Re: [patch 03/10] rseq: Dont advertise time slice extensions if disabled

[Dmitry Vyukov]

On Wed, 29 Apr 2026 at 01:33, Thomas Gleixner <tglx@xxxxxxxxxx> wrote:
>
> If time slice extensions have been disabled on the kernel command line,
> then advertising them in RSEQ flags is wrong.
>
> Adjust the conditionals to reflect reality, fixup the misleading comments
> about the gap of these flags and the rseq::flags field.
>
> Fixes: d6200245c75e ("rseq: Allow registering RSEQ with slice extension")
> Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx

Reviewed-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>

> ---
>  include/uapi/linux/rseq.h |    5 ++++-
>  kernel/rseq.c             |    9 +++++----
>  2 files changed, 9 insertions(+), 5 deletions(-)
>
> --- a/include/uapi/linux/rseq.h
> +++ b/include/uapi/linux/rseq.h
> @@ -28,7 +28,7 @@ enum rseq_cs_flags_bit {
>         RSEQ_CS_FLAG_NO_RESTART_ON_PREEMPT_BIT  = 0,
>         RSEQ_CS_FLAG_NO_RESTART_ON_SIGNAL_BIT   = 1,
>         RSEQ_CS_FLAG_NO_RESTART_ON_MIGRATE_BIT  = 2,
> -       /* (3) Intentional gap to put new bits into a separate byte */
> +       /* (3) Intentional gap to keep new bits separate */
>
>         /* User read only feature flags */
>         RSEQ_CS_FLAG_SLICE_EXT_AVAILABLE_BIT    = 4,
> @@ -161,6 +161,9 @@ struct rseq {
>          *      - RSEQ_CS_FLAG_NO_RESTART_ON_PREEMPT
>          *      - RSEQ_CS_FLAG_NO_RESTART_ON_SIGNAL
>          *      - RSEQ_CS_FLAG_NO_RESTART_ON_MIGRATE
> +        *
> +        * It is now used for feature status advertisement by the kernel.
> +        * See: enum rseq_cs_flags_bit for further information.
>          */
>         __u32 flags;
>
> --- a/kernel/rseq.c
> +++ b/kernel/rseq.c
> @@ -462,10 +462,11 @@ SYSCALL_DEFINE4(rseq, struct rseq __user
>                 return -EFAULT;
>
>         if (IS_ENABLED(CONFIG_RSEQ_SLICE_EXTENSION)) {
> -               rseqfl |= RSEQ_CS_FLAG_SLICE_EXT_AVAILABLE;
> -               if (rseq_slice_extension_enabled() &&
> -                   (flags & RSEQ_FLAG_SLICE_EXT_DEFAULT_ON))
> -                       rseqfl |= RSEQ_CS_FLAG_SLICE_EXT_ENABLED;
> +               if (rseq_slice_extension_enabled()) {
> +                       rseqfl |= RSEQ_CS_FLAG_SLICE_EXT_AVAILABLE;
> +                       if (flags & RSEQ_FLAG_SLICE_EXT_DEFAULT_ON)
> +                               rseqfl |= RSEQ_CS_FLAG_SLICE_EXT_ENABLED;
> +               }
>         }
>
>         scoped_user_write_access(rseq, efault) {
>