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

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Message-ID: <CAKwvOdkeVmcoD7WusKE83MmRjX+h73rnHDY=Rn_a7LXsULPsrw@mail.gmail.com>
Date:   Thu, 7 Mar 2019 16:35:26 -0800
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Nathan Chancellor <natechancellor@...il.com>
Cc:     Jonathan Cameron <jic23@...nel.org>,
        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, 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>

> ---
>  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
>


-- 
Thanks,
~Nick Desaulniers