linux-kernel - Re: BUG: mmapfile/writev spurious zero bytes (x86

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Message-ID: <20080617213618.GL28946@ZenIV.linux.org.uk>
Date:	Tue, 17 Jun 2008 22:36:18 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Andi Kleen <andi@...stfloor.org>,
	Bron Gondwana <brong@...tmail.fm>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Nick Piggin <npiggin@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Rob Mueller <robm@...tmail.fm>, Ingo Molnar <mingo@...e.hu>
Subject: Re: BUG: mmapfile/writev spurious zero bytes (x86_64/not i386,
	bisected, reproducable)

On Tue, Jun 17, 2008 at 02:24:39PM -0700, Linus Torvalds wrote:

> The whole *and*only* reason for copy_to/from_user() existing AT ALL is 
> exactly the fact that the source or destination access can fault. 

Erm...  The reason for copy_to_user() existing is that dereferencing
a user-supplied address in the kernel does not have to access any
memory reachable in user mode, regardless of any faults.  WTF are you
guys talking about?
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