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Date:   Tue, 30 Jul 2019 20:51:14 +0200
From:   Pavel Machek <pavel@denx.de>
To:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc:     linux-kernel@vger.kernel.org, stable@vger.kernel.org,
        Bastien Nocera <hadess@hadess.net>,
        Jonathan Cameron <Jonathan.Cameron@huawei.com>,
        Sasha Levin <sashal@kernel.org>
Subject: Re: [PATCH 4.19 044/113] iio: iio-utils: Fix possible incorrect mask
 calculation
Message-ID: <20190730185114.GA9061@amd>
References: <20190729190655.455345569@linuxfoundation.org>
 <20190729190706.217460325@linuxfoundation.org>
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Hi!

> [ Upstream commit 208a68c8393d6041a90862992222f3d7943d44d6 ]
>=20
> 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:
>=20
> *mask =3D (1 << 32) - 1;
>=20
> 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 =3D (endianchar =3D=3D 'b');
>  			*bytes =3D padint / 8;
>  			if (*bits_used =3D=3D 64)
> -				*mask =3D ~0;
> +				*mask =3D ~(0ULL);
>  			else
> -				*mask =3D (1ULL << *bits_used) - 1;
> +				*mask =3D (1ULL << *bits_used) - 1ULL;
> =20

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
--=20
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blo=
g.html

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