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Subject: bug#41755: feature/native-comp (master?): temacs crash in GC during
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Cc: Paul Eggert <eggert@cs.ucla.edu>, 41755@debbugs.gnu.org,
 Andrea Corallo <akrl@sdf.org>
To: Nicolas =?UTF-8?Q?B=C3=A9rtolo?= <nicolasbertolo@gmail.com>
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Xref: news.gmane.io gmane.emacs.bugs:181747
Archived-At: <http://permalink.gmane.org/gmane.emacs.bugs/181747>

Nicolas B=C3=A9rtolo <nicolasbertolo@gmail.com> writes:

>> I'm wondering what we could do to make such bugs easier to find...
>
> We could add a canary to stack based strings and conses. Then while
> marking if we
> come across a stack based string or cons we check that the canary is
> intact. If
> it is not, then we can be sure that the memory has been written over.

I believe we should never be marking stack-based objects. If we do
that's a GC bug.

Code like

  AUTO_STRING (s, "foo");
  Lisp_Object c =3D Fcons (s, s);
  garbage_collect ();
  ...
  Fsetcar (c, Qnil);
  Fsetcdr (c, Qnil);

shouldn't work. I hope it doesn't :-) (With GC_CHECK_MARKED_OBJECTS, it
should abort; without, it would leave the mark bit of s set, so the
"..." code would presumably crash).

> Something like this:
>
> struct Stack_String
> {
>   struct Lisp_String string;
>   uint64_t canary =3D 0x12341234;
> };
>
>> Would GC_CHECK_MARKED_OBJECTS have caught this?
>
> As far as I can see, during a GC we can't know if a stack-based string
> is still alive.

But we can know whether a string is stack-based or not; if it is, we
shouldn't be marking it, so we can abort in that case...