linux-kernel - Re: Unsigned widening casts of binary "not" operations..

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Message-Id: <20130423.135657.824070337983596004.davem@davemloft.net>
Date:	Tue, 23 Apr 2013 13:56:57 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	torvalds@...ux-foundation.org
Cc:	David.Laight@...lab.com, mingo@...nel.org, hpa@...or.com,
	tglx@...utronix.de, tytso@....edu, linux-kernel@...r.kernel.org,
	x86@...nel.org, netdev@...r.kernel.org, linux-ext4@...r.kernel.org
Subject: Re: Unsigned widening casts of binary "not" operations..

From: Linus Torvalds <torvalds@...ux-foundation.org>
Date: Tue, 23 Apr 2013 10:52:33 -0700

> On Tue, Apr 23, 2013 at 10:37 AM, David Miller <davem@...emloft.net> wrote:
>>
>> I just want to mention that this is dangerous in different ways, we
>> just recently got a patch in the networking that removed such a cast.
>> The problem is when the cast narrows, f.e.:
>>
>>         ~(u8)0
>>
>> doesn't do what you think it does.  That doesn't evaluate to 0xff.
> 
> Yeah, sparse will get that right, but won't warn about it even with my
> patch. The normal "all arithmetic is done in *at*least* 'int'" will
> always kick any C expression like that up to 'int' before the binary
> not op is done. So in your example, the implicit cast is widening the
> value *before* the binary not, not after.

If you're not bored, and could add a check for that kind of narrowing
situation, I'd really appreciate it.
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