From: Guenter Roeck <linux@roeck-us.net>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: linux-kernel@vger.kernel.org, lm-sensors@lm-sensors.org,
Juergen Beisert <jbe@pengutronix.de>,
Jean Delvare <khali@linux-fr.org>
Subject: Re: [lm-sensors] [PATCH v2] linux/kernel.h: Fix DIV_ROUND_CLOSEST with unsigned divisors
Date: Wed, 19 Dec 2012 22:41:22 +0000 [thread overview]
Message-ID: <20121219224122.GA26863@roeck-us.net> (raw)
In-Reply-To: <20121219134721.bbde212f.akpm@linux-foundation.org>
On Wed, Dec 19, 2012 at 01:47:21PM -0800, Andrew Morton wrote:
> On Wed, 19 Dec 2012 06:40:15 -0800
> Guenter Roeck <linux@roeck-us.net> wrote:
>
> > Commit 263a523 fixes a warning seen with W=1 due to change in
> > DIV_ROUND_CLOSEST. Unfortunately, the C compiler converts divide operations
> > with unsigned divisors to unsigned, even if the dividend is signed and
> > negative (for example, -10 / 5U = 858993457). The C standard says "If one
> > operand has unsigned int type, the other operand is converted to unsigned
> > int", so the compiler is not to blame.
> > As a result, DIV_ROUND_CLOSEST(0, 2U) and similar operations now return
> > bad values, since the automatic conversion of expressions such as "0 - 2U/2"
> > to unsigned was not taken into account.
> >
> > Fix by checking for the divisor variable type when deciding which operation
> > to perform. This fixes DIV_ROUND_CLOSEST(0, 2U), but still returns bad values
> > for negative dividends divided by unsigned divisors. Mark the latter case as
> > unsupported.
>
> The changelog didn't describe the end-user visible effects of the bug.
> Please always include this information. Because...
>
One observed effect is that the s2c_hwmon driver reports a value of 4198403
instead of 0 if the ADC reads 0.
Other impact is unpredictable. Problem is seen if the divisor is an unsigned
variable or constant and the dividend is less than (divisor/2).
> The patch is applicable to 3.7.x. Should we backport it?
>
Yes. DIV_ROUND_CLOSEST is used throughout the kernel, and impact is
unpredictable.
3.6 needs it as well.
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WARNING: multiple messages have this Message-ID (diff)
From: Guenter Roeck <linux@roeck-us.net>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: linux-kernel@vger.kernel.org, lm-sensors@lm-sensors.org,
Juergen Beisert <jbe@pengutronix.de>,
Jean Delvare <khali@linux-fr.org>
Subject: Re: [PATCH v2] linux/kernel.h: Fix DIV_ROUND_CLOSEST with unsigned divisors
Date: Wed, 19 Dec 2012 14:41:22 -0800 [thread overview]
Message-ID: <20121219224122.GA26863@roeck-us.net> (raw)
In-Reply-To: <20121219134721.bbde212f.akpm@linux-foundation.org>
On Wed, Dec 19, 2012 at 01:47:21PM -0800, Andrew Morton wrote:
> On Wed, 19 Dec 2012 06:40:15 -0800
> Guenter Roeck <linux@roeck-us.net> wrote:
>
> > Commit 263a523 fixes a warning seen with W=1 due to change in
> > DIV_ROUND_CLOSEST. Unfortunately, the C compiler converts divide operations
> > with unsigned divisors to unsigned, even if the dividend is signed and
> > negative (for example, -10 / 5U = 858993457). The C standard says "If one
> > operand has unsigned int type, the other operand is converted to unsigned
> > int", so the compiler is not to blame.
> > As a result, DIV_ROUND_CLOSEST(0, 2U) and similar operations now return
> > bad values, since the automatic conversion of expressions such as "0 - 2U/2"
> > to unsigned was not taken into account.
> >
> > Fix by checking for the divisor variable type when deciding which operation
> > to perform. This fixes DIV_ROUND_CLOSEST(0, 2U), but still returns bad values
> > for negative dividends divided by unsigned divisors. Mark the latter case as
> > unsupported.
>
> The changelog didn't describe the end-user visible effects of the bug.
> Please always include this information. Because...
>
One observed effect is that the s2c_hwmon driver reports a value of 4198403
instead of 0 if the ADC reads 0.
Other impact is unpredictable. Problem is seen if the divisor is an unsigned
variable or constant and the dividend is less than (divisor/2).
> The patch is applicable to 3.7.x. Should we backport it?
>
Yes. DIV_ROUND_CLOSEST is used throughout the kernel, and impact is
unpredictable.
3.6 needs it as well.
next prev parent reply other threads:[~2012-12-19 22:41 UTC|newest]
Thread overview: 20+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-12-19 14:40 [lm-sensors] [PATCH v2] linux/kernel.h: Fix DIV_ROUND_CLOSEST with unsigned divisors Guenter Roeck
2012-12-19 14:40 ` Guenter Roeck
2012-12-19 21:47 ` [lm-sensors] " Andrew Morton
2012-12-19 21:47 ` Andrew Morton
2012-12-19 22:41 ` Guenter Roeck [this message]
2012-12-19 22:41 ` Guenter Roeck
2012-12-20 10:22 ` [lm-sensors] " Jean Delvare
2012-12-20 10:22 ` Jean Delvare
2012-12-20 10:30 ` [lm-sensors] " Juergen Beisert
2012-12-20 10:30 ` Juergen Beisert
2012-12-20 11:00 ` [lm-sensors] " Jean Delvare
2012-12-20 11:00 ` Jean Delvare
2012-12-20 14:13 ` [lm-sensors] " Guenter Roeck
2012-12-20 14:13 ` Guenter Roeck
2012-12-19 22:21 ` [lm-sensors] " Jean Delvare
2012-12-19 22:21 ` Jean Delvare
2012-12-19 23:01 ` [lm-sensors] " Guenter Roeck
2012-12-19 23:01 ` Guenter Roeck
2012-12-20 11:48 ` [lm-sensors] " Jean Delvare
2012-12-20 11:48 ` Jean Delvare
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