linux-kernel - Re: [PATCH] selftests/rseq: Fix handling of glibc without rseq support

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Message-ID: <0e4bfd16-76da-430d-a7a4-f1d31448ea43@linuxfoundation.org>
Date: Wed, 15 Jan 2025 10:57:53 -0700
From: Shuah Khan <skhan@...uxfoundation.org>
To: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc: linux-kernel@...r.kernel.org, Raghavendra Rao Ananta
 <rananta@...gle.com>, Peter Zijlstra <peterz@...radead.org>,
 Boqun Feng <boqun.feng@...il.com>, "Paul E. McKenney" <paulmck@...nel.org>,
 Carlos O'Donell <carlos@...hat.com>, Florian Weimer <fweimer@...hat.com>,
 Michael Jeanson <mjeanson@...icios.com>, linux-kselftest@...r.kernel.org,
 stable@...r.kernel.org, Shuah Khan <skhan@...uxfoundation.org>
Subject: Re: [PATCH] selftests/rseq: Fix handling of glibc without rseq
 support

On 1/14/25 17:45, Mathieu Desnoyers wrote:
> On 2025-01-14 19:14, Shuah Khan wrote:
>> On 1/14/25 07:51, Mathieu Desnoyers wrote:
>>> When porting librseq commit:
>>>
>>> commit c7b45750fa85 ("Adapt to glibc __rseq_size feature detection")
>>>
>>> from librseq to the kernel selftests, the following line was missed
>>> at the end of rseq_init():
>>>
>>>    rseq_size = get_rseq_kernel_feature_size();
>>>
>>> which effectively leaves rseq_size initialized to -1U when glibc does not
>>> have rseq support. glibc supports rseq from version 2.35 onwards.
>>>
>>> In a following librseq commit
>>>
>>> commit c67d198627c2 ("Only set 'rseq_size' on first thread registration")
>>>
>>> to mimic the libc behavior, a new approach is taken: don't set the
>>> feature size in 'rseq_size' until at least one thread has successfully
>>> registered. This allows using 'rseq_size' in fast-paths to test for both
>>> registration status and available features. The caveat is that on libc
>>> either all threads are registered or none are, while with bare librseq
>>> it is the responsability of the user to register all threads using rseq.
>>>
>>> This combines the changes from the following librseq commits:
>>>
>>> commit c7b45750fa85 ("Adapt to glibc __rseq_size feature detection")
>>> commit c67d198627c2 ("Only set 'rseq_size' on first thread registration")
>>>
>>> Fixes: 73a4f5a704a2 ("selftests/rseq: Fix mm_cid test failure")

Fixed this commit id
commit c7b45750fa85 ("Adapt to glibc __rseq_size feature detection")

>>> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
>>
>> Hi Mathieu,
>>
>> Can you double check these commits and make sure these are right
>> ones in the mainline rc7?
>>
>> I am seeing "Unknown commit id" warnings on all of these - my
>> repo is at 6.13 rc7
> 
> This is because those are commits in the librseq project at
> https://git.kernel.org/pub/scm/libs/librseq/librseq.git/
> which is a different tree from the Linux kernel. I am not
> sure what is the preferred approach when citing a
> commit ID from a different project ?
> 
> I've been keeping both rseq selftests and librseq in
> sync since 2018. I plan to eventually add a dependency
> of the rseq selftests on librseq, but this cannot
> happen until we freeze the API and cut a librseq
> release.
> 
> This was premature before we reached the major milestone
> of having extensible rseq support in glibc.
> 
> Now that it's merged into glibc (as of last week),
> we can start looking forward to a librseq release,
> which should eventually eliminate code duplication
> with rseq selftests.
> 
> Perhaps we should add a Link: to the librseq
> repository ?
> 
>>
>> Also would you like to add Reported-by for Raghavendra Rao Ananta?

Added. The patch is now in linux-kselftest next

thanks,
-- Shuah