[RFC PATCH glibc] Linux: Use fixed rseq_len value for rseq registration

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

The rseq registration system call expects a fixed-size argument of 32
for the rseq_len.  We are currently discussing schemes to extend
struct rseq beyond that size, and those involve using fields within
struct rseq, without any changes to the rseq_len argument.

Building a glibc with an updated, larger, struct rseq in the Linux
kernel UAPI headers should not break registration.  Therefore, use
a fixed-size of 32 as rseq_len parameter.

See struct rseq extension discussion:

https://lore.kernel.org/lkml/1305865358.10354.1594665620975.JavaMail.zimbra@efficios.com/
---
 sysdeps/unix/sysv/linux/rseq-internal.h | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/sysdeps/unix/sysv/linux/rseq-internal.h b/sysdeps/unix/sysv/linux/rseq-internal.h
index 8f6772ca1d..3522668f3a 100644
--- a/sysdeps/unix/sysv/linux/rseq-internal.h
+++ b/sysdeps/unix/sysv/linux/rseq-internal.h
@@ -33,8 +33,9 @@ rseq_register_current_thread (void)
   if (__rseq_abi.cpu_id != RSEQ_CPU_ID_UNINITIALIZED)
     __libc_fatal ("glibc fatal error: "
                   "rseq already initialized for this thread\n");
-  ret = INTERNAL_SYSCALL_CALL (rseq, &__rseq_abi, sizeof (struct rseq),
-                               0, RSEQ_SIG);
+  /* The rseq_len parameter does not allow extending struct rseq.  Fix its
+     value to 32 as expected by the Linux kernel.  */
+  ret = INTERNAL_SYSCALL_CALL (rseq, &__rseq_abi, 32, 0, RSEQ_SIG);
   if (INTERNAL_SYSCALL_ERROR_P (ret))
     {
       const char *msg = NULL;
-- 
2.17.1