Re: [PATCH v12 01/17] nptl: Add rseq auxvals

[Florian Weimer <fweimer@redhat.com>]

* Michael Jeanson:

> On 2024-08-06 09:32, Michael Jeanson wrote:
>> Before the removal of _dl_rseq_feature_size there was no behavior
>> change introduced until "Move the rseq area to the 'extra TLS'
>> block" which is where _rseq_size is set to 0 on failure. I agree it
>> would make more sense to have it in this patch, I'll queue this
>> change for the next round.
>
> I was wondering if this patch with the code to set _rseq_size to zero
> on registration failure, plus an additional modification to cap the
> _rseq_size to 32 (the size of the rseq area in the struct pthread)
> could be backported to 2.40 and earlier? It would give application
> code access to the 'node_id' and 'mm_cid' fields without introducing
> the whole 'extra tls' stuff.
>
> Thoughts?

I thought about this some more.  I think we should also increase the
reservation inside struct pthread beyond 32 bytes if we do this, so that
we avoid the __rseq_size == 32 ambiguity.  It currently doesn't break
the GLIBC_PRIVATE ABI because rseq_area is at the end of struct pthread.
(As long as we assume that ld.so gets updated before libc.so because it
comes first in lexicographic order.)

Thanks,
Florian