From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754411Ab0GWIia (ORCPT ); Fri, 23 Jul 2010 04:38:30 -0400 Received: from cantor.suse.de ([195.135.220.2]:55293 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752090Ab0GWIi1 (ORCPT ); Fri, 23 Jul 2010 04:38:27 -0400 Date: Fri, 23 Jul 2010 10:37:59 +0200 From: Jan Kara To: shenghui Cc: Jan Kara , linux-ext4 , linux-kernel , kernel-janitors Subject: Re: [PATCH] check name_len before down_read xattr_sem and sb_read in ext2_xattr_get Message-ID: <20100723083759.GA3305@quack.suse.cz> References: <201007122229025316610@gmail.com> <20100721174457.GE1215@atrey.karlin.mff.cuni.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: User-Agent: Mutt/1.5.20 (2009-06-14) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu 22-07-10 08:03:18, shenghui wrote: > 2010/7/22 Jan Kara : > >> Hi, > >>        I walked through ext2_xattr_get, and felt that we can > >> do some optimization on it. For name_len check, it's done > >> after down xattr_sem and sb_read, both of which are time > >> consuming operation compared with strlen: > >>          down_read(&EXT2_I(inode)->xattr_sem); > >>  ... > >>          bh = sb_bread(inode->i_sb, EXT2_I(inode)->i_file_acl); > >>  ... > >>          /* find named attribute */ > >>          name_len = strlen(name); > >> > >>          error = -ERANGE; > >>          if (name_len > 255) > >>                  goto cleanup; > >> > >>        Most of the case, you'll get one valid block, but if the > >> name len > 255, then the xattr_sem down and sb_bread operation > >> can be seen as a waste of time. So I think we'd better do > >> name len check as early as possible. > >>        Following is my patch, and it's against 2.6.35-rc4. > >> Please check it. > >> > >> Signed-off-by: Wang Sheng-Hui > >> --- > >>  fs/ext2/xattr.c |   12 +++++++----- > >>  1 files changed, 7 insertions(+), 5 deletions(-) > >> > >> diff --git a/fs/ext2/xattr.c b/fs/ext2/xattr.c > >> index 7c39157..0b94d61 100644 > >> --- a/fs/ext2/xattr.c > >> +++ b/fs/ext2/xattr.c > >> @@ -161,6 +161,13 @@ ext2_xattr_get(struct inode *inode, int name_index, const char *name, > >> > >>       if (name == NULL) > >>               return -EINVAL; > >> + > >> +     /* find named attribute */ > >> +     name_len = strlen(name); > >> +     error = -ERANGE; > >> +     if (name_len > 255) > >> +             goto cleanup; > >  But you cannot go to cleanup here because you don't hold xattr_sem... > > > > Sorry, I'm a little confused by your words. > The patch just checks name_len, and it > doesn't need xattr_sem. 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. Honza -- Jan Kara SUSE Labs, CR