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: stable@vger.kernel.org Suggested-by: Florian Weimer fweimer@redhat.com Reported-by: Thomas Gleixner tglx@linutronix.de Closes: https://lore.kernel.org/all/87frdoybk4.ffs@tglx Signed-off-by: Sean Christopherson seanjc@google.com
+ CC: Michael Jeanson mjeanson@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.desnoyers@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