linux-kernel - Re: [PATCH 4.19 044/113] iio: iio-utils: Fix possible incorrect mask calculation

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Message-ID: <20190730185114.GA9061@amd>
Date:   Tue, 30 Jul 2019 20:51:14 +0200
From:   Pavel Machek <pavel@...x.de>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        Bastien Nocera <hadess@...ess.net>,
        Jonathan Cameron <Jonathan.Cameron@...wei.com>,
        Sasha Levin <sashal@...nel.org>
Subject: Re: [PATCH 4.19 044/113] iio: iio-utils: Fix possible incorrect mask
 calculation

Hi!

> [ 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.

Ok, it would be problem if code was like that. But it is not:

> @@ -159,9 +159,9 @@ 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;
>  

Note 1ULL already being there before the change. AFAICT this does not
change anything; 1ULL << foo will already have long long type, so
substraction will be long long too.

AFAICT this does not change the binary code at all, so it can't fix a
bug...

Best regards,

									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

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