[PATCH] nptl: Unconditionally use a 32-byte rseq area

[Florian Weimer <fweimer@redhat.com>]

If the kernel headers provide a larger struct rseq, we used that
size as the argument to the rseq system call.  As a result,
rseq registration would fail on older kernels which only accept
size 32.

Tested on x86_64-linux-gnu.  Built with build-many-glibcs.py.
This needs to be backported all the way to glibc 2.35.

---
 nptl/descr.h | 15 +++++++++++++--
 1 file changed, 13 insertions(+), 2 deletions(-)

diff --git a/nptl/descr.h b/nptl/descr.h
index d06abd6ad9..abc9282ef7 100644
--- a/nptl/descr.h
+++ b/nptl/descr.h
@@ -405,8 +405,19 @@ struct pthread
   /* Used on strsignal.  */
   struct tls_internal_t tls_state;
 
-  /* rseq area registered with the kernel.  */
-  struct rseq rseq_area;
+  /* rseq area registered with the kernel.  Use a custom definition
+     here to isolate from kernel struct rseq changes.  The
+     implementation of sched_getcpu needs acccess to the cpu_id field;
+     the other fields are unused and not included here.  */
+  union
+  {
+    struct
+    {
+      uint32_t cpu_id_start;
+      uint32_t cpu_id;
+    };
+    char pad[32];		/* Original rseq area size.  */
+  } rseq_area __attribute__ ((aligned (32)));
 
   /* Amount of end padding, if any, in this structure.
      This definition relies on rseq_area being last.  */

base-commit: 3edc4ff2ceff4a59587ebecb94148d3bcfa1df62