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

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Message-ID: <alpine.LFD.1.10.0806171412470.2907@woody.linux-foundation.org>
Date:	Tue, 17 Jun 2008 14:14:40 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Andi Kleen <andi@...stfloor.org>
cc:	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, 17 Jun 2008, Andi Kleen wrote:
> 
> The x86-64 copy_*_user functions were always designed to return errors
> both ways (as in both for load and for store).

That's not the problem, Andi.

The problem is that it returns THE WRONG VALUE!

If the fault happened on the second load, but the first load was never 
actually paired up with a store (because of unrolling the loop), then YOU 
MUST NOT CLAIM THAT YOU DID A 8-BYTE COPY! Because you have copied exactly 
_zero_ bytes, even though you _loaded_ 8 bytes successfully!

See?

Claiming that you copied 8 bytes when you didn't do anything at all is 
WRONG. It's so incredibly wrong that it is scary. 

			Linus
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