bug#28211: Stack marking issue in multi-threaded code, 2020 edition

[Andy Wingo <wingo@igalia.com>]

On Thu 12 Mar 2020 22:59, Ludovic Courtès <ludo@gnu.org> writes:

> I think I’ve found another race condition involving stack marking, as a
> followup to <https://issues.guix.gnu.org/issue/28211> (this time on
> 3.0.1+, but the code is almost the same.)
>
> ‘abort_to_prompt’ does this:
>
>   fp = vp->stack_top - fp_offset;
>   sp = vp->stack_top - sp_offset;
>
>   /* Continuation gets nargs+1 values: the one more is for the cont.  */
>   sp = sp - nargs - 1;
>
>   /* Shuffle abort arguments down to the prompt continuation.  We have
>      to be jumping to an older part of the stack.  */
>   if (sp < vp->sp)
>     abort ();
>   sp[nargs].as_scm = cont;
>   while (nargs--)
>     sp[nargs] = vp->sp[nargs];
>
>   /* Restore VM regs */
>   vp->fp = fp;
>   vp->sp = sp;
>   vp->ip = vra;
>
>
> What if ‘scm_i_vm_mark_stack’ walks the stack right before the ‘vp->fp’
> assignment?  It can determine that one of the just-assigned ‘sp[nargs]’
> is a dead slot, and thus set it to SCM_UNSPECIFIED.

I think you're right here.

Given that the most-recently-pushed frame is marked conservatively, I
think it would be sufficient to reset vp->fp before shuffling stack
args; that would make it so that the frame includes the values to
shuffle, their target locations, and probably some other crap in
between.  Given that marking the crap is harmless, I think that would be
enough.  WDYT?

In a more perfect world, initiating GC should tell threads to reach a
safepoint and mark their own stacks -- preserves thread locality and
prevents this class of bug.  But given that libgc uses signals to stop
threads, we have to be less precise.

Cheers,

Andy