Re: tools/testing/selftests/kvm/rseq_test and glibc 2.35

Gavin Shan
2022-08-09T01:58:30Z

On 8/9/22 10:57 AM, Mathieu Desnoyers wrote:
>
> ----- Gavin Shan <gshan@redhat.com> wrote:
>> Hi Florian,
>>
>> On 8/9/22 2:01 AM, Florian Weimer wrote:
>>> It has come to my attention that the KVM rseq test apparently needs to
>>> be ported to glibc 2.35. The background is that on aarch64, rseq is the
>>> only way to get a practically useful sched_getcpu. (There's no hidden
>>> per-task CPU state the vDSO could reveal as the CPU ID.)
>>>
>>
>> Yes, kvm/selftests/rseq needs to support glibc 2.35. The question is
>> about glibc 2.34 or 2.35 because kvm/selftest/rseq fails on glibc 2.34
>>
>> I would guess upstream-glibc-2.35 feature is enabled on downstream
>> glibc-2.34?
>>
>> # ./rseq_test
>> ==== Test Assertion Failure ====
>> rseq_test.c:60: !r
>> pid=112043 tid=112043 errno=22 - Invalid argument
>> 1 0x0000000000401973: main at rseq_test.c:226
>> 2 0x0000ffff84b6c79b: ?? ??:0
>> 3 0x0000ffff84b6c86b: ?? ??:0
>> 4 0x0000000000401b6f: _start at ??:?
>> rseq failed, errno = 22 (Invalid argument)
>> # rpm -aq | grep glibc-2
>> glibc-2.34-39.el9.aarch64
>>
>>
>>> The main rseq tests have already been adjusted via:
>>>
>>> commit 233e667e1ae3e348686bd9dd0172e62a09d852e1
>>> Author: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
>>> Date: Mon Jan 24 12:12:45 2022 -0500
>>>
>>> selftests/rseq: Uplift rseq selftests for compatibility with glibc-2.35
>>>
>>> glibc-2.35 (upcoming release date 2022-02-01) exposes the rseq per-thread
>>> data in the TCB, accessible at an offset from the thread pointer, rather
>>> than through an actual Thread-Local Storage (TLS) variable, as the
>>> Linux kernel selftests initially expected.
>>>
>>> The __rseq_abi TLS and glibc-2.35's ABI for per-thread data cannot
>>> actively coexist in a process, because the kernel supports only a single
>>> rseq registration per thread.
>>>
>>> Here is the scheme introduced to ensure selftests can work both with an
>>> older glibc and with glibc-2.35+:
>>>
>>> - librseq exposes its own "rseq_offset, rseq_size, rseq_flags" ABI.
>>>
>>> - librseq queries for glibc rseq ABI (__rseq_offset, __rseq_size,
>>> __rseq_flags) using dlsym() in a librseq library constructor. If those
>>> are found, copy their values into rseq_offset, rseq_size, and
>>> rseq_flags.
>>>
>>> - Else, if those glibc symbols are not found, handle rseq registration
>>> from librseq and use its own IE-model TLS to implement the rseq ABI
>>> per-thread storage.
>>>
>>> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
>>> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
>>> Link: https://lkml.kernel.org/r/20220124171253.22072-8-mathieu.desnoyers@efficios.com
>>>
>>> But I don't see a similar adjustment for
>>> tools/testing/selftests/kvm/rseq_test.c. As an additional wrinkle,
>>> you'd have to start calling getcpu (glibc function or system call)
>>> because comparing rseq.cpu_id against sched_getcpu won't test anything
>>> anymore once glibc implements sched_getcpu using rseq.
>>>
>>> We noticed this because our downstream glibc version, while based on
>>> 2.34, enables rseq registration by default. To facilitate coordination
>>> with rseq application usage, we also backported the __rseq_* ABI
>>> symbols, so the selftests could use that even in our downstream version.
>>> (We enable the glibc tunables downstream, but they are an optional
>>> glibc feature, so it's probably better in the long run to fix the kernel
>>> selftests rather than using the tunables as a workaround.)
>>>
>>
>> Thanks for the pointer. It makes sense. So it means rseq registration has
>> been done by glibc TLS? In this case, kvm/selftests/rseq is unable to
>> register again.
>
> The registration is done by glibc initialization and thread startup code.
>
>>
>> I will come up something similiar for kvm/selftest/rseq.
>
> Make sure to chech the rseq selftests fixes recently pulled in the current merge window as well. One is relevant:
>
> https://github.com/torvalds/linux/commit/d1a997ba4c1bf65497d956aea90de42a6398f73a
>
> We may want to find a way to remove this duplicated rseq.c code eventually.
>

Thanks, Mathieu. The check for 'rseq-size' will be included either. I almost
have something working. I will post the fixes after some tests.

Thanks,
Gavin