linux-kernel - Re: [PATCH] iio: ssp_sensors: avoid uninitialized variable usage

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Date:   Fri, 22 Mar 2019 08:01:19 -0700
From:   Nathan Chancellor <natechancellor@...il.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Jonathan Cameron <jic23@...nel.org>,
        clang-built-linux@...glegroups.com,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Hartmut Knaack <knaack.h@....de>,
        Lars-Peter Clausen <lars@...afoo.de>,
        Peter Meerwald-Stadler <pmeerw@...erw.net>,
        linux-iio@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] iio: ssp_sensors: avoid uninitialized variable usage

On Fri, Mar 22, 2019 at 03:09:22PM +0100, Arnd Bergmann wrote:
> clang points out that 'calculated_time' is only sometimes
> initialized here, which leads to incorrect data being
> passed into another function:
> 
> drivers/iio/common/ssp_sensors/ssp_iio.c:95:6: error: variable 'calculated_time' is used uninitialized whenever 'if' condition is false [-Werror,-Wsometimes-uninitialized]
>         if (indio_dev->scan_timestamp) {
>             ^~~~~~~~~~~~~~~~~~~~~~~~~
> drivers/iio/common/ssp_sensors/ssp_iio.c:102:9: note: uninitialized use occurs here
>                                                   calculated_time);
>                                                   ^~~~~~~~~~~~~~~
> drivers/iio/common/ssp_sensors/ssp_iio.c:95:2: note: remove the 'if' if its condition is always true
>         if (indio_dev->scan_timestamp) {
>         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> drivers/iio/common/ssp_sensors/ssp_iio.c:84:25: note: initialize the variable 'calculated_time' to silence this warning
>         int64_t calculated_time;
>                                ^
> The data is subsequently ignored by iio_push_to_buffers_with_timestamp(),
> but the warning still feels legitimate and to work around it, we can
> initialize the time in the other case.
> 
> Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=18501
> Signed-off-by: Arnd Bergmann <arnd@...db.de>
> ---
>  drivers/iio/common/ssp_sensors/ssp_iio.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/drivers/iio/common/ssp_sensors/ssp_iio.c b/drivers/iio/common/ssp_sensors/ssp_iio.c
> index 645f2e3975db..81e8f4844c90 100644
> --- a/drivers/iio/common/ssp_sensors/ssp_iio.c
> +++ b/drivers/iio/common/ssp_sensors/ssp_iio.c
> @@ -96,6 +96,8 @@ int ssp_common_process_data(struct iio_dev *indio_dev, void *buf,
>  		memcpy(&time, &((char *)buf)[len], SSP_TIME_SIZE);
>  		calculated_time =
>  			timestamp + (int64_t)le32_to_cpu(time) * 1000000;
> +	} else {
> +		calculated_time = 0;
>  	}
>  
>  	return iio_push_to_buffers_with_timestamp(indio_dev, spd->buffer,
> -- 
> 2.20.0
> 

I sent a similar change, which is sitting in Jonathan's testing branch:

https://git.kernel.org/pub/scm/linux/kernel/git/jic23/iio.git/commit/?id=0643039b4fee4aa54a233ead15dc0b2286f059d7

You made a good point previously that initializing the variable at the
beginning of a function may not always be the best choice. I don't have
a personal preference for which patch stays around so:

Reviewed-by: Nathan Chancellor <natechancellor@...il.com>

Just in case.

Nathan