linux-kernel - Re: [PATCH AUTOSEL 4.4 18/35] iio: iio-utils: Fix possible incorrect mask calculation

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Date:   Sun, 21 Jul 2019 18:27:58 +0100
From:   Jonathan Cameron <jic23@...23.retrosnub.co.uk>
To:     Sasha Levin <sashal@...nel.org>
Cc:     linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        Bastien Nocera <hadess@...ess.net>,
        Jonathan Cameron <Jonathan.Cameron@...wei.com>,
        linux-iio@...r.kernel.org
Subject: Re: [PATCH AUTOSEL 4.4 18/35] iio: iio-utils: Fix possible
 incorrect mask calculation

On Fri, 19 Jul 2019 00:14:06 -0400
Sasha Levin <sashal@...nel.org> wrote:

> From: Bastien Nocera <hadess@...ess.net>
> 
> [ Upstream commit 208a68c8393d6041a90862992222f3d7943d44d6 ]
> 
> On some machines, iio-sensor-proxy was returning all 0's for IIO sensor
> values. It turns out that the bits_used for this sensor is 32, which makes
> the mask calculation:
> 
> *mask = (1 << 32) - 1;
> 
> If the compiler interprets the 1 literals as 32-bit ints, it generates
> undefined behavior depending on compiler version and optimization level.
> On my system, it optimizes out the shift, so the mask value becomes
> 
> *mask = (1) - 1;
> 
> With a mask value of 0, iio-sensor-proxy will always return 0 for every axis.
> 
> Avoid incorrect 0 values caused by compiler optimization.
> 
> See original fix by Brett Dutro <brett.dutro@...il.com> in
> iio-sensor-proxy:
> https://github.com/hadess/iio-sensor-proxy/commit/9615ceac7c134d838660e209726cd86aa2064fd3
> 
> Signed-off-by: Bastien Nocera <hadess@...ess.net>
> Signed-off-by: Jonathan Cameron <Jonathan.Cameron@...wei.com>
> Signed-off-by: Sasha Levin <sashal@...nel.org>
Good catch, I should have cc'd stable on this one.

Thanks,

Jonathan

> ---
>  tools/iio/iio_utils.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/tools/iio/iio_utils.c b/tools/iio/iio_utils.c
> index 5eb6793f3972..2d0dcd6fc64c 100644
> --- a/tools/iio/iio_utils.c
> +++ b/tools/iio/iio_utils.c
> @@ -163,9 +163,9 @@ int iioutils_get_type(unsigned *is_signed, unsigned *bytes, unsigned *bits_used,
>  			*be = (endianchar == 'b');
>  			*bytes = padint / 8;
>  			if (*bits_used == 64)
> -				*mask = ~0;
> +				*mask = ~(0ULL);
>  			else
> -				*mask = (1ULL << *bits_used) - 1;
> +				*mask = (1ULL << *bits_used) - 1ULL;
>  
>  			*is_signed = (signchar == 's');
>  			if (fclose(sysfsfp)) {