Subject: Re: [PATCH v5 10/10] powerpc/signal64: Use __get_user() to copy sigset_t

From: Daniel Axtens <[email-masked]>
Date: Sat, 13 Feb 2021 08:21:16 +1100

"Christopher M. Riedl" <[email-masked]> writes:

> Usually sigset_t is exactly 8B which is a "trivial" size and does not
> warrant using __copy_from_user(). Use __get_user() directly in
> anticipation of future work to remove the trivial size optimizations
> from __copy_from_user(). Calling __get_user() also results in a small
> boost to signal handling throughput here.

Modulo the comments from Christophe, this looks good to me. It seems to
do what it says on the tin.

Reviewed-by: Daniel Axtens <[email-masked]>

Do you know if this patch is responsible for the slightly increase in
radix performance you observed in your cover letter? The rest of the
series shouldn't really make things faster than the no-KUAP case...

Kind regards,
Daniel

>
> Signed-off-by: Christopher M. Riedl <[email-masked]>
> ---
>  arch/powerpc/kernel/signal_64.c | 14 ++++++++++++--
>  1 file changed, 12 insertions(+), 2 deletions(-)
>
> diff --git a/arch/powerpc/kernel/signal_64.c b/arch/powerpc/kernel/signal_64.c
> index 817b64e1e409..42fdc4a7ff72 100644
> --- a/arch/powerpc/kernel/signal_64.c
> +++ b/arch/powerpc/kernel/signal_64.c
> @@ -97,6 +97,14 @@ static void prepare_setup_sigcontext(struct task_struct *tsk, int ctx_has_vsx_re
>  #endif /* CONFIG_VSX */
>  }
>  
> +static inline int get_user_sigset(sigset_t *dst, const sigset_t *src)
> +{
> +	if (sizeof(sigset_t) <= 8)
> +		return __get_user(dst->sig[0], &src->sig[0]);
> +	else
> +		return __copy_from_user(dst, src, sizeof(sigset_t));
> +}
> +
>  /*
>   * Set up the sigcontext for the signal frame.
>   */
> @@ -701,8 +709,9 @@ SYSCALL_DEFINE3(swapcontext, struct ucontext __user *, old_ctx,
>  	 * We kill the task with a SIGSEGV in this situation.
>  	 */
>  
> -	if (__copy_from_user(&set, &new_ctx->uc_sigmask, sizeof(set)))
> +	if (get_user_sigset(&set, &new_ctx->uc_sigmask))
>  		do_exit(SIGSEGV);
> +
>  	set_current_blocked(&set);
>  
>  	if (!user_read_access_begin(new_ctx, ctx_size))
> @@ -740,8 +749,9 @@ SYSCALL_DEFINE0(rt_sigreturn)
>  	if (!access_ok(uc, sizeof(*uc)))
>  		goto badframe;
>  
> -	if (__copy_from_user(&set, &uc->uc_sigmask, sizeof(set)))
> +	if (get_user_sigset(&set, &uc->uc_sigmask))
>  		goto badframe;
> +
>  	set_current_blocked(&set);
>  
>  #ifdef CONFIG_PPC_TRANSACTIONAL_MEM
> -- 
> 2.26.1