rseq notes from Cauldron

[Florian Weimer]

* Mathieu Desnoyers:

>> We need to disable symbols in pthread_create, so that signal handlers
>
> I think you meant "disable signals".

Sorry, you are right.

>> We need a comment somehwere in NPTL about the reliance of the rseq
>> area on implicit deregistration.  This is currently not a problem
>> because it lives on the stack (as static TLS), and the stack outlives
>> the thread.  But if we ever change the TCB allocation, additional
>> steps may be required.  It is NOT sufficient to block signals because
>> the kernel may write to the rseq area even without signals.  This
>> should go into the rseq patch as well.
>
> This adds a hard requirement on using the IE TLS model AFAIU. Another
> action item on my end.

Yes, but given that glibc already has initial-exec TLS, the rest has
to be initial-exec, too, I think.  At least in the current
implementation in the loader.