netdev - Re: [PATCH][CAN]: Fix copy_from_user() results interpretation.

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Date:	Sat, 26 Apr 2008 15:04:30 +0200
From:	Wolfgang Grandegger <wg@...ndegger.com>
To:	David Miller <davem@...emloft.net>
CC:	sam@...nborg.org, oliver.hartkopp@...kswagen.de,
	socketcan-core@...ts.berlios.de, netdev@...r.kernel.org,
	urs.thuermann@...kswagen.de, xemul@...nvz.org
Subject: Re: [PATCH][CAN]: Fix copy_from_user() results interpretation.

David Miller wrote:
> From: Sam Ravnborg <sam@...nborg.org>
> Date: Sat, 26 Apr 2008 08:40:07 +0200
> 
>> On Sat, Apr 26, 2008 at 08:19:31AM +0200, Wolfgang Grandegger wrote:
>>> What about removing the assignment "err =" in that case.
>> Preferred.
>>
>> See sample patch below (made on top of -linus
>> so it does likely not apply but made it only to
>> see the difference anyway).
> 
> I want to make one comment, directed at Wolfgang.
> 
> You are absolutely wrong, Wolfgang, in saying that Pavel's
> original patch isn't easier to review than just changing
> this code to go:
> 
> 	if (copy_*_user())
> 		return -EFAULT;
> 
> In fact, recoding things like this is an immense extra hardship on a
> reviewer.  I'll explain why.
> 
> If I see a patch that changes:
> 
> 	err = SOMETHING;
> 	break;
> 
> into:
> 
> 	err = SOMETHING_ELSE;
> 	break;
> 
> I know, WITH JUST READING THE PATCH, exactly what the side effects of
> this change are.
> 
> I DO NOT need to bring the code into my editor and validate side
> effects to the surrounding code.
> 
> I know that the assignment to 'err' is being changed, and that's it.
> 
> Whereas if you change:
> 
> 	err = SOMETHING;
> 	break;
> 
> into:
> 
> 	if (SOMETHING)
> 		return -SOME_ERROR;
> 	break;
> 
> I now have to bring the code into an editor and make sure that the
> control flow change doesn't break things.
> 
> For example, maybe the exit of the switch statement was important, to
> make sure cleanup code runs at the end of the function to release
> locks, free allocated memory, etc.
> 
> With Pavel's patch it is not necessary to make such validations so
> it's INFINITELY easier to validate.

OK, I see, if you have to review a lot of patches, it does matter, indeed.

Wolfgang.

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