Date:   Mon, 10 Jan 2022 16:20:03 +0000
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Geert Uytterhoeven <geert@linux-m68k.org>
Cc:     "Eric W. Biederman" <ebiederm@xmission.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Linux-Arch <linux-arch@vger.kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Oleg Nesterov <oleg@redhat.com>,
        Kees Cook <keescook@chromium.org>,
        Linux API <linux-api@vger.kernel.org>
Subject: Re: [PATCH 08/17] ptrace/m68k: Stop open coding ptrace_report_syscall
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On Mon, Jan 10, 2022 at 04:26:57PM +0100, Geert Uytterhoeven wrote:
> On Mon, Jan 3, 2022 at 10:33 PM Eric W. Biederman <ebiederm@xmission.com> wrote:
> > The generic function ptrace_report_syscall does a little more
> > than syscall_trace on m68k.  The function ptrace_report_syscall
> > stops early if PT_TRACED is not set, it sets ptrace_message,
> > and returns the result of fatal_signal_pending.
> >
> > Setting ptrace_message to a passed in value of 0 is effectively not
> > setting ptrace_message, making that additional work a noop.
> >
> > Returning the result of fatal_signal_pending and letting the caller
> > ignore the result becomes a noop in this change.
> >
> > When a process is ptraced, the flag PT_PTRACED is always set in
> > current->ptrace.  Testing for PT_PTRACED in ptrace_report_syscall is
> > just an optimization to fail early if the process is not ptraced.
> > Later on in ptrace_notify, ptrace_stop will test current->ptrace under
> > tasklist_lock and skip performing any work if the task is not ptraced.
> >
> > Cc: Geert Uytterhoeven <geert@linux-m68k.org>
> > Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
> 
> As this depends on the removal of a parameter from
> ptrace_report_syscall() earlier in this series:
> Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>

FWIW, I would suggest taking it a bit further: make syscall_trace_enter()
and syscall_trace_leave() in m68k ptrace.c unconditional, replace the
calls of syscall_trace() in entry.S with syscall_trace_enter() and
syscall_trace_leave() resp. and remove syscall_trace().

Geert, do you see any problems with that?  The only difference is that
current->ptrace_message would be set to 1 for ptrace stop on entry and
2 - on leave.  Currently m68k just has it 0 all along.

It is user-visible (the whole point is to let the tracer see which
stop it is - entry or exit one), so somebody using PTRACE_GETEVENTMSG
on syscall stops would start seeing 1 or 2 instead of "0 all along".
That's how it works on all other architectures (including m68k-nommu),
and I doubt that anything in userland will get broken.

Behaviour of PTRACE_GETEVENTMSG for other stops (fork, etc.) remains
as-is, of course.