[PATCH] iio: common: ssp_sensors: Initialize calculated_time in ssp_common_process_data

[PATCH] iio: common: ssp_sensors: Initialize calculated_time in ssp_common_process_data

[Nathan Chancellor]

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@gmail.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

Re: [PATCH] iio: common: ssp_sensors: Initialize calculated_time in ssp_common_process_data

[Nick Desaulniers]

On Thu, Mar 7, 2019 at 1:46 PM Nathan Chancellor
<natechancellor@gmail.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@gmail.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@google.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

Re: [PATCH] iio: common: ssp_sensors: Initialize calculated_time in ssp_common_process_data

[Jonathan Cameron]

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

> On Thu, Mar 7, 2019 at 1:46 PM Nathan Chancellor
> <natechancellor@gmail.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@gmail.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@google.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
> >  
> 
> 

Re: [PATCH] iio: common: ssp_sensors: Initialize calculated_time in ssp_common_process_data

[Arnd Bergmann]

On Sun, Mar 10, 2019 at 10:37 AM Jonathan Cameron <jic23@kernel.org> wrote:
> On Thu, 7 Mar 2019 16:35:26 -0800 Nick Desaulniers <ndesaulniers@google.com> wrote:
> > On Thu, Mar 7, 2019 at 1:46 PM Nathan Chancellor <natechancellor@gmail.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@gmail.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@google.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

Re: [PATCH] iio: common: ssp_sensors: Initialize calculated_time in ssp_common_process_data

[Nathan Chancellor]

On Mon, Mar 11, 2019 at 12:10:18PM +0100, Arnd Bergmann wrote:
> On Sun, Mar 10, 2019 at 10:37 AM Jonathan Cameron <jic23@kernel.org> wrote:
> > On Thu, 7 Mar 2019 16:35:26 -0800 Nick Desaulniers <ndesaulniers@google.com> wrote:
> > > On Thu, Mar 7, 2019 at 1:46 PM Nathan Chancellor <natechancellor@gmail.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@gmail.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@google.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

Yes, -Wsometimes-uninitialized is not on by default because
-Wuninitialized is disabled. You can either pass it via KCFLAGS or
modify scripts/Makefile.extrawarn, which is the eventual goal because
Clang can catch things that GCC can't:

https://github.com/ClangBuiltLinux/linux/issues/381

Cheers,
Nathan