Re: [RFC PATCH glibc 1/4] glibc: Perform rseq(2) registration at nptl init and thread creation (v4)

Mathieu Desnoyers
2019-01-14T19:22:42Z

----- On Jan 14, 2019, at 10:55 AM, Florian Weimer fweimer@redhat.com wrote:

> * Mathieu Desnoyers:
>
>> Therefore, both symbols will end up in
>> sysdeps/unix/sysv/linux/Versions.
>
> I'm not sure what you mean by that. The physical location in the
> directory tree has little effect on which shared object the symbol is
> placed in; that will need other changes.

I'm currently moving the symbol definitions to csu/rseq-sym.c. On Linux,
its content is overridden by a new sysdeps/unix/sysv/linux/rseq-sym.c
which contains both __rseq_abi and __rseq_refcount symbols. On other
platforms, it is a stub file.

>>> By the way, you could avoid the need for unregistration if you allocated
>>> the rseq areas persistently, index by TID. They are quite small, so
>>> with the typical PID range, maybe the wasted memory due to changing TIDs
>>> would be acceptable?
>>
>> Would we be able to access those __rseq_abi as normal TLS IE model
>> variables ? The overhead of indexing an array matters for a
>> fast-path.
>
> No, that wouldn't be possible in this case. You would need another
> indirection.

Thanks for the clarification!

Mathieu

--
Mathieu Desnoyers
EfficiOS Inc.