[PATCH] nptl: clear the whole rseq area before registration

[Florian Weimer]

* Michael Jeanson:

> Due to the extensible nature of the rseq area we can't explictly
> initialize fields that are not part of the ABI yet. It was agreed with
> upstream that all new fields will be documented as zero initialized by
> userspace. Future kernels configured with CONFIG_DEBUG_RSEQ will
> validate the content of all fields during registration.
>
> Replace the explicit field initialization with a memset of the whole
> rseq area which will cover fields as they are added to future kernels.
>
> Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
> ---
>  sysdeps/unix/sysv/linux/rseq-internal.h | 11 +++++------
>  1 file changed, 5 insertions(+), 6 deletions(-)
>
> diff --git a/sysdeps/unix/sysv/linux/rseq-internal.h b/sysdeps/unix/sysv/linux/rseq-internal.h
> index f89e784243..d2ab4cb829 100644
> --- a/sysdeps/unix/sysv/linux/rseq-internal.h
> +++ b/sysdeps/unix/sysv/linux/rseq-internal.h
> @@ -108,13 +108,12 @@ rseq_register_current_thread (struct pthread *self, bool do_rseq)
>        if (size < RSEQ_AREA_SIZE_INITIAL)
>          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.  */
> +      /* Initialize the whole rseq area to zero prior to registration.  */
> +      memset (RSEQ_SELF (), 0, size);
> +
> +      /* Set the cpu_id field to RSEQ_CPU_ID_UNINITIALIZED, this is checked by
> +         the kernel at registration when CONFIG_DEBUG_RSEQ is enabled.  */
>        RSEQ_SETMEM (cpu_id, RSEQ_CPU_ID_UNINITIALIZED);
> -      RSEQ_SETMEM (cpu_id_start, 0);
> -      RSEQ_SETMEM (rseq_cs, 0);
> -      RSEQ_SETMEM (flags, 0);
>  
>        int ret = INTERNAL_SYSCALL_CALL (rseq, RSEQ_SELF (), size, 0, RSEQ_SIG);
>        if (!INTERNAL_SYSCALL_ERROR_P (ret))

The change looks fine to me in principle, just one observation:

I think it might be safer to add #include <dl-symbol-redir-ifunc.h>
to sysdeps/nptl/dl-tls_init_tp.c (where rseq_register_current_thread is
called on the main thread) because memset might need a more initialzed
TCB than what we have at this point.

Thanks,
Florian