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

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Message-ID: <20140425213414.GC18016@ZenIV.linux.org.uk>
Date:	Fri, 25 Apr 2014 22:34:14 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
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:

> I have  found some codes in the driver which use put_user() in loop.
> Can we avoid the overhead of checking the same memory area( where
> put_user() writes) again n again using __put_user() in side loop and
> checking permission using access_ok before entering the loop?

>                         if (put_user(type, dst) ||
>                             put_user(chs_bytes, dst + 1))
>                                 return -EFAULT;
>                         dst += 2;
			  ^^^^^^^^^
Note that increment.  It's *not* "the same memory area" next time
around.  Sure, you can check the whole range once before the loop
and switch the stuff inside to __put_user()/__copy_to_user(), but
it's not guaranteed to buy you any speedup.

BTW, you might be a bit confused about the work done by access_ok() - e.g.
on an architectures with separate kernel and userland MMU contexts it might
very well be a no-op (always return true).  It's *not* checking if user has
permissions of some sort.
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