[PATCH v2 1/2] ARM: ptrace: Restore syscall restart tracing

[Arnd Bergmann]

On Thu, Aug 10, 2023, at 21:54, Kees Cook wrote:
> Since commit 4e57a4ddf6b0 ("ARM: 9107/1: syscall: always store
> thread_info->abi_syscall"), the seccomp selftests "syscall_restart" has
> been broken. This was caused by the restart syscall not being stored to
> "abi_syscall" during restart setup before branching to the "local_restart"
> label. Tracers would see the wrong syscall, and scno would get overwritten
> while returning from the TIF_WORK path. Add the missing store.
>

> diff --git a/arch/arm/kernel/entry-common.S b/arch/arm/kernel/entry-common.S
> index bcc4c9ec3aa4..5c31e9de7a60 100644
> --- a/arch/arm/kernel/entry-common.S
> +++ b/arch/arm/kernel/entry-common.S
> @@ -90,6 +90,7 @@ slow_work_pending:
>  	cmp	r0, #0
>  	beq	no_work_pending
>  	movlt	scno, #(__NR_restart_syscall - __NR_SYSCALL_BASE)
> +	str	scno, [tsk, #TI_ABI_SYSCALL]	@ make sure tracers see update
>  	ldmia	sp, {r0 - r6}			@ have to reload r0 - r6
>  	b	local_restart			@ ... and off we go
>  ENDPROC(ret_fast_syscall)

Looks good to me, as in my previous reply this is safe for all cases
with the current restartable syscalls, though it would be wrong if
we had a restartblock version of epoll_wait on an OABI_COMPAT task,
which needs to look at the high bits of the number to tell which
structure layout to use.

I also looked at my original patch now and see what I did wrong
there.

Reviewed-by: Arnd Bergmann <arnd at arndb.de>