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

[Sean Christopherson <seanjc@google.com>]

On Mon, Aug 18, 2025, Florian Weimer wrote:
> * 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.

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) {
                libc_rseq_offset_p = dlsym(RTLD_NEXT, "__rseq_offset");
                libc_rseq_size_p = dlsym(RTLD_NEXT, "__rseq_size");
                libc_rseq_flags_p = dlsym(RTLD_NEXT, "__rseq_flags");

> >>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.

I vaguely recall exploring that option when trying to get static linking working.
I think I shied away from it because I wanted to have something that Just Worked
for all users, and didn't like the idea of hardcoding that into the KVM selftests
makefile.