Re: [PATCH 1/2] alpha/ptrace: Record and handle the absence of switch_stack

Michael Schmitz
2021-06-21T04:09:29Z

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
>