[tip: core/rseq] rseq/selftests: Use weak symbol reference, not definition, to link with glibc

["tip-bot2 for Sean Christopherson" <tip-bot2@linutronix.de>]

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");