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

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Date:	Tue, 17 Jun 2008 23:30:44 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Linus Torvalds <torvalds@...ux-foundation.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)

Linus Torvalds wrote:
> 
> On Tue, 17 Jun 2008, Andi Kleen wrote:
>> If that fixes anything:
>> - The caller is broken because it shouldn't pass a faulting source to copy_to_user()
>> - And you broken copy_from_user error reporting which shares the same code
> 
> Andi, I'm sorry I cc'd you. You are the author of that crap, but the bug 
> seems to be that you never even understood what copy_from_user() is 
> supposed to do.
> 
> 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. 

yes, but only one of them (destination for copy_to_user and source for
copy_from_user)

Or are you're describing copy_in_user()?

> I don't really see why you continually start arguing about things that are 
> OBVIOUSLY BUGGY, as if they weren't buggy. Once somebody has debugged a 
> buggy routine, you shouldn't argue against it. 
> 
> So here's a hint: next time I claim some code of yours is buggy, either 
> just acknowledge the bug, or stay silent. You'll look smarter that way.

Ok if I'm really wrong on this (but frankly I don't see the mistake, sorry)
for my person edification: what's a legitimate case for copy_to_user()
where the source can fault?

-Andi


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