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:

quoted

* Mathieu Desnoyers:

quoted

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