Re: [RFC] entry: Untangle the return value of syscall_enter_from_user_mode from syscall NR

[Thomas Gleixner]

On Wed, Jul 01 2026 at 19:42, Michal Suchánek wrote:
> The return value of syscall_enter_from_user_mode is used both for the
> adjusted syscall number and the indicator that a syscall should be
> skipped.
>
> As seccomp can be invoked on any syscall, including invalid ones this
> somewhat undermines seccomp.
>
> While the seccomp variants that terminate the process do not need to
> care about this for the filter that sets the syscall return value this
> disctinction is required.

You completely fail to explain why and what actual problem you are
trying to solve. At least I can't figure it out from the above word
salad.

> Pass the syscall number as a pointer to the inline entry functions, and
> use the return value exclusively for the indication that the syscall is
> already handled.
>
> This should avoid the need for the s390 PIF_SYSCALL_RET_SET which is the
> workaround for exactly this deficiency.
>
> If this is desirable the patch could be split into some series that
> adjusts the code flow where needed so that the final change is mostly
> mechanical.

That's not a matter of desire. That's mandatory.

> -	instrumentation_begin();
> -	if (!invoke_syscall(regs, nr) && nr != -1)
> -	 	result_reg(regs) = __sys_ni_syscall(regs);
> -	instrumentation_end();
> +	/* Skip syscall when -1 is returned */
> +	if (!syscall_enter_from_user_mode(regs, &nr)) {

Seriously?

If we go and separate the syscall number from the return value, then the
return value 0 means success and anything else fail. Which in other
words is a boolean. So instead of tastelessly adding a completely
nonsensical comment about -1 here, syscall_enter_from_user_mode() wants
to have the return value type bool with a proper boolean logic: true =
success, false = abort.

> @@ -168,8 +168,7 @@ __visible noinstr void do_int80_emulation(struct pt_regs *regs)
>  	nr = syscall_32_enter(regs);
>  
>  	local_irq_enable();
> -	nr = syscall_enter_from_user_mode_work(regs, nr);
> -	do_syscall_32_irqs_on(regs, nr);
> +	syscall_enter_from_user_mode_work(regs, &nr);

How exactly is this ever going to invoke a valid syscall?

> +	if (!syscall_enter_from_user_mode_work(regs, &nr)) {
> +		nr &= GENMASK(31, 0);
> +		do_syscall_32_irqs_on(regs, nr);

  do_syscall_32_irqs_on(regs, (int)nr);

would be too simple, right?

Thanks,

        tglx