On 2025-08-19 18:29, Sean Christopherson wrote:
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: sta...@vger.kernel.org
Suggested-by: Florian Weimer <fwei...@redhat.com>
Reported-by: Thomas Gleixner <t...@linutronix.de>
Closes: https://lore.kernel.org/all/87frdoybk4.ffs@tglx
Signed-off-by: Sean Christopherson <sea...@google.com>
+ CC: Michael Jeanson <mjean...@efficios.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.
We should therefore update the comment above to e.g.:
/*
* Define weak symbol references to play nice with binaries that are
* statically linked against a libc that doesn't support registering its
* own rseq.
*/
For the rest:
Reviewed-by: Mathieu Desnoyers <mathieu.desnoy...@efficios.com>
Michael, can you try it out ?
Kienan, we may want to add a configuration forcing "-fno-common" to our
CI, this problematic pattern may be hiding other issues elsewhere. I'm
thinking of LTTng-UST tracepoint headers and libside headers.
Sean, do you want to contribute the fix to librseq as well ?
Thanks everyone for looking into this. I'll be back from vacation next
week and will resume normal operations. :-)
Mathieu
*/
-__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
--
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com
