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Subject: Re: [PATCH 1/2] alpha/ptrace: Record and handle the absence of
 switch_stack
To:     Linus Torvalds <torvalds@linux-foundation.org>
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Cc:     "Eric W. Biederman" <ebiederm@xmission.com>,
        linux-arch <linux-arch@vger.kernel.org>,
        Jens Axboe <axboe@kernel.dk>, Oleg Nesterov <oleg@redhat.com>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Richard Henderson <rth@twiddle.net>,
        Ivan Kokshaysky <ink@jurassic.park.msu.ru>,
        Matt Turner <mattst88@gmail.com>,
        alpha <linux-alpha@vger.kernel.org>,
        Geert Uytterhoeven <geert@linux-m68k.org>,
        linux-m68k <linux-m68k@lists.linux-m68k.org>,
        Arnd Bergmann <arnd@kernel.org>,
        Ley Foon Tan <ley.foon.tan@intel.com>,
        Tejun Heo <tj@kernel.org>, Kees Cook <keescook@chromium.org>
From:   Michael Schmitz <schmitzmic@gmail.com>
Message-ID: <700d8acd-d3df-a026-8b1d-55eeae836eea@gmail.com>
Date:   Mon, 21 Jun 2021 16:08:35 +1200
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Hi Linus,

I realized that the patch is still incomplete when answering Al...

Am 21.06.2021 um 15:37 schrieb Linus Torvalds:
> On Sun, Jun 20, 2021 at 8:18 PM Michael Schmitz <schmitzmic@gmail.com> wrote:
>>
>> I hope that makes more sense?
>
> So the problem is in your debug patch: you don't set that
> TIS_SWITCH_STACK in nearly enough places.
>
> In this particular example, I think it's that you don't set it in
> do_trace_exit, so when you strace the process, the system call exit -
> which is where the return value will be picked up - gets that warning.
>
> You did set TIS_SWITCH_STACK on trace_entry, but then it's cleared
> again during the system call, and not set at the trace_exit path.
> Oddly, your debug patch also _clears_ it on the exit path, but it
> doesn't set it when do_trace_exit does the SAVE_SWITCH_STACK.
>
> You oddly also set it for __sys_exit, but not all the other special
> system calls that also do that SAVE_SWITCH_STACK.

That's the one I used to test whether my debug patch had any ill side 
effects (i.e. smashing the stack) late yesterday. Forgot to add that to 
the other cases.

>
> Really, pretty much every single case of SAVE_SWITCH_STACK would need
> to set it. Not just do_trace_enter/exit

Yes - done that now and the warning is gone.

> It's why I didn't like Eric's debug patch either. It's quite expensive
> to do, partly because you look up that curptr thing. All very nasty.

I need to talk to Geert and Andreas to find where register a1 is 
preserved, but if I have to reload a1 all the time, this won't be useful 
except for debugging.

> It would be *much* better to make the flag be part of the stack frame,
> but sadly at least on alpha we had exported the format of that stack
> frame to user space.

Same on m68k, but can we push a flag _after_ the switch stack?

> Anyway, I think these debug patches are not just expensive but the
> m68k one most definitely is also very incomplete.

Yes, I've seen that in the meantime. Need to triple check my work next 
time.

Sorry for the extra noise!

Cheers,

	Michael

>
>              Linus
>