linux-kernel - Re[2]: [PATCH] fork_init: fix division by zero

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Date:	Wed, 10 Dec 2008 13:29:12 +0300
From:	Yuri Tikhonov <yur@...raft.com>
To:	Al Viro <viro@...IV.linux.org.uk>
CC:	Geert Uytterhoeven <Geert.Uytterhoeven@...ycom.com>,
	<linux-kernel@...r.kernel.org>, <linuxppc-dev@...abs.org>,
	Detlev Zundel <dzu@...x.de>, Wolfgang Denk <wd@...x.de>,
	Milton Miller <miltonm@....com>, Ilya Yanok <yanok@...raft.com>
Subject: Re[2]: [PATCH] fork_init: fix division by zero


 Hello Al,

On Wednesday, December 10, 2008 you wrote:

> On Wed, Dec 10, 2008 at 01:01:13PM +0300, Yuri Tikhonov wrote:

>> >> +     max_threads = mempages * PAGE_SIZE / (8 * THREAD_SIZE);
>> >                       ^^^^^^^^^^^^^^^^^^^^
>> >> +#endif
>> 
>> > Can't this overflow, e.g. on 32-bit machines with HIGHMEM?
>> 
>>  The multiplier here is not PAGE_SIZE, but [PAGE_SIZE / (8 * 
>> THREAD_SIZE)], and this value is expected to be rather small (2, 4, or 
>> so).

> x * y / z is parsed as (x * y) / z, not x * (y / z).

 Here we believe in preprocessor: since all PAGE_SIZE, 8, and 
THREAD_SIZE are the constants we expect it will calculate this.

 E.g. here is the result from this line as produced by cross-gcc 
4.2.2:

        lis     r9,0
        rlwinm  r29,r29,2,16,29
        stw     r29,0(r9)

 As you see - only rotate-left, i.e. multiplication to the constant.

 In any case, adding braces as follows probably would be better:

+     max_threads = mempages * (PAGE_SIZE / (8 * THREAD_SIZE));

 Right ?

> Only assignment operators (and ?:, in a sense that a ? b : c ? d : e is
> parsed as a ? b : (c ? d : e)) are right-to-left.  The rest is left-to-right.



 Regards, Yuri

 --
 Yuri Tikhonov, Senior Software Engineer
 Emcraft Systems, www.emcraft.com

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