linux-kernel - Re: Clarification needed on use of put

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Message-ID: <20140425190531.GD3636@mguzik.redhat.com>
Date:	Fri, 25 Apr 2014 21:05:32 +0200
From:	Mateusz Guzik <mguzik@...hat.com>
To:	Kumar Gaurav <kumargauravgupta3@...il.com>
Cc:	kernel-janitors@...r.kernel.org,
	Dan Carpenter <dan.carpenter@...cle.com>,
	Greg KH <gregkh@...uxfoundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: Clarification needed on use of put_user inside a loop

On Fri, Apr 25, 2014 at 09:39:57PM +0530, Kumar Gaurav wrote:
> Hence when transferring data involves loops then checking permission
> (using access_ok()) once should be good to go then after we can
> simply transfer data using __put_user(), instead of using put_user()
> itself in loop.
> 

Well, I can't tell you whether this is a good idea, but:

This looks correct and other code is doing this already.

However, put_user calls might_fault, but __put_user consumers I found
(e.g. copy_siginfo_to_user) don't do that.

While it has only debugging purposes and would not change anything for
those consumers, it seems to be a bug to not include it.

Thus I suggest adding access_ok variant which calls might_fault.

-- 
Mateusz Guzik
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