bug#58042: 29.0.50; ASAN use-after-free in re_match_2_internal

[Eli Zaretskii <eliz@gnu.org>]

> From: Gerd Möllmann <gerd.moellmann@gmail.com>
> Cc: 58042@debbugs.gnu.org
> Date: Sun, 25 Sep 2022 07:50:17 +0200
> 
> > The GC was triggered by
> > redisplay, but how did redisplay start while regexp match was in
> > progress?  Do you see any code in regexp that could trigger redisplay?
> 
> I'm afraid, I don't follow.  Why do you think redisplay comes into play
> here?

Because of this part of your message in
https://debbugs.gnu.org/cgi/bugreport.cgi?bug=58042#8:

  freed by thread T0 here:
      #0 0x103332de4 in wrap_free+0x98 (libclang_rt.asan_osx_dynamic.dylib:arm64e+0x3ede4)
      #1 0x100985e38 in rpl_free free.c:48
      #2 0x1005b71a4 in lisp_free alloc.c:1038
      #3 0x1005cbda4 in compact_small_strings alloc.c:2191
      #4 0x1005c9f24 in sweep_strings alloc.c:2072
      #5 0x1005bd028 in gc_sweep alloc.c:7397
      #6 0x1005bb178 in garbage_collect alloc.c:6245
      #7 0x1005ba694 in maybe_garbage_collect alloc.c:6090
      #8 0x1006505ac in maybe_gc lisp.h:5624
      #9 0x100648ffc in Ffuncall eval.c:2972
      #10 0x10064bcd0 in internal_condition_case_n eval.c:1555
      #11 0x1000cdc8c in safe__call xdisp.c:3026
      #12 0x1000cdfc4 in safe__call1 xdisp.c:3062
      #13 0x1001d6404 in prepare_menu_bars xdisp.c:13572
      #14 0x1000f2340 in redisplay_internal xdisp.c:16523
      #15 0x100108f34 in redisplay xdisp.c:16105

AFAIU, this says that the GC which freed the string data was caused by
safe__call1 inside prepare_menu_bars, which was called from
redisplay_internal.

> Anyways, my working hypotheses currently goes like this:
> 
> We match using some Lisp string S and get its data pointer, say D.
> Since D is not null, S must be a live string.
> 
> (Actually I didn't check that this is still the case, but I think I've
> been setting s.data to null for free strings right from the start, and I
> can't imagine why anyone would change that.)
> 
> Between the point we get D, and the point of the crash, a GC happens.
> We know in principle that a GC can happen while matching since
> bug#56108.  I'm taking that as a given.  The GC compacts strings and
> changes S's data pointer.
> 
> After GC, S.data != D.

Yes, but I have difficulty with the fact that GC was caused by
redisplay, and redisplay cannot be invoked while we are in
re_match_2_internal, AFAIK.  So something else is missing here (or
maybe I'm misinterpreting the ASAN report you posted).