linux-kernel - Re: Word-at-a-time dcache name accesses (was Re: .. anybody know of any filesystems that depend on the exact VFS 'namehash' implementation?)

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Message-ID: <4F516DBA.2030809@zytor.com>
Date:	Fri, 02 Mar 2012 17:02:50 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Andi Kleen <andi@...stfloor.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	Al Viro <viro@...iv.linux.org.uk>
Subject: Re: Word-at-a-time dcache name accesses (was Re: .. anybody know
 of any filesystems that depend on the exact VFS 'namehash' implementation?)

On 03/02/2012 04:57 PM, Linus Torvalds wrote:
> On Fri, Mar 2, 2012 at 4:38 PM, H. Peter Anvin <hpa@...or.com> wrote:
>>
>> My biggest concern is what happens when this happens to be at the end of
>> mapped kernel memory and we overrun the page?
> 
> Yes. It's very unlikely, and it never happens with the dentry data
> itself (the name is always aligned for those).
> 
> But it *can* happen if:
> 
>  - the page contains the filename we copied from user space
> 
>  - the page is the last page mapped
> 
>  - the filename is PATH_MAX in size (or very close)
> 
>  - the last component is sufficiently unaligned
> 
> but I was thinking we'd just make sure not to free the last page, and
> just solve it that way.
> 
> I was playing around with other ideas (take the page fault and fix it
> up), but those are all really complicated when the notion of "don't
> use the last page" is so much simpler.
> 

Note that does mean we need a guard page after each and every
discontiguous RAM range, not just the last one.  Raising that issue
since we have had serious bugs in that area in the past.

	-hpa


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