Re: [PATCH 1/5] glibc: Perform rseq(2) registration at C startup and thread creation (v10)

[Florian Weimer <fweimer@redhat.com>]

* Mathieu Desnoyers:

>> It's the registration from libc.so which needs some care.  In
>> particular, we must not override an existing registration.
>
> OK, so it could check if __rseq_abi.cpu_id is -1, and only
> perform registration if it is the case. Or do you have another
> approach in mind ?

No, __rseq_abi will not be shared with the outer libc, so the inner libc
will always see -1 there, even if the outer libc has performed
registration.

libio/vtables.c has some example what you can do:

  /* In case this libc copy is in a non-default namespace, we always
     need to accept foreign vtables because there is always a
     possibility that FILE * objects are passed across the linking
     boundary.  */
  {
    Dl_info di;
    struct link_map *l;
    if (!rtld_active ()
        || (_dl_addr (_IO_vtable_check, &di, &l, NULL) != 0
            && l->l_ns != LM_ID_BASE))
      return;
  }

_IO_vtable_check would have to be replaced with your own function; the
actual function doesn't really matter.

The rtld_active check covers the static dlopen case, where
rtld_active () is false in the inner libc.

Thanks,
Florian