----- On Feb 2, 2022, at 3:41 AM, Florian Weimer [email protected] wrote:
> * Florian Weimer:
>
>> * Chris Kennelly:
>>
>>> Thanks for the heads up.
>>>
>>> I did have a question about whether the new protocol would introduce
>>> an extra memory reference while initializing a critical section.
>>>
>>> * With initial-exec TLS, I can directly reference __rseq_abi.
>>> * With the new ABI, I might need to ask glibc for the address of the
>>> registered rseq structure in its thread data.
>>
>> You can write __rseq_offset to a static/hidden variable in an ELF
>> constructor, and then use pretty much the same assembler sequences as
>> for initial-exec TLS on most architectures.
>
> And now I'm kind of worried that we should be using ptrdiff_t for
> __rseq_offset because that's what the initial-exec relocations use. 8-/
I suspect the underlying question here is: how likely is it that a libc
requires an offset of more than 2GB either way from the thread pointer
to allocate its rseq thread area on a 64-bit architecture ?
Thanks,
Mathieu
--
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com
----- On Feb 2, 2022, at 6:36 AM, Mathieu Desnoyers [email protected] wrote:
> ----- On Feb 2, 2022, at 3:41 AM, Florian Weimer [email protected] wrote:
>
>> * Florian Weimer:
>>
>>> * Chris Kennelly:
>>>
>>>> Thanks for the heads up.
>>>>
>>>> I did have a question about whether the new protocol would introduce
>>>> an extra memory reference while initializing a critical section.
>>>>
>>>> * With initial-exec TLS, I can directly reference __rseq_abi.
>>>> * With the new ABI, I might need to ask glibc for the address of the
>>>> registered rseq structure in its thread data.
>>>
>>> You can write __rseq_offset to a static/hidden variable in an ELF
>>> constructor, and then use pretty much the same assembler sequences as
>>> for initial-exec TLS on most architectures.
>>
>> And now I'm kind of worried that we should be using ptrdiff_t for
>> __rseq_offset because that's what the initial-exec relocations use. 8-/
>
> I suspect the underlying question here is: how likely is it that a libc
> requires an offset of more than 2GB either way from the thread pointer
> to allocate its rseq thread area on a 64-bit architecture ?
More to the point: is ptrdiff_t the correct type here ? I think so.
Do we want to revert the ABI and wait another 6 months before we
bring back rseq into glibc just for this ? I'm not sure this limitation
justifies it.
So if there is a quick way to fix that before the official 2.35 release,
I'm all for it, otherwise I cannot say that __rseq_offset being an "int"
rather than a "ptrdiff_t" will make much real-life difference (unless
I'm proven wrong). But we will be stuck with this quirk forever.
Thanks,
Mathieu
--
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com