The following commit has been merged into the core/rseq branch of tip:

Commit-ID: a001cd248ab244633c5fabe4f7c707e13fc1d1cc Gitweb: https://git.kernel.org/tip/a001cd248ab244633c5fabe4f7c707e13fc1d1cc Author: Sean Christopherson seanjc@google.com AuthorDate: Tue, 19 Aug 2025 15:29:44 -07:00 Committer: Thomas Gleixner tglx@linutronix.de CommitterDate: Sat, 13 Sep 2025 19:51:59 +02:00

rseq/selftests: Use weak symbol reference, not definition, to link with glibc

Add "extern" to the glibc-defined weak rseq symbols to convert the rseq selftest's usage from weak symbol definitions to weak symbol _references_. Effectively re-defining the glibc symbols wreaks havoc when building with -fno-common, e.g. generates segfaults when running multi-threaded programs, as dynamically linked applications end up with multiple versions of the symbols.

Building with -fcommon, which until recently has the been the default for GCC and clang, papers over the bug by allowing the linker to resolve the weak/tentative definition to glibc's "real" definition.

Note, the symbol itself (or rather its address), not the value of the symbol, is set to 0/NULL for unresolved weak symbol references, as the symbol doesn't exist and thus can't have a value. Check for a NULL rseq size pointer to handle the scenario where the test is statically linked against a libc that doesn't support rseq in any capacity.

Fixes: 3bcbc20942db ("selftests/rseq: Play nice with binaries statically linked against glibc 2.35+") Reported-by: Thomas Gleixner tglx@linutronix.de Suggested-by: Florian Weimer fweimer@redhat.com Signed-off-by: Sean Christopherson seanjc@google.com Signed-off-by: Thomas Gleixner tglx@linutronix.de Reviewed-by: Mathieu Desnoyers mathieu.desnoyers@efficios.com Cc: stable@vger.kernel.org Closes: https://lore.kernel.org/all/87frdoybk4.ffs@tglx --- tools/testing/selftests/rseq/rseq.c | 8 ++++---- 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/tools/testing/selftests/rseq/rseq.c b/tools/testing/selftests/rseq/rseq.c index 663a9ce..dcac5cb 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_size_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");