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

Michal Suchánek [off-list ref] writes:

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.

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.

I'm not sure whether PIF_SYSCALL_RET_SET can be removed - the syscall
return might still get set by PTRACE_SET_SYSCALL_INFO when the tracee is
stopped. This might be a positive number which can't be distinguished
from a syscall number. But maybe i'm missing something? It's been quite
a while since I touched all that ptrace stuff.