On Tue, Jan 09, 2024 at 02:07:26PM -0500, Rich Felker wrote:
> On Tue, Jan 09, 2024 at 03:37:17PM +0100, jvoisin wrote:
> > Ohai,
> > 
> > as discussed on irc, Android's bionic has a check to prevent
> > use-after-destroy on phtread mutexes
> > (https://github.com/LineageOS/android_bionic/blob/e0aac7df6f58138dae903b5d456c947a3f8092ea/libc/bionic/pthread_mutex.cpp#L803),
> > and musl doesn't.
> > 
> > While odds are that this is a super-duper common bug, it would still be
> > nice to have this kind of protection, since it's cheap, and would
> > prevent/make it easy to diagnose weird states.
> > 
> > Is this something that should/could be implemented?
> > 
> > o/
> 
> I think you meant that the odds are it's not common. There's already
> enough complexity in the code paths for supporting all the different
> mutex types that my leaning would be, if we do any hardening for
> use-after-destroy, that it should probably just take the form of
> putting the object in a state that will naturally deadlock or error
> rather than adding extra checks to every path where it's used.
> 
> If OTOH we do want it to actually trap in all cases where it's used
> after destroy, the simplest way to achieve that is probably to set it
> up as a non-robust non-PI recursive or errorchecking mutex with
> invalid prev/next pointers and owner of 0x3fffffff. Then the only
> place that would actually have to have an explicit trap is trylock in
> the code path:
> 
>         if (own == 0x3fffffff) return ENOTRECOVERABLE;
> 
> where it could trap if type isn't robust. The unlock code path would
> trap on accessing invalid prev/next pointers.

Draft attached in case anyone wants to play with it. This could
probably be something we could consider to adopt.