linux-kernel - [PATCH] rseq/selftests: Use weak symbol reference, not definition, to link with glibc

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Message-ID: <20250819222945.3052711-1-seanjc@google.com>
Date: Tue, 19 Aug 2025 15:29:44 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, Peter Zijlstra <peterz@...radead.org>, 
	"Paul E. McKenney" <paulmck@...nel.org>, Boqun Feng <boqun.feng@...il.com>, Shuah Khan <shuah@...nel.org>
Cc: linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org, 
	Florian Weimer <fweimer@...hat.com>, Thomas Gleixner <tglx@...utronix.de>, 
	Sean Christopherson <seanjc@...gle.com>
Subject: [PATCH] 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+")
Cc: stable@...r.kernel.org
Suggested-by: Florian Weimer <fweimer@...hat.com>
Reported-by: Thomas Gleixner <tglx@...utronix.de>
Closes: https://lore.kernel.org/all/87frdoybk4.ffs@tglx
Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 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 663a9cef1952..dcac5cbe7933 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");

base-commit: c17b750b3ad9f45f2b6f7e6f7f4679844244f0b9
-- 
2.51.0.rc1.167.g924127e9c0-goog