linux-kernel - Re: BUG: rseq selftests and librseq vs. glibc fail

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Message-ID: <87349oxk2n.ffs@tglx>
Date: Tue, 19 Aug 2025 01:54:08 +0200
From: Thomas Gleixner <tglx@...utronix.de>
To: Sean Christopherson <seanjc@...gle.com>, Florian Weimer
 <fweimer@...hat.com>
Cc: Jens Axboe <axboe@...nel.dk>, LKML <linux-kernel@...r.kernel.org>,
 Michael Jeanson <mjeanson@...icios.com>, Mathieu Desnoyers
 <mathieu.desnoyers@...icios.com>, Peter Zijlstra <peterz@...radead.org>,
 "Paul E. McKenney" <paulmck@...nel.org>, Boqun Feng
 <boqun.feng@...il.com>, Wei Liu <wei.liu@...nel.org>, Samuel Thibault
 <sthibault@...ian.org>
Subject: Re: BUG: rseq selftests and librseq vs. glibc fail

On Mon, Aug 18 2025 at 13:27, Sean Christopherson wrote:
> On Mon, Aug 18, 2025, Florian Weimer wrote:
>> You need both (extern and weak) to get a weak symbol reference instead
>> of a weak symbol definition.  You still need to check &__rseq_offset, of
>> course.
>
> Ooh, you're saying add "extern" to the existing __weak symbol, not replace it.
> Huh, TIL weak symbol references are a thing.
>
> This works with static and dynamic linking, with and without an rseq-aware glibc.
>
> Thomas, does this fix the problem you were seeing?
>
> diff --git a/tools/testing/selftests/rseq/rseq.c b/tools/testing/selftests/rseq/rseq.c
> index 663a9cef1952..d17ded120d48 100644
> --- a/tools/testing/selftests/rseq/rseq.c
> +++ b/tools/testing/selftests/rseq/rseq.c
> @@ -40,9 +40,9 @@
>   * Define weak versions to play nice with binaries that are statically linked
>   * against a libc that doesn't support registering its own rseq.
>   */
> -__weak ptrdiff_t __rseq_offset;
> -__weak unsigned int __rseq_size;
> -__weak unsigned int __rseq_flags;
> +extern __weak ptrdiff_t __rseq_offset;
> +extern __weak unsigned int __rseq_size;
> +extern __weak unsigned int __rseq_flags;
>  
>  static const ptrdiff_t *libc_rseq_offset_p = &__rseq_offset;
>  static const unsigned int *libc_rseq_size_p = &__rseq_size;
> @@ -209,7 +209,7 @@ void rseq_init(void)
>          * libc not having registered a restartable sequence.  Try to find the
>          * symbols if that's the case.
>          */
> -       if (!*libc_rseq_size_p) {
> +       if (!libc_rseq_offset_p || !*libc_rseq_size_p) {

If I make that:

+       if (!*libc_rseq_offset_p || !*libc_rseq_size_p) {

then it makes sense and actually works. The pointer can hardly be NULL,
even when statically linked, no?

Thanks,

        tglx