commit 350db2839387659e1500a54d276e401c9c6b2dee
Author: Michael Jeanson <mjeanson@efficios.com>
Date:   Wed Nov 20 14:15:42 2024 -0500

    nptl: initialize cpu_id_start prior to rseq registration
    
    When adding explicit initialization of rseq fields prior to
    registration, I glossed over the fact that 'cpu_id_start' is also
    documented as initialized by user-space.
    
    While current kernels don't validate the content of this field on
    registration, future ones could.
    
    Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
    Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
    (cherry picked from commit d9f40387d3305d97e30a8cf8724218c42a63680a)

diff --git a/sysdeps/unix/sysv/linux/rseq-internal.h b/sysdeps/unix/sysv/linux/rseq-internal.h
index 37a8f630b6519ff0..ef3eab1fefd4d90d 100644
--- a/sysdeps/unix/sysv/linux/rseq-internal.h
+++ b/sysdeps/unix/sysv/linux/rseq-internal.h
@@ -56,6 +56,7 @@ rseq_register_current_thread (struct pthread *self, bool do_rseq)
          registration, there is no guarantee that struct pthread is
          cleared on all architectures.  */
       THREAD_SETMEM (self, rseq_area.cpu_id, RSEQ_CPU_ID_UNINITIALIZED);
+      THREAD_SETMEM (self, rseq_area.cpu_id_start, 0);
       THREAD_SETMEM (self, rseq_area.rseq_cs, 0);
       THREAD_SETMEM (self, rseq_area.flags, 0);