Re: [RFC PATCH v4 1/5] glibc: Perform rseq(2) registration at nptl init and thread creation

[Rich Felker]

On Thu, Nov 22, 2018 at 04:11:45PM +0100, Florian Weimer wrote:
> * Mathieu Desnoyers:
> 
> > Thoughts ?
> >
> >   /* Unregister rseq TLS from kernel. */
> >   if (has_rseq && __rseq_unregister_current_thread ())
> >     abort();
> >
> >   advise_stack_range (pd->stackblock, pd->stackblock_size, (uintptr_t) pd,
> >                       pd->guardsize);
> >
> >   /* If the thread is detached free the TCB.  */
> >   if (IS_DETACHED (pd))
> >     /* Free the TCB.  */
> >     __free_tcb (pd);
> 
> Considering that we proceed to free the TCB, I really hope that all
> signals are blocked at this point.  (I have not checked this, though.)
> 
> Wouldn't this address your concern about access to the rseq area?

I'm not familiar with glibc's logic here, but for other reasons, I
don't think freeing it is safe until the kernel task exit futex (set
via clone or set_tid_address) has fired. I would guess __free_tcb just
sets up for it to be reclaimable when this happens rather than
immediately freeing it for reuse.

Rich