linux-kernel - Re: [PATCH] Avoid overflows in kernel/time.c

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Message-Id: <20071129175440.ead82de6.akpm@linux-foundation.org>
Date:	Thu, 29 Nov 2007 17:54:40 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] Avoid overflows in kernel/time.c

On Thu, 29 Nov 2007 16:19:51 -0800 "H. Peter Anvin" <hpa@...or.com> wrote:

> When the conversion factor between jiffies and milli- or microseconds
> is not a single multiply or divide, as for the case of HZ == 300, we
> currently do a multiply followed by a divide.  The intervening
> result, however, is subject to overflows, especially since the
> fraction is not simplified (for HZ == 300, we multiply by 300 and
> divide by 1000).
> 
> This is exposed to the user when passing a large timeout to poll(),
> for example.
> 
> This patch replaces the multiply-divide with a reciprocal
> multiplication on 32-bit platforms.  When the input is an unsigned
> long, there is no portable way to do this on 64-bit platforms there is
> no portable way to do this since it requires a 128-bit intermediate
> result (which gcc does support on 64-bit platforms but may generate
> libgcc calls, e.g. on 64-bit s390), but since the output is a 32-bit
> integer in the cases affected, just simplify the multiply-divide
> (*3/10 instead of *300/1000).
> 
> The reciprocal multiply used can have off-by-one errors in the upper
> half of the valid output range.  This could be avoided at the expense
> of having to deal with a potential 65-bit intermediate result.  Since
> the intent is to avoid overflow problems and most of the other time
> conversions are only semiexact, the off-by-one errors were considered
> an acceptable tradeoff.
> 
> NOTE: This patch uses a bc(1) script to compute the appropriate
> constants.

Does this add the first dependency upon the availability of bc?
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