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

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Date:   Mon, 11 Mar 2019 12:10:18 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Jonathan Cameron <jic23@...nel.org>
Cc:     Nick Desaulniers <ndesaulniers@...gle.com>,
        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 Sun, Mar 10, 2019 at 10:37 AM Jonathan Cameron <jic23@...nel.org> wrote:
> 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.

I think it's actually a correct warning, it just doesn't trigger
on gcc because of a known gcc bug.

I'm not sure however why I don't see the warning with clang-8
on my local build. Is this something I have to enable
separately?

       Arnd