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

[Florian Weimer <fweimer@redhat.com>]

* 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.

Right, but in case of user-supplied stacks, we actually free TLS memory
at this point, so signals need to be blocked because the TCB is
(partially) gone after that.

Thanks,
Florian