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

Gavin Shan
2022-08-08T23:47:18Z

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.

I will come up something similiar for kvm/selftest/rseq.

Thanks,
Gavin