linux-kernel - Re: [patch 0/5] Support for sanitization flag in low-level page allocator

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Message-ID: <4A20EF5C.10702@cs.helsinki.fi>
Date:	Sat, 30 May 2009 11:33:32 +0300
From:	Pekka Enberg <penberg@...helsinki.fi>
To:	"Larry H." <research@...reption.com>
CC:	Alan Cox <alan@...rguk.ukuu.org.uk>, Ingo Molnar <mingo@...e.hu>,
	Rik van Riel <riel@...hat.com>, linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...l.org>, linux-mm@...ck.org,
	Ingo Molnar <mingo@...hat.com>, pageexec@...email.hu,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [patch 0/5] Support for sanitization flag in low-level page	allocator

Hi Larry,

On 10:53 Sat 30 May, Pekka Enberg wrote:
>>> That's hopeless, and kzfree is broken. Like I said in my earlier reply,
>>> please test that yourself to see the results. Whoever wrote that ignored
>>> how SLAB/SLUB work and if kzfree had been used somewhere in the kernel
>>> before, it should have been noticed long time ago.
>> An open-coded version of kzfree was being used in the kernel:
>>
>> http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commitdiff;h=00fcf2cb6f6bb421851c3ba062c0a36760ea6e53
>>
>> Can we now get to the part where you explain how it's broken because I 
>> obviously "ignored how SLAB/SLUB works"?

Larry H. wrote:
> You can find the answer in the code of sanitize_obj, within my kfree
> patch. Besides, it would have taken less time for you to write a simple
> module that kmallocs and kzfrees a buffer, than writing these two
> emails.
> 
> Consider the inuse, size, objsize and offset members of a kmem_cache
> structure, for further hints. Test the module on a system with SLUB,
> though the issue should replicate over SLAB too. And don't dare test it
> on SLOB and its wonderful ksize, or even look at the freelist pointer
> management within SLUB.

Thank you for the lesson in slab internals! That said, I did go over 
your patch and am still as confused as ever. I am afraid I have to 
consider this discussion done unless you're willing to share your 
knowledge on the subject.

			Pekka
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