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

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Date:	Tue, 17 Jun 2008 13:16:39 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Bron Gondwana <brong@...tmail.fm>
cc:	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>, Andi Kleen <ak@...e.de>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: BUG: mmapfile/writev spurious zero bytes (x86_64/not i386,
 bisected, reproducable)

On Tue, 17 Jun 2008, Linus Torvalds wrote:
> 
> Bron, does this untested patch hide the bug?

Ok, it does hide the bug.

> I don't think this patch is correct, because it doesn't really fix the 
> basic issue (the code should do the right thing even if a page isn't 
> there), but it might hide it by faulting in the whole "bytes" range rather 
> than just the first iov.
> 
> So Nick, it's still over to you, but if this does hide it, then that's an 
> interesting detail in itself.

I actually am starting to think that the bug is in 
__copy_to_user_inatomic_nocache().

The return value of that thing in the case of a failure seems to be 
totally wrong. In particular, since that function does an unrolled loop of 
accesses like so:

	.Ls1:   movq (%rsi),%r11
	.Ls2:   movq 1*8(%rsi),%r8
	.Ls3:   movq 2*8(%rsi),%r9
	.Ls4:   movq 3*8(%rsi),%r10
	.Ld1:   movnti %r11,(%rdi)
	.Ld2:   movnti %r8,1*8(%rdi)
	.Ld3:   movnti %r9,2*8(%rdi)
	.Ld4:   movnti %r10,3*8(%rdi)

it may have done three of the 64-bit loads, and then trap on the fouth: 
but since it hasn't done a _single_ of the stores, it shouldn't count as 
any different whether it traps on any of .Ls[1-4]. But that code 
definitely seems to think it makes a difference whether the exception 
happened at Ls1 or at Ls4, even though both points have copied _exactly_ 
as many bytes when the exception happens.

So I'm starting to think the bug is all in there, not in the VM itself. 
See arch/x86/lib/copy_user_nocache.S.

In fact, even the comment there implies that the author didn't know or 
care about returning the correct value.

Andi and Ingo added to Cc.

			Linus
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