linux-kernel - Re: [PATCH] applicom: Prevent unsigned wrap in ac

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Date:	Tue, 11 Aug 2009 11:00:15 +0200
From:	Jiri Slaby <jirislaby@...il.com>
To:	roel kluin <roel.kluin@...il.com>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] applicom: Prevent unsigned wrap in ac_interrupt()

On 08/11/2009 10:54 AM, roel kluin wrote:
>>> unsigned i wraps if this occurs in the first iteration.
>>
>> Could you elaborate? I don't quite understand the point.
> 
> `i' is unsigned. The last test in the loop is:
> 
> if(readb(apbs[i].RamIO + RAM_IT_TO_PC)) {
>         /* There's another int waiting on this card */
>         spin_unlock(&apbs[i].mutex);
>         i--;
> } else {
>         spin_unlock(&apbs[i].mutex);
> }
> 
> In the first iteration `i' is 0, so if this condition evaluates to true
> then `i' becomes 0xffffffff (since it's unsigned), the for loop test
> fails and the i++ never occurs.

Hmm, no. This is not how three `for' expressions are evaluated. The CFG
of "for (a; b; c) d;" is "a->(b->d->c)*". Read 6.8.5.3 of ANSI C99.

I.e. 0xff increments back to 0.

Am I still missing something?
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