netdev - Re: [net-next 06/16] i40e: Properly cast type for arithmetic

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Message-ID: <4F7AD1BB-DF5D-4391-88F4-F0599A2AF90C@intel.com>
Date:	Wed, 25 Nov 2015 17:12:35 +0000
From:	"Rustad, Mark D" <mark.d.rustad@...el.com>
To:	"Zhang, Helin" <helin.zhang@...el.com>
CC:	"Kirsher, Jeffrey T" <jeffrey.t.kirsher@...el.com>,
	"David S. Miller" <davem@...emloft.net>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"nhorman@...hat.com" <nhorman@...hat.com>,
	"sassmann@...hat.com" <sassmann@...hat.com>,
	"jogreene@...hat.com" <jogreene@...hat.com>,
	"Joe Perches" <joe@...ches.com>
Subject: Re: [net-next 06/16] i40e: Properly cast type for arithmetic

Joe Perches <joe@...ches.com> wrote:

> On Wed, 2015-11-25 at 02:46 -0800, Jeff Kirsher wrote:
>> On Tue, 2015-11-24 at 16:43 -0800, Joe Perches wrote:
>>> On Tue, 2015-11-24 at 16:04 -0800, Jeff Kirsher wrote:
>>>> From: Helin Zhang <helin.zhang@...el.com>
>>>> 
>>>> Pointer of type void * shouldn't be used in arithmetic, which may
>>>> result in compilation error. Casting of (u8 *) can be added to fix
>>>> that.
>>> 
>>> void * arithmetic is used quite frequently in the kernel.
>>> 
>>> What compiler emits an error?
>> 
>> When you use the gcc -Wpointer-arith, it generates the warning.
> 
> make W=3 does that
> 
> $ make help
> [...]
> make W=n   [targets] Enable extra gcc checks, n=1,2,3 where
> 		1: warnings which may be relevant and do not occur too often
> 		2: warnings which occur quite often but may still be relevant
> 		3: more obscure warnings, can most likely be ignored
> 
> but I think it should be ignored.

Yes. That doesn't mean that -Wpointer-arith can't catch other possible errors that may be worth fixing, but this particular type is an accepted usage in the kernel and so should be ignored - unless you find something logically wrong, which does not seem to be the case here at all.

If one cared about portability to other compilers that treat void * very differently in this usage, then you would want to deal with all of these. That is not the case with the kernel.

--
Mark Rustad, Networking Division, Intel Corporation

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