevsubfs.sh start
root@debian:~# echo 3 > /proc/sys/vm/drop_caches
[ 913.560000] bash (1024): drop_caches: 3
root@debian:~# sh /etc/init.d/mountdevsubfs.sh start
root@debian:~# sh /etc/init.d/mountdevsubfs.sh start
root@debian:~# sh /etc/init.d/mountdevsubfs.sh start
root@debian:~# sh /etc/init.d/mountdevsubfs.sh start
*** stack smashing detected ***: terminated
Aborted (core dumped)
root@debian:~# sh -c "md5sum < /proc/self/maps"
baacbaf944fb01d3200d924da7f7a815 -
root@debian:~# sh -c "md5sum < /proc/self/maps"
baacbaf944fb01d3200d924da7f7a815 -
root@debian:~# sh -c "md5sum < /proc/self/maps"
baacbaf944fb01d3200d924da7f7a815 -
root@debian:~# sh -c "md5sum < /proc/self/maps"
baacbaf944fb01d3200d924da7f7a815 -
root@debian:~# sh -c "cat < /proc/self/maps"
c0000000-c0021000 r-xp 00000000 08:06 38780 /usr/lib/m68k-linux-gnu/ld.so.1
c0021000-c0023000 rw-p 00000000 00:00 0
c0023000-c0024000 r--p 00021000 08:06 38780 /usr/lib/m68k-linux-gnu/ld.so.1
c0024000-c0026000 rw-p 00022000 08:06 38780 /usr/lib/m68k-linux-gnu/ld.so.1
c002a000-c0199000 r-xp 00000000 08:06 38786 /usr/lib/m68k-linux-gnu/libc.so.6
c0199000-c019a000 ---p 0016f000 08:06 38786 /usr/lib/m68k-linux-gnu/libc.so.6
c019a000-c019c000 r--p 00170000 08:06 38786 /usr/lib/m68k-linux-gnu/libc.so.6
c019c000-c01a0000 rw-p 00172000 08:06 38786 /usr/lib/m68k-linux-gnu/libc.so.6
c01a0000-c01aa000 rw-p 00000000 00:00 0
d0000000-d0019000 r-xp 00000000 08:06 32713 /usr/bin/dash
d001b000-d001c000 r--p 00019000 08:06 32713 /usr/bin/dash
d001c000-d001d000 rw-p 0001a000 08:06 32713 /usr/bin/dash
d001d000-d001f000 rwxp 00000000 00:00 0 [heap]
d001f000-d0040000 rwxp 00000000 00:00 0 [heap]
effdf000-f0000000 rw-p 00000000 00:00 0 [stack]

So I guess this bug has more to do with timing and little to do with
state, contrary to my guesswork above. And no doubt I will have to
contradict myself again if/when it turns out that uninitialized memory is
a factor :-/