netdev - Re: [PATCH] intel: i40e: fix confused code

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Message-ID: <87wpuim2c9.fsf@rasmusvillemoes.dk>
Date:	Tue, 20 Oct 2015 09:22:14 +0200
From:	Rasmus Villemoes <linux@...musvillemoes.dk>
To:	"Nelson\, Shannon" <shannon.nelson@...el.com>
Cc:	"Kirsher\, Jeffrey T" <jeffrey.t.kirsher@...el.com>,
	"Brandeburg\, Jesse" <jesse.brandeburg@...el.com>,
	"Wyborny\, Carolyn" <carolyn.wyborny@...el.com>,
	"Skidmore\, Donald C" <donald.c.skidmore@...el.com>,
	"Vick\, Matthew" <matthew.vick@...el.com>,
	"Ronciak\, John" <john.ronciak@...el.com>,
	"Williams\, Mitch A" <mitch.a.williams@...el.com>,
	"intel-wired-lan\@lists.osuosl.org" 
	<intel-wired-lan@...ts.osuosl.org>,
	"netdev\@vger.kernel.org" <netdev@...r.kernel.org>,
	"linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] intel: i40e: fix confused code

On Mon, Oct 19 2015, "Nelson, Shannon" <shannon.nelson@...el.com> wrote:

>> From: Rasmus Villemoes [mailto:linux@...musvillemoes.dk]
>> Sent: Saturday, October 17, 2015 1:58 PM
>> Subject: [PATCH] intel: i40e: fix confused code
>> 
>> This code is pretty confused. The variable name 'bytes_not_copied'
>> clearly indicates that the programmer knew the semantics of
>> copy_{to,from}_user, but then the return value is checked for being
>> negative and used as a -Exxx return value.
>> 
>> I'm not sure this is the proper fix, but at least we get rid of the
>> dead code which pretended to check for access faults.
>> 
>> Signed-off-by: Rasmus Villemoes <linux@...musvillemoes.dk>
>
> I believe this patch is unnecessary: if the value is negative, then it
> already is an error code giving some potentially useful information.
> When I dig into the copy_to_user() code, I see in the comments for
> put_user() that -EFAULT is the error being returned.

Thanks, this was precisely the kind of confusion I'm talking about:
copy_{from,to}_user _never_ returns a negative value. It returns
precisely what the very explicit variable name hints.

This is in contrast to the single-scalar functions get_user/put_user,
which do return -EFAULT for error and 0 for success.

(See also lines 479-519 of Documentation/DocBook/kernel-hacking.tmpl).

In the entire kernel source tree, two files contain a check for the
return value from copy_{from,to}_user being negative. It will never
trigger, so might as well be removed - except if it was _supposed_ to be
checking for access violations, in which case one should probably
replace it with actually handling it. Try

  git grep -C2 -E 'copy_(from|to)_user' drivers/net/ethernet/

Rasmus
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