linux-kernel - Re: [PATCH] Avoid overflows in kernel/time.c (version 5)

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Date:	Sat, 22 Dec 2007 18:58:22 -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>,
	Linux Arch Mailing List <linux-arch@...r.kernel.org>,
	Ralf Baechle <ralf@...ux-mips.org>,
	Sam Ravnborg <sam@...nborg.org>,
	Paul Mundt <lethal@...ux-sh.org>,
	Richard Henderson <rth@...ddle.net>,
	Michael Starvik <starvik@...s.com>,
	David Howells <dhowells@...hat.com>,
	Yoshinori Sato <ysato@...rs.sourceforge.jp>,
	Hirokazu Takata <takata@...ux-m32r.org>,
	Geert Uytterhoeven <geert@...ux-m68k.org>,
	Roman Zippel <zippel@...ux-m68k.org>,
	"William L. Irwin" <sparclinux@...r.kernel.org>,
	Chris Zankel <chris@...kel.net>,
	Jan Engelhardt <jengelh@...putergmbh.de>
Subject: Re: [PATCH] Avoid overflows in kernel/time.c (version 5)

On Mon, 17 Dec 2007 13:53:23 -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).


alpha allmodconfig:

kernel/time.c: In function 'jiffies_to_msecs':
kernel/time.c:248: error: 'HZ_TO_MSEC_NUM' undeclared (first use in this function)
kernel/time.c:248: error: (Each undeclared identifier is reported only once
kernel/time.c:248: error: for each function it appears in.)
kernel/time.c:248: error: 'HZ_TO_MSEC_DEN' undeclared (first use in this function)
kernel/time.c: In function 'jiffies_to_usecs':
kernel/time.c:264: error: 'HZ_TO_USEC_NUM' undeclared (first use in this function)
kernel/time.c:264: error: 'HZ_TO_USEC_DEN' undeclared (first use in this function)
kernel/time.c: In function 'usecs_to_jiffies':
kernel/time.c:486: error: 'm' undeclared (first use in this function)
kernel/time.c:489: warning: control reaches end of non-void function
kernel/time.c: In function 'jiffies_to_usecs':
kernel/time.c:267: warning: control reaches end of non-void function
kernel/time.c: In function 'jiffies_to_msecs':
kernel/time.c:251: warning: control reaches end of non-void function

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