Bringing rseq back into glibc

[Florian Weimer]

* Noah Goldstein:

> On Thu, Nov 18, 2021 at 4:17 AM Florian Weimer via Libc-alpha
> <libc-alpha@sourceware.org> wrote:
>>
>> I would like to bring back rseq for glibc 2.35.  I propose the following
>> steps:
>>
>> 1. Enable rseq registration in glibc, for internal use only.  This time,
>>    put the rseq area into struct pthread, not into a initial-exec TLS
>>    symbol.  (This helps to avoid with initial-exec TLS bloat with dlopen
>>    and simplifies initialization somewhat.)
>
> Isn't THREAD_SELF also implemented in TLS? Or am I missing
> something?

THREAD_SELF uses a pointer in the thread control block, and that pointer
is not replicated for different libc.so.6 copies with dlmopen (like the
rest of the TCB and struct pthread).

Thanks,
Florian