linux-kernel - Re: [PATCH] fix sysfs_readdir oops (was Re: sysfs reclaim crash)

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Message-ID: <461756D8.8070002@gmail.com>
Date:	Sat, 07 Apr 2007 17:31:20 +0900
From:	Tejun Heo <htejun@...il.com>
To:	maneesh@...ibm.com
CC:	Ethan Solomita <solo@...gle.com>,
	Dipankar Sarma <dipankar@...ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Greg KH <greg@...ah.com>, Martin Bligh <mbligh@...gle.com>,
	Rohit Seth <rohitseth@...gle.com>, viro@....linux.org.uk,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] fix sysfs_readdir oops (was Re: sysfs reclaim crash)

Hello, Maneesh.

Maneesh Soni wrote:
> o sysfs_d_iput() is invoked in dentry reclaim path under memory pressure. This
>   happens without i_mutex. It also nullifies s_dentry to indicate that
>   the associated dentry is evicted. sysfs_readdir() accesses the s_dentry,
>   and gets the inode number from the associated dentry->d_inode, if 
>   there is one, else it invokes iunique(). This can create a race situation,
>   and crash while accessing the d_inode in sysfs_readdir(). 
> 
> o The race happens when the dentry is getting reclaimed and detached from
>   the corresponding sysfs_dirent though sysfs_dirent is still a valid
>   node. Accessing dentry fields are ok as it is under RCU but the inode is
>   not hence we may see oops accessing dentry->d_inode->i_no.
> 
> o The following patch always use i_unique() to get the inode number in
>   sysfs_readdir. This is ok as sysfs doesnot have permanent inode numbering.
>   It could be slower but avoids the oops. 

This isn't correct as i_unique() assumes the inode is in inode hash
table which isn't true for sysfs.  This can result in duplicate inode
numbers.  Please take a look at the following alternative fix.

  http://article.gmane.org/gmane.linux.kernel/513325

Thanks.

-- 
tejun
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