On Fri, Sep 04, 2020 at 04:31:44PM -0400, Gabriel Krisman Bertazi wrote:

Syscall User Dispatch (SUD) must take precedence over seccomp, since the
use case is emulation (it can be invoked with a different ABI) such that
seccomp filtering by syscall number doesn't make sense in the first
place.  In addition, either the syscall is dispatched back to userspace,
in which case there is no resource for seccomp to protect, or the
syscall will be executed, and seccomp will execute next.

Regarding ptrace, I experimented with before and after, and while the
same ABI argument applies, I felt it was easier to debug if I let ptrace
happen for syscalls that are dispatched back to userspace.  In addition,
doing it after ptrace makes the code in syscall_exit_work slightly
simpler, since it doesn't require special handling for this feature.

I think I'm with Andy that this should be before ptrace(). ptrace()
users will attempt to interpret things like they're regular syscalls,
and that's definitely not the case.