Re: BUG: rseq selftests and librseq vs. glibc fail

[Florian Weimer <fweimer@redhat.com>]

* Sean Christopherson:

> On Mon, Aug 18, 2025, Florian Weimer wrote:
>> You could add the extern keyword and check &__rseq_offset for NULL if
>> you want to probe for the availability of the signal?
>
> That will fail to link if the glibc version doesn't support rseq in
> any capacity, which is why I added the __weak crud.

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.

>>Or use:
>> 
>> #if __has_include(<sys/rseq.h>)
>> #include <sys/rseq.h>
>> /* Code that depends on glibc's rseq support goes here. */
>
> FWIW, the code in question doesn't depend on rseq per se, rather the problem is
> that attempting to register a restartable sequence fails if glibc has already
> "claimed" rseq.

You can set GLIBC_TUNABLES=glibc.pthread.rseq=0 as an environment
variable to prevent glibc from registering it.  For a test that's
probably okay?  It won't help with other libcs that might use rseq
eventually.

> What about something horrific like this?  Or if __has_include(<sys/rseq.h>) is
> preferrable to checking the glibc version, go with that.  The idea with checking
> LIBC_RSEQ_STATIC_LINK is to give folks a way to force static linking if their
> libc registers rseq, but doesn't satisfy the glibc version checks for whatever
> reason.
>
> diff --git a/tools/testing/selftests/rseq/rseq.c b/tools/testing/selftests/rseq/rseq.c
> index 663a9cef1952..1a88352fcff3 100644
> --- a/tools/testing/selftests/rseq/rseq.c
> +++ b/tools/testing/selftests/rseq/rseq.c
> @@ -36,17 +36,18 @@
>  #include "../kselftest.h"
>  #include "rseq.h"
>  
> -/*
> - * 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;
> +#if defined(LIBC_RSEQ_STATIC_LINK) || __GLIBC__ > 2 || (__GLIBC__ == 2 && __GLIBC_MINOR__ >= 35)
> +extern ptrdiff_t __rseq_offset;
> +extern unsigned int __rseq_size;
> +extern unsigned int __rseq_flags;
> +#define GLIBC_RSEQ_PTR(x) &x
> +#else
> +#define GLIBC_RSEQ_PTR(x) NULL
> +#endif

That doesn't work for a couple of distributions nowadays that are
nominally based on glibc 2.34.  The AArch64 performance improvement for
sched_getcpu was just too sweet to give up, so these distributions have
__rseq_offset@@GLIBC_2.35, too.  We added these symbols specifically so
that applications that need to interact with rseq can do so safely, even
though glibc claimed it for the sched_getcpu acceleration.

Thanks,
Florian