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

[Gavin Shan]

On 8/9/22 10:57 AM, Mathieu Desnoyers wrote:
> ----- Gavin Shan <gshan@xxxxxxxxxx> 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@xxxxxxxxxxxx>
> > > 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@xxxxxxxxxxxx>
> > >       Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> > >       Link: https://lkml.kernel.org/r/20220124171253.22072-8-mathieu.desnoyers@xxxxxxxxxxxx
> > >
> > > 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