Re: [PATCH v2 seccomp 2/6] asm/syscall.h: Add syscall_arches[] array

[Jann Horn]

On Fri, Sep 25, 2020 at 2:18 AM Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote:
> On Fri, Sep 25, 2020 at 02:15:50AM +0200, Jann Horn wrote:
> > On Fri, Sep 25, 2020 at 2:01 AM Kees Cook <keescook@xxxxxxxxxxxx> wrote:
> > > 2) seccomp needs to handle "multiplexed" tables like x86_x32 (distros
> > >    haven't removed CONFIG_X86_X32 widely yet, so it is a reality that
> > >    it must be dealt with), which means seccomp's idea of the arch
> > >    "number" can't be the same as the AUDIT_ARCH.
> >
> > Sure, distros ship it; but basically nobody uses it, it doesn't have
> > to be fast. As long as we don't *break* it, everything's fine. And if
> > we ignore the existence of X32 in the fastpath, that'll just mean that
> > syscalls with the X32 marker bit always hit the seccomp slowpath
> > (because it'll look like the syscall number is out-of-bounds ) - no
> > problem.
>
> You do realize that X32 is amd64 counterpart of mips n32, right?  And that's
> not "basically nobody uses it"...

What makes X32 weird for seccomp is that it has the syscall tables for
X86-64 and X32 mushed together, using the single architecture
identifier AUDIT_ARCH_X86_64. I believe that's what Kees referred to
by "multiplexed tables".

As far as I can tell, MIPS is more well-behaved there and uses the
separate architecture identifiers
AUDIT_ARCH_MIPS|__AUDIT_ARCH_64BIT
and
AUDIT_ARCH_MIPS|__AUDIT_ARCH_64BIT|__AUDIT_ARCH_CONVENTION_MIPS64_N32.

(But no, I did not actually realize that that's what N32 is. Thanks
for the explanation, I was wondering why MIPS was the only
architecture with three architecture identifiers...)