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

[Mathieu Desnoyers]

----- 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

> 
> Thanks,
> Gavin
> 

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com