linux-kernel - Re: [PATCH] iio: common: ssp_sensors: Initialize calculated_time in ssp_common_process

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Date:   Sun, 10 Mar 2019 09:37:38 +0000
From:   Jonathan Cameron <jic23@...nel.org>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Nathan Chancellor <natechancellor@...il.com>,
        Hartmut Knaack <knaack.h@....de>,
        Lars-Peter Clausen <lars@...afoo.de>,
        Peter Meerwald-Stadler <pmeerw@...erw.net>,
        linux-iio@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
        clang-built-linux@...glegroups.com
Subject: Re: [PATCH] iio: common: ssp_sensors: Initialize calculated_time in
 ssp_common_process_data

On Thu, 7 Mar 2019 16:35:26 -0800
Nick Desaulniers <ndesaulniers@...gle.com> wrote:

> On Thu, Mar 7, 2019 at 1:46 PM Nathan Chancellor
> <natechancellor@...il.com> wrote:
> >
> > When building with -Wsometimes-uninitialized, Clang warns:
> >
> > drivers/iio/common/ssp_sensors/ssp_iio.c:95:6: warning: variable
> > 'calculated_time' is used uninitialized whenever 'if' condition is false
> > [-Wsometimes-uninitialized]
> >
> > While it isn't wrong, this will never be a problem because
> > iio_push_to_buffers_with_timestamp only uses calculated_time
> > on the same condition that it is assigned (when scan_timestamp
> > is not zero). While iio_push_to_buffers_with_timestamp is marked
> > as inline, Clang does inlining in the optimization stage, which
> > happens after the semantic analysis phase (plus inline is merely
> > a hint to the compiler).
> >
> > Fix this by just zero initializing calculated_time.
> >
> > Link: https://github.com/ClangBuiltLinux/linux/issues/394
> > Signed-off-by: Nathan Chancellor <natechancellor@...il.com>  
> 
> Knowing that the same invariant holds across function boundaries to
> protect access of unitialized values and thus undefined behavior
> sounds tricky to diagnose accurately.  Thanks for the patch.
> Reviewed-by: Nick Desaulniers <ndesaulniers@...gle.com>
Applied to the togreg branch of iio.git and pushed out as testing
for the autobuilders to play with it.

Note this is going to be a while before it hits mainline but as
it is a false (if reasonable!) warning I'm not going to rush
it in as a fix.

Thanks,

Jonathan

> 
> > ---
> >  drivers/iio/common/ssp_sensors/ssp_iio.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/drivers/iio/common/ssp_sensors/ssp_iio.c b/drivers/iio/common/ssp_sensors/ssp_iio.c
> > index 645f2e3975db..e38f704d88b7 100644
> > --- a/drivers/iio/common/ssp_sensors/ssp_iio.c
> > +++ b/drivers/iio/common/ssp_sensors/ssp_iio.c
> > @@ -81,7 +81,7 @@ int ssp_common_process_data(struct iio_dev *indio_dev, void *buf,
> >                             unsigned int len, int64_t timestamp)
> >  {
> >         __le32 time;
> > -       int64_t calculated_time;
> > +       int64_t calculated_time = 0;
> >         struct ssp_sensor_data *spd = iio_priv(indio_dev);
> >
> >         if (indio_dev->scan_bytes == 0)
> > --
> > 2.21.0
> >  
> 
>