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

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Message-ID: <CAMuHMdXzNMt63pOa3136Fmrs2tPQzh5SGe2YfCn+yGtzOyMqoQ@mail.gmail.com>
Date:	Wed, 24 Apr 2013 14:36:15 +0200
From:	Geert Uytterhoeven <geert@...ux-m68k.org>
To:	David Miller <davem@...emloft.net>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	David.Laight@...lab.com, Ingo Molnar <mingo@...nel.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"Theodore Ts'o" <tytso@....edu>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"the arch/x86 maintainers" <x86@...nel.org>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	linux-ext4@...r.kernel.org
Subject: Re: Unsigned widening casts of binary "not" operations..

On Tue, Apr 23, 2013 at 7:37 PM, David Miller <davem@...emloft.net> wrote:
> From: Linus Torvalds <torvalds@...ux-foundation.org>
> Date: Tue, 23 Apr 2013 08:42:49 -0700
>
>> An explicit cast fixes it, and shows that you were aware of the issue:
>>
>>    foo &= ~(foo_t)bar;
>>
>> and gcc will generate the right logic. Of course, casts then have
>> their own problems, which your thing avoids (as would just having a
>> "andn" operation in C)
>
> 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.

This is the definition of MAC802154_CHAN_NONE?

We _should_ have noticed this earlier, as old gcc (e.g. 4.1.2) emits a
warning when comparing it to a u8:

net/mac802154/monitor.c: In function ‘mac802154_monitor_xmit’:
net/mac802154/monitor.c:49: warning: comparison is always false due to
limited range of data type
net/mac802154/wpan.c: In function ‘mac802154_wpan_xmit’:
net/mac802154/wpan.c:323: warning: comparison is always false due to
limited range of data type

Interestingly, none of this is seen in the build logs of the linux-next build
service, which uses gcc 4.2.3, 4.2.4, 4.5.1, and 4.6...

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds
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