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

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.

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