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Date: Mon, 12 May 2014 14:41:49 -0700 (PDT)
From: Don Lewis <truckman@FreeBSD.org>
Subject: Re: svn commit: r265901 - stable/10/sys/kern
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On 12 May, Andrey Chernov wrote:
> On 12.05.2014 21:03, Jilles Tjoelker wrote:
>> On Mon, May 12, 2014 at 08:04:32PM +0400, Andrey Chernov wrote:
>>> On 12.05.2014 8:27, Don Lewis wrote:
>>>> +	if (start + amask < start) {
>>>> +		DPRINTF(("start+amask wrapped around\n"));
>>>> +		goto out;
>>>> +	}
>> 
>>> Checking for overflow _after_ it happens is unportable and dangerous,
>>> since wrapping around is not only one possible result. They should be
>>> rewritten like that:
>> 
>>> if (start > ULONG_MAX - amask)
>> 
>> Unsigned types wrap around per the C standard. Overflow checking after
>> it happens is fine.
>> 
>> You are right for signed types.
>> 
> 
> You are right. The C Standard, 6.2.5, paragraph 9 [ISO/IEC 9899:2011],
> states:
> 
> "A computation involving unsigned operands can never overflow, because a
> result that cannot be represented by the resulting unsigned integer type
> is reduced modulo the number that is one greater than the largest value
> that can be represented by the resulting type."
> 
> I was initially confused by "integer overflow" phrase in the commit's
> comment, mechanically producing example above which supposed to be for
> signed types.

I went ahead and changed the code.  I think the new version makes the
intent of the code clearer.  The compiler is also likely to recognize
that "ULONG_MAX - amask" is loop invariant.