linux-kernel - Re: [patch 4/9] fs, exportfs: Fix nil dereference if no s_export

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Message-ID: <20120823123422.GB9112@moon>
Date:	Thu, 23 Aug 2012 16:34:22 +0400
From:	Cyrill Gorcunov <gorcunov@...nvz.org>
To:	"J. Bruce Fields" <bfields@...ldses.org>
Cc:	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	Al Viro <viro@...iv.linux.org.uk>,
	Alexey Dobriyan <adobriyan@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Pavel Emelyanov <xemul@...allels.com>,
	James Bottomley <jbottomley@...allels.com>,
	Matthew Helsley <matt.helsley@...il.com>,
	aneesh.kumar@...ux.vnet.ibm.com
Subject: Re: [patch 4/9] fs, exportfs: Fix nil dereference if no s_export_op
 present

On Thu, Aug 23, 2012 at 08:12:30AM -0400, J. Bruce Fields wrote:
> On Thu, Aug 23, 2012 at 02:43:27PM +0400, Cyrill Gorcunov wrote:
> > If there is no s_export_op present in a target superblock
> > we might have nil dereference.
> 
> Is that NULL dereference possible with current code, or is it a check
> you're adding to account for a new caller that you're about to add?
> 
> I believe it's the latter, but this would be a good thing to make clear
> in the changelog.

With the current code it seems to be impossible (well, i can't be sure
about nfs caller) because do_sys_name_to_handle does check for s_export_op
to exist. Updated changelog below. After all I think not checking
s_export_op was a mistake in general -- this routine is exported to
other modules but has no a single line of comment about possibility
of nil dereference.
---
From: Cyrill Gorcunov <gorcunov@...nvz.org>
Subject: fs, exportfs: Escape nil dereference if no s_export_op present

This routine will be used to generate a file handle in fdinfo
output for inotify subsystem, where if no s_export_op present
the general export_encode_fh should be used. Thus add
a test if s_export_op present inside exportfs_encode_fh itself.

Signed-off-by: Cyrill Gorcunov <gorcunov@...nvz.org>
CC: Pavel Emelyanov <xemul@...allels.com>
CC: Al Viro <viro@...IV.linux.org.uk>
CC: Alexey Dobriyan <adobriyan@...il.com>
CC: Andrew Morton <akpm@...ux-foundation.org>
CC: James Bottomley <jbottomley@...allels.com>
CC: "Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
CC: Alexey Dobriyan <adobriyan@...il.com>
CC: Matthew Helsley <matt.helsley@...il.com>
CC: "J. Bruce Fields" <bfields@...ldses.org>
CC: "Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
---
 fs/exportfs/expfs.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

Index: linux-2.6.git/fs/exportfs/expfs.c
===================================================================
--- linux-2.6.git.orig/fs/exportfs/expfs.c
+++ linux-2.6.git/fs/exportfs/expfs.c
@@ -357,7 +357,7 @@ int exportfs_encode_fh(struct dentry *de
 		 */
 		parent = p->d_inode;
 	}
-	if (nop->encode_fh)
+	if (nop && nop->encode_fh)
 		error = nop->encode_fh(inode, fid->raw, max_len, parent);
 	else
 		error = export_encode_fh(inode, fid, max_len, parent);
--
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