linux-kernel - Re: 2.6.27-rc7-sha1: EIP at proc_sys

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Date:	Sun, 28 Sep 2008 20:05:22 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Hugh Dickins <hugh@...itas.com>, Al Viro <viro@...IV.linux.org.uk>,
	Alexey Dobriyan <adobriyan@...il.com>,
	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: 2.6.27-rc7-sha1: EIP at proc_sys_compare+0x36/0x50

Linus Torvalds <torvalds@...ux-foundation.org> writes:

> I actually like my second patch better - it looks simpler, and it means 
> that the rules for filesystems using d_compare() are a bit clearer: at 
> least we'll only pass them dentries to look at that haven't gone through 
> d_drop (and we do hold dentry->d_lock that serializes all of that).
>
> So here it is again (I sent it out just minutes ago, but you weren't on 
> that cc, you must have picked this up off the kernel list)
>
> NOTE! Totally untested patch! It looks sane and really obvious, but maybe 
> it has some insane and non-obvious bug.

We definitely have a race between d_kill setting dentry->d_inode = NULL
and proc_sys_compare reading d_inode. 

We don't generate negative dentries for /proc/sys.

In dput atomic_dec_and_lock takes the lock before setting the count to 0.
So there is no race there.

Testing for d_unhashed and getting us out of rcu limbo before calling
into the filesystem methods makes the reasoning a lot clearer.

Looks good to me.

Reviewed-by: "Eric W. Biederman" <ebiederm@...ssion.com>

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