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
