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

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Date:	Thu, 23 Aug 2012 11:22:01 -0400
From:	"J. Bruce Fields" <bfields@...ldses.org>
To:	Cyrill Gorcunov <gorcunov@...nvz.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 04:34:22PM +0400, Cyrill Gorcunov wrote:
> 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.

Fine, just make sure that's explained in the changelog.

For distributors looking for patches to backport, "fix nil dereference"
may set off alarm bells unnecessarily.

--b.

> ---
> 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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