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

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Message-Id: <200806181610.21017.nickpiggin@yahoo.com.au>
Date:	Wed, 18 Jun 2008 16:10:20 +1000
From:	Nick Piggin <nickpiggin@...oo.com.au>
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 Wednesday 18 June 2008 07:46, Linus Torvalds wrote:
> On Tue, 17 Jun 2008, Andi Kleen wrote:

> So the patch _fixes_ copy_from_user(), exactly because it says that even
> if you've loaded 24 bytes, but you faulted on the fourth load, you've
> still _copied_ exactly zero bytes, because you didn't actually store the
> 24 bytes you loaded.

Yes, the new filemap.c code does not require an exact byte count, but
it won't work if there is an under-estimation of the number of bytes
left to copy.

The old filemap.c code actually also relies on the byte count in some
cases, I can't remember off the top of my head, but I *think* it was a
security measure to prevent uninitialized data leak.

Most other cases of course only care about complete success or not, but
there are others. filemap_xip, splice are a couple that pop up.

Thanks for working that all out before I even read my email :)
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