linux-kernel - Re: [PATCH] media: i2c: fix an uninitialized error code

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Date:   Fri, 4 Dec 2020 09:22:02 +0100
From:   Arnd Bergmann <arnd@...nel.org>
To:     Dongchun Zhu <dongchun.zhu@...iatek.com>
Cc:     Mauro Carvalho Chehab <mchehab@...nel.org>,
        Nathan Chancellor <natechancellor@...il.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Sakari Ailus <sakari.ailus@...ux.intel.com>,
        Arnd Bergmann <arnd@...db.de>,
        Linux Media Mailing List <linux-media@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        tfiga@...gle.com, sj.huang@...iatek.com
Subject: Re: [PATCH] media: i2c: fix an uninitialized error code

On Fri, Dec 4, 2020 at 3:10 AM Dongchun Zhu <dongchun.zhu@...iatek.com> wrote:
>
> Hi Arnd,
>
> Thanks for the patch.
>
> On Thu, 2020-12-03 at 23:29 +0100, Arnd Bergmann wrote:
> > From: Arnd Bergmann <arnd@...db.de>
> >
> > Clang points out that the error handling in ov02a10_s_stream() is
> > broken, and just returns a random error code:
> >
> > drivers/media/i2c/ov02a10.c:537:6: warning: variable 'ret' is used uninitialized whenever 'if' condition is true [-Wsometimes-uninitialized]
> >         if (ov02a10->streaming == on)
> >             ^~~~~~~~~~~~~~~~~~~~~~~~
> > drivers/media/i2c/ov02a10.c:568:9: note: uninitialized use occurs here
> >         return ret;
> >                ^~~
> > drivers/media/i2c/ov02a10.c:537:2: note: remove the 'if' if its condition is always false
> >         if (ov02a10->streaming == on)
> >         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > drivers/media/i2c/ov02a10.c:533:9: note: initialize the variable 'ret' to silence this warning
> >         int ret;
> >
> > I assume that -EBUSY is the intended error code, so use that.
> >
> > Fixes: 91807efbe8ec ("media: i2c: add OV02A10 image sensor driver")
> > Signed-off-by: Arnd Bergmann <arnd@...db.de>
> > ---
> >  drivers/media/i2c/ov02a10.c | 4 +++-
> >  1 file changed, 3 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/media/i2c/ov02a10.c b/drivers/media/i2c/ov02a10.c
> > index 391718136ade..7ee9c904d9b5 100644
> > --- a/drivers/media/i2c/ov02a10.c
> > +++ b/drivers/media/i2c/ov02a10.c
> > @@ -534,8 +534,10 @@ static int ov02a10_s_stream(struct v4l2_subdev *sd, int on)
> >
> >       mutex_lock(&ov02a10->mutex);
> >
> > -     if (ov02a10->streaming == on)
> > +     if (ov02a10->streaming == on) {
> > +             ret = -EBUSY;
> >               goto unlock_and_return;
> > +     }
> >
> >       if (on) {
> >               ret = pm_runtime_get_sync(&client->dev);
>
> Only if sensor fails to stream on, ret can return a negative error code.
> Thus ret above needs to be initialized to '0'.

Ok, I sent a version 2.

> Also you could fix the clang error like this.
>
> static int ov02a10_s_stream(struct v4l2_subdev *sd, int on)
> {
>         struct ov02a10 *ov02a10 = to_ov02a10(sd);
>         ...
>         int ret = 0;
>
>         ...
>         if (ov02a10->streaming == on)
>                 goto unlock_and_return;
>
>         ...
> }

Sorry, I should have removed the last line of the output from
clang that suggested doing that. Initializing a local variable
in the declaration is generally a bad idea precisely because it
prevents the compiler from warning about a case like this where
the author forgot to set the correct error value.

We should really fix clang to not propagate that nonsense.

        Arnd