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From:   Ard Biesheuvel <ardb@kernel.org>
Date:   Fri, 11 Nov 2022 09:55:38 +0100
Message-ID: <CAMj1kXH994YRCZ9mYfjfN2JdJ_-6_too66bLYGYres5zfXvRCw@mail.gmail.com>
Subject: Re: arm: TI BeagleBoard X15 : Unable to handle kernel NULL pointer
 dereference at virtual address 00000369 - Internal error: Oops: 5 [#1] SMP ARM
To:     Arnd Bergmann <arnd@arndb.de>
Cc:     Naresh Kamboju <naresh.kamboju@linaro.org>,
        linux-stable <stable@vger.kernel.org>,
        open list <linux-kernel@vger.kernel.org>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        lkft-triage@lists.linaro.org,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Sasha Levin <sashal@kernel.org>,
        Linus Walleij <linus.walleij@linaro.org>,
        Mark Brown <broonie@kernel.org>,
        Liam Girdwood <lgirdwood@gmail.com>
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On Fri, 11 Nov 2022 at 09:45, Arnd Bergmann <arnd@arndb.de> wrote:
>
> On Fri, Nov 11, 2022, at 07:28, Naresh Kamboju wrote:
> > On Thu, 10 Nov 2022 at 03:33, Arnd Bergmann <arnd@arndb.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?
>

It at least needs the preceding patch, which tracks the location of LR
on the stack when using CONFIG_UNWINDER_ARM.

But I'd take the whole series for good measure.