linux-kernel - Re: [PATCH 08/17] ptrace/m68k: Stop open coding ptrace_report

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Message-ID: <acf7b627-2dec-c76c-2aa0-6b4c6addd793@gmail.com>
Date:   Tue, 11 Jan 2022 14:33:50 +1300
From:   Michael Schmitz <schmitzmic@...il.com>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>,
        Al Viro <viro@...iv.linux.org.uk>
Cc:     "Eric W. Biederman" <ebiederm@...ssion.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux-Arch <linux-arch@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Oleg Nesterov <oleg@...hat.com>,
        Kees Cook <keescook@...omium.org>,
        Linux API <linux-api@...r.kernel.org>,
        linux-m68k <linux-m68k@...ts.linux-m68k.org>
Subject: Re: [PATCH 08/17] ptrace/m68k: Stop open coding ptrace_report_syscall

Hi Geert,

Am 11.01.2022 um 06:54 schrieb Geert Uytterhoeven:
> Hi Al,
>
> CC Michael/m68k,
>
> On Mon, Jan 10, 2022 at 5:20 PM Al Viro <viro@...iv.linux.org.uk> wrote:
>> 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@...ssion.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@...ux-m68k.org>
>>>> Signed-off-by: "Eric W. Biederman" <ebiederm@...ssion.com>
>>>
>>> As this depends on the removal of a parameter from
>>> ptrace_report_syscall() earlier in this series:
>>> Acked-by: Geert Uytterhoeven <geert@...ux-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.
>
> In fact Michael did so in "[PATCH v7 1/2] m68k/kernel - wire up
> syscall_trace_enter/leave for m68k"[1], but that's still stuck...
>
> [1] https://lore.kernel.org/r/1624924520-17567-2-git-send-email-schmitzmic@gmail.com/

That patch (for reasons I never found out) did interact badly with 
Christoph Hellwig's 'remove set_fs' patches (and Al's signal fixes which 
Christoph's patches are based upon). Caused format errors under memory 
stress tests quite reliably, on my 030 hardware.

Probably needs a fresh look - the signal return path got changed by Al's 
patches IIRC, and I might have relied on offsets to data on the stack 
that are no longer correct with these patches. Or there's a race between 
the syscall trap and signal handling when returning from interrupt 
context ...

Still school hols over here so I won't have much peace and quiet until 
February.

Cheers,

	Michael


>
> Gr{oetje,eeting}s,
>
>                         Geert
>
> --
> Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org
>
> In personal conversations with technical people, I call myself a hacker. But
> when I'm talking to journalists I just say "programmer" or something like that.
>                                 -- Linus Torvalds
>