Re: [PATCH v8 2/8] Add rseq extensible ABI support

[DJ Delorie <dj@redhat.com>]

Michael Jeanson <mjeanson@efficios.com> writes:
>> The TLS doesn't get resized, does it?  Or am I thinking of the DTV?
>
> The terminology is a bit confusing and could be clarified, my 
> understanding is that the 'static TLS block' is the area where the 'TLS 
> blocks' from shared objects and the main executable are allocated and 
> the DTV is the array of descriptors that point to each of these 'TLS 
> blocks' inside the 'static TLS block'.

I might be thinking of where TLS data for dlopen()'d objects go, after
the static TLS block is filled.

>> Isn't __rseq_size always at least 32 ?
>
> It's at least 32 when the registration succeeded, however if the 
> registration failed or was disabled by tunable then it's set to 0.

Are we sure?

  /* Default to the rseq ABI minimum sizes, this will reduce TLS usage to 32
     bytes when rseq is disabled by tunables.  */
  size_t rseq_size = TLS_DL_RSEQ_MIN_SIZE;
  size_t rseq_align = TLS_DL_RSEQ_MIN_ALIGN;
  bool do_rseq = true;
  do_rseq = TUNABLE_GET_FULL (glibc, pthread, rseq, int, NULL);
  if (do_rseq)
    {
      rseq_align = GLRO(dl_tls_rseq_align);
      /* Make sure the rseq area size is at least the minimum ABI size and a
         multiple of the requested aligment. */
      rseq_size = roundup (MAX (GLRO(dl_tls_rseq_feature_size),
			      TLS_DL_RSEQ_MIN_SIZE), rseq_align);
    }


static inline bool
rseq_register_current_thread (struct pthread *self, bool do_rseq)
{
  if (do_rseq)
    {
      /* The kernel expects 'rseq_area->rseq_cs == NULL' on registration, zero
         the whole rseq area.  */
      memset(rseq_get_area(), 0, GLRO (dl_tls_rseq_size));
      int ret = INTERNAL_SYSCALL_CALL (rseq, rseq_get_area(),
                                       GLRO (dl_tls_rseq_size),
                                       0, RSEQ_SIG);
      if (!INTERNAL_SYSCALL_ERROR_P (ret))
        return true;
    }
  RSEQ_SETMEM (rseq_get_area(), cpu_id, RSEQ_CPU_ID_REGISTRATION_FAILED);
  return false;
}