linux-kernel - Re: arm: TI BeagleBoard X15 : Unable to handle kernel NULL pointer dereference at virtual address 00000369

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Date:   Fri, 11 Nov 2022 09:44:55 +0100
From:   "Arnd Bergmann" <arnd@...db.de>
To:     "Naresh Kamboju" <naresh.kamboju@...aro.org>
Cc:     linux-stable <stable@...r.kernel.org>,
        "open list" <linux-kernel@...r.kernel.org>,
        "Linux ARM" <linux-arm-kernel@...ts.infradead.org>,
        lkft-triage@...ts.linaro.org,
        "Greg Kroah-Hartman" <gregkh@...uxfoundation.org>,
        "Sasha Levin" <sashal@...nel.org>,
        "Linus Walleij" <linus.walleij@...aro.org>,
        "Mark Brown" <broonie@...nel.org>,
        "Liam Girdwood" <lgirdwood@...il.com>,
        "Ard Biesheuvel" <ardb@...nel.org>
Subject: Re: arm: TI BeagleBoard X15 : Unable to handle kernel NULL pointer dereference
 at virtual address 00000369 - Internal error: Oops: 5 [#1] SMP ARM

On Fri, Nov 11, 2022, at 07:28, Naresh Kamboju wrote:
> On Thu, 10 Nov 2022 at 03:33, Arnd Bergmann <arnd@...db.de> wrote:
>>
>> One more idea I had is the unwinder: since this kernel is built
>> with the frame-pointer unwinder, I think the stack usage per
>> function is going to be slightly larger than with the arm unwinder.
>>
>> Naresh, how hard is it to reproduce this bug intentionally?
>> Can you try if it still happens if you change the .config to
>> use these:?
>>
>> # CONFIG_FUNCTION_GRAPH_TRACER is not set
>> # CONFIG_UNWINDER_FRAME_POINTER is not set
>> CONFIG_UNWINDER_ARM=y
>
> I have done this experiment and reported crash not reproduced
> after eight rounds of testing [1].
>
> https://lkft.validation.linaro.org/scheduler/job/5835922#L1993

Ok, good to hear. In this case, I see three possible ways forward
to prevent this from coming back on your system:

a) use asynchronous probing for one or more of the drivers as
   Dmitry suggested. This means fixing it upstream first and then
   backporting the fix to all stable kernels. We should probably
   do this anyway, but this will need more testing on your side.

b) Change your kernel config permanently with the options above,
   if LKFT does not actually rely on CONFIG_FUNCTION_GRAPH_TRACER.
   I don't know if it does.

c) backport commit 41918ec82eb6 ("ARM: ftrace: enable the graph
   tracer with the EABI unwinder") from 5.17. This was part of 
   a longer series from Ard, and while the patch itself looks
   simple enough to be backported, I suspect we'd have to
   backport the entire series, which is probably not going to
   be realistic. Ard, any comments on this?

       Arnd