linux-kernel - Re: [PATCH] spi: rockchip: avoid objtool warning

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Date:   Fri, 26 Feb 2021 10:49:04 +0100
From:   Arnd Bergmann <arnd@...nel.org>
To:     Pratyush Yadav <p.yadav@...com>
Cc:     Mark Brown <broonie@...nel.org>, Heiko Stuebner <heiko@...ech.de>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Emil Renner Berthing <kernel@...il.dk>,
        Arnd Bergmann <arnd@...db.de>,
        Jon Lin <jon.lin@...k-chips.com>,
        Chris Ruehl <chris.ruehl@...ys.com.hk>,
        Alexander Kochetkov <al.kochet@...il.com>,
        Johan Jonker <jbx6244@...il.com>,
        Vincent Pelletier <plr.vincent@...il.com>,
        linux-spi <linux-spi@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        "open list:ARM/Rockchip SoC support" 
        <linux-rockchip@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH] spi: rockchip: avoid objtool warning

On Fri, Feb 26, 2021 at 9:16 AM 'Pratyush Yadav' via Clang Built Linux
<clang-built-linux@...glegroups.com> wrote:
>
> Hi,
>
> On 25/02/21 01:55PM, Arnd Bergmann wrote:
> > From: Arnd Bergmann <arnd@...db.de>
> >
> > Building this file with clang leads to a an unreachable code path
> > causing a warning from objtool:
> >
> > drivers/spi/spi-rockchip.o: warning: objtool: rockchip_spi_transfer_one()+0x2e0: sibling call from callable instruction with modified stack frame
> >
> > Use BUG() instead of unreachable() to avoid the undefined behavior
> > if it does happen.
> >
> > Fixes: 65498c6ae241 ("spi: rockchip: support 4bit words")
> > Signed-off-by: Arnd Bergmann <arnd@...db.de>
> > ---
> >  drivers/spi/spi-rockchip.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/drivers/spi/spi-rockchip.c b/drivers/spi/spi-rockchip.c
> > index 936ef54e0903..972beac1169a 100644
> > --- a/drivers/spi/spi-rockchip.c
> > +++ b/drivers/spi/spi-rockchip.c
> > @@ -521,7 +521,7 @@ static void rockchip_spi_config(struct rockchip_spi *rs,
> >                * ctlr->bits_per_word_mask, so this shouldn't
> >                * happen
> >                */
> > -             unreachable();
> > +             BUG();
>
> checkpatch says:
>
>   Avoid crashing the kernel - try using WARN_ON & recovery code rather
>   than BUG() or BUG_ON()
>
> Which makes sense to me. This is not something bad enough to justify
> crashing the kernel.

I thought about rewriting it more thoroughly when I wrote the patch, but
couldn't come up with a good alternative, so I did the simplest change
in this direction, replacing the silent crash with a loud one.

Should we just dev_warn() and return instead, hoping that it won't do
more harm?

The backtrace from WARN_ON() probably doesn't help here.

       Arnd