commit 9a0e174a39a3a65f628c6a55e29fe35f6d67bf42
Author: Michael Jeanson <mjeanson@efficios.com>
Date:   Thu Nov 7 22:23:49 2024 +0100

    nptl: initialize rseq area prior to registration
    
    Per the rseq syscall documentation, 3 fields are required to be
    initialized by userspace prior to registration, they are 'cpu_id',
    'rseq_cs' and 'flags'. Since we have no guarantee that 'struct pthread'
    is cleared on all architectures, explicitly set those 3 fields prior to
    registration.
    
    Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
    Reviewed-by: Florian Weimer <fweimer@redhat.com>
    (cherry picked from commit 97f60abd25628425971f07e9b0e7f8eec0741235)

diff --git a/nptl/descr.h b/nptl/descr.h
index 4697f633e16c7359..a83df327e4bcba2e 100644
--- a/nptl/descr.h
+++ b/nptl/descr.h
@@ -417,6 +417,8 @@ struct pthread
     {
       uint32_t cpu_id_start;
       uint32_t cpu_id;
+      uint64_t rseq_cs;
+      uint32_t flags;
     };
     char pad[32];		/* Original rseq area size.  */
   } rseq_area __attribute__ ((aligned (32)));
diff --git a/sysdeps/unix/sysv/linux/rseq-internal.h b/sysdeps/unix/sysv/linux/rseq-internal.h
index 7ea935b4adab8c20..37a8f630b6519ff0 100644
--- a/sysdeps/unix/sysv/linux/rseq-internal.h
+++ b/sysdeps/unix/sysv/linux/rseq-internal.h
@@ -51,11 +51,21 @@ rseq_register_current_thread (struct pthread *self, bool do_rseq)
         /* The initial implementation used only 20 bytes out of 32,
            but still expected size 32.  */
         size = RSEQ_AREA_SIZE_INITIAL;
+
+      /* Initialize the rseq fields that are read by the kernel on
+         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.rseq_cs, 0);
+      THREAD_SETMEM (self, rseq_area.flags, 0);
+
       int ret = INTERNAL_SYSCALL_CALL (rseq, &self->rseq_area,
                                        size, 0, RSEQ_SIG);
       if (!INTERNAL_SYSCALL_ERROR_P (ret))
         return true;
     }
+  /* When rseq is disabled by tunables or the registration fails, inform
+     userspace by setting 'cpu_id' to RSEQ_CPU_ID_REGISTRATION_FAILED.  */
   THREAD_SETMEM (self, rseq_area.cpu_id, RSEQ_CPU_ID_REGISTRATION_FAILED);
   return false;
 }