linux-kernel - Re: [PATCH] check name_len before down_read xattr_sem and sb_read in ext2_xattr

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Message-ID: <20100723124657.GA13090@thunk.org>
Date:	Fri, 23 Jul 2010 08:46:57 -0400
From:	Ted Ts'o <tytso@....edu>
To:	Jan Kara <jack@...e.cz>
Cc:	shenghui <crosslonelyover@...il.com>,
	linux-ext4 <linux-ext4@...r.kernel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	kernel-janitors <kernel-janitors@...r.kernel.org>
Subject: Re: [PATCH] check name_len before down_read xattr_sem and sb_read
 in ext2_xattr_get

On Fri, Jul 23, 2010 at 10:37:59AM +0200, Jan Kara wrote:
>   Checking of name_len is fine as you did it. But I wanted to point out
> that if name_len is greater than 255, you then go to 'cleanup' label which
> tries to do up_read(&EXT2_I(inode)->xattr_sem). But that's a bug because
> after you moved the code, we don't hold xattr_sem at the moment we check
> name_len.

Yup, you could just return -ERANGE right there.

The simpler fix though might be to just delete the check altogether.
Neither ext3 nor ext4 checks for the length of the xattr name in their
_xattr_get() function.  Instead they'll just do the search, and then
return -ENODATA.  That seems legit; there can be no entries larger
than 255, so saying the extended attribute doesn't exist is quite
correct.  There is a check in the _xattr_set() functions for both ext3
and ext4, which is quite correct and proper.

Does that mean we'll end up doing a search before returning an error
--- yes, but I don't think that matters.  Why should we care about
optimizing an error case?  It's not like this is going to be in a
timing sensitive part of an application....  (of course the same
consideration could apply to your patch as well).

	      	    	     	  	- Ted
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