[PATCH v2] nptl: clear the whole rseq area before registration

[Michael Jeanson]

On 2025-02-14 13:54, Michael Jeanson wrote:
> 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>

Florian are you ok with me pushing this? I would also backport it to the
relevant stable branches.

Thanks,

Michael

> ---
> Changes since v1:
> * Add include of dl-symbol-redir-ifunc.h to dl-tls_init_tp.c to use
>   memset() prior to the full initialization of the TCB.
> ---
>  sysdeps/nptl/dl-tls_init_tp.c           |  1 +
>  sysdeps/unix/sysv/linux/rseq-internal.h | 11 +++++------
>  2 files changed, 6 insertions(+), 6 deletions(-)
> 
> diff --git a/sysdeps/nptl/dl-tls_init_tp.c b/sysdeps/nptl/dl-tls_init_tp.c
> index db99082b29..47566dce4f 100644
> --- a/sysdeps/nptl/dl-tls_init_tp.c
> +++ b/sysdeps/nptl/dl-tls_init_tp.c
> @@ -23,6 +23,7 @@
>  #include <tls.h>
>  #include <rseq-internal.h>
>  #include <thread_pointer.h>
> +#include <dl-symbol-redir-ifunc.h>
>  
>  #define TUNABLE_NAMESPACE pthread
>  #include <dl-tunables.h>
> 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))