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

["Michal Suchánek" <msuchanek@suse.de>]

On Thu, Jul 02, 2026 at 10:45:54PM +0200, Thomas Gleixner wrote:
> On Thu, Jul 02 2026 at 13:45, Michal Suchánek wrote:
> > On Thu, Jul 02, 2026 at 01:24:57PM +0200, Thomas Gleixner wrote:
> >> 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.
> >
> > syscall_enter_from_user_mode returns the new syscall number after doing
> > something arbitrarry with it, including running seccomp.
> >
> > Wehn the syscall is already handled, eg. by seccomp filtering it returns
> > -1 as the new syscall number. -1 is an invalid syscall number but it can
> > still be filtered by seccomp.
> 
> Once syscall_enter_from_user_mode() returns -1 nothing can filter it
> anymore.
> 
> > When the syscall number was -1 to start with it's not possible to
> > determine if the syscall was fileterd from the return value. s390
> > returns the filtered state in a flag it sets on the regs structure,
> > avoiding this problem.
> 
> What needs to determine whether the syscall was filtered or not?

The code that executes syscall_enter_from_user_mode() needs to determine
that.

After syscall_enter_from_user_mode() returns the syscall needs to be
executed or skipped.

'Executing' an invalid syscall boils down to setting the return value to
-ENOSYS.

But if the syscall number returned is -1 was the syscall filtered and
the return value set by syscall_enter_from_user_mode() or should it be
set by the caller to -ENOSYS?

> 
> > However, the API should be specified in a way that does not require
> > everyone implementing such flag.
> 
> Which exact problem does the flag solve?

To be able to tell if the syscall was handled or no, the return value
from syscall_enter_from_user_mode() is inconclusive.

That's what the flag is for. To be able to tell if the syscall was
handled without relying on the ambiguous return value of
syscall_enter_from_user_mode().

Thanks

Michal