Re: [PATCH V2] powerpc/syscall: Fix seccomp errno handling with GENERIC_ENTRY

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

On Wed, Jul 01, 2026 at 10:01:49AM +0200, Michal Suchánek wrote:
> On Wed, Jul 01, 2026 at 09:41:57AM +0200, Michal Suchánek wrote:
> > On Wed, Jul 01, 2026 at 11:57:00AM +0530, Mukesh Kumar Chaurasiya wrote:
> > > On Wed, Jul 01, 2026 at 01:41:09AM +0530, Shrikanth Hegde wrote:
> > > > Hi Mukesh.
> > > > 
> > > > On 6/29/26 11:59 PM, Mukesh Kumar Chaurasiya (IBM) wrote:

> > > > > diff --git a/arch/powerpc/kernel/syscall.c b/arch/powerpc/kernel/syscall.c
> > > > > index a9da2af6efa8..36d73933a311 100644
> > > > > --- a/arch/powerpc/kernel/syscall.c
> > > > > +++ b/arch/powerpc/kernel/syscall.c
> > > > > @@ -20,7 +20,6 @@ notrace long system_call_exception(struct pt_regs *regs, unsigned long r0)
> > > > >   	syscall_fn f;
> > > > >   	add_random_kstack_offset();
> > > > > -	r0 = syscall_enter_from_user_mode(regs, r0);
> > > > >   	if (unlikely(r0 >= NR_syscalls)) {
> > > > >   		if (unlikely(trap_is_unsupported_scv(regs))) {
> > > > > @@ -31,6 +30,12 @@ notrace long system_call_exception(struct pt_regs *regs, unsigned long r0)
> > > > >   		return -ENOSYS;
> > > > >   	}
> > > > > +	r0 = syscall_enter_from_user_mode(regs, r0);
> > > > > +
> > > > 
> > > > I see many arch first do syscall_enter_from_user_mode and then check for return value.
> > > > take x86 for example,
> > > > 
> > > > __visible noinstr bool do_syscall_64(struct pt_regs *regs, int nr)
> > > > {
> > > >         nr = syscall_enter_from_user_mode(regs, nr);
> > > > 
> > > >         if (!do_syscall_x64(regs, nr) && !do_syscall_x32(regs, nr) && nr != -1) {
> > > >                 /* Invalid system call, but still a system call. */
> > > >                 regs->ax = __x64_sys_ni_syscall(regs);
> > > >         }
> > > > 
> > > > }
> > > > 
> > > > So seccomp fails silently there if initial nr was -1?
> > > > 
> > > Hey,
> > > 
> > > No the -1 syscall ignores the error silently and returns 0.
> > > 
> > 
> > There seems to be some inconsistency with the invalid syscalls.
> > 
> > Adapting the example from seccomp man page to ignore architecture I get
> > on x86_64 (presumably with GENERIC_ENTRY since long ago):
> > 
> > ./a.out -2 55 /usr/bin/perl -MPOSIX -e '$!=0; my $r = syscall(-2, 0); print "ret=$r errno=".($!+0)." ($!)\n"'
> > ret=-1 errno=55 (No anode)
> > 
> > but on ppc64le (with GENEREC_ENTRY):
> > 
> > ./a.out -2 55 /usr/bin/perl -MPOSIX -e '$!=0; my $r = syscall(-2, 0); print "ret=$r errno=".($!+0)." ($!)\n"'
> > ret=-1 errno=38 (Function not implemented)
> > 
> > That said, behavior of seccomp on invalid syscalls is not particularly
> > concerning. The tools that people typically use for constructing those
> > filters typically require a valid syscall number.
> > 
> > It would be nice to align, though.
> 
> It is more concerning for SECCOMP_SET_MODE_STRICT or similar. So it
> should be resolved to correctly execute seccomp even on invalid
> syscalls. The syscall_enter_from_user_mode API is not particularly
> well-suited for that, though.

In particular the fixup per
https://lore.kernel.org/linuxppc-dev/akJzuEJRLniHk4Fi@kunlun.suse.cz/

handles some cases

./a.out -2 55 /usr/bin/perl -MPOSIX -e '$!=0; my $r = syscall(-2, 0); print "ret=$r errno=".($!+0)." ($!)\n"'
ret=-1 errno=55 (No anode)

but not -1

./a.out -1 55 /usr/bin/perl -MPOSIX -e '$!=0; my $r = syscall(-1, 0); print "ret=$r errno=".($!+0)." ($!)\n"'
ret=-1 errno=38 (Function not implemented)

which is the direct result of the ambiguous return value of
syscall_enter_from_user_mode

Thanks

Michal