linux-kernel - Re: [PATCH 0/7] Permit filesystem local caching and NFS superblock sharing [try #13]

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Date:	Mon, 04 Sep 2006 22:23:47 -0400
From:	Trond Myklebust <trond.myklebust@....uio.no>
To:	David Howells <dhowells@...hat.com>
Cc:	Andrew Morton <akpm@...l.org>, torvalds@...l.org,
	steved@...hat.com, linux-fsdevel@...r.kernel.org,
	linux-cachefs@...hat.com, nfsv4@...ux-nfs.org,
	linux-kernel@...r.kernel.org, Ian Kent <raven@...maw.net>
Subject: Re: [PATCH 0/7] Permit filesystem local caching and NFS superblock
	sharing [try #13]

On Mon, 2006-09-04 at 12:52 +0100, David Howells wrote:
> Andrew Morton <akpm@...l.org> wrote:
> 
> > sony:/home/akpm> ls -l /net/bix/usr/src
> > total 0
> > 
> > sony:/home/akpm> showmount -e bix
> > Export list for bix:
> > /           *
> > /usr/src    *
> > /mnt/export *
> 
> Yes, but what's your /etc/exports now?  Not all options appear to showmount.
> 
> Can you add "nohide" to the /usr/src and /mnt/export lines and "fsid=0" to the
> / line if you don't currently have them and try again?
> 
> > iirc, we decided this is related to the fs-cache infrastructure work which
> > went into git-nfs.  I think David can reproduce this?
> 
> I'd only reproduced it with SELinux in enforcing mode.
> 
> Under such conditions, unless there's a readdir on the root directory, the
> subdirs under which exports exist will remain as incorrectly negative
> dentries.
> 
> The problem is a conjunction of circumstances:
> 
>  (1) nfs_lookup() has a shortcut in it that skips contact with the server if
>      we're doing a lookup with intent to create.  This leaves an incorrectly
>      negative dentry if there _is_ actually an object on the server.
> 
>  (2) The mkdir procedure is aborted between the lookup() op and the mkdir() op
>      by SELinux (see vfs_mkdir()).  Note that SELinux isn't the _only_ method
>      by which the abort can occur.
> 
>  (3) One of my patches correctly assigns the security label to the automounted
>      root dentry.
> 
>  (4) SELinux then aborts the automounter's mkdir() call because the automounter
>      does _not_ carry the correct security label to write to the NFS directory.
> 
>  (5) The incorrectly set up dentry from (1) remains because the the mkdir() op
>      is not invoked to set it right.
> 
> The only bit I added was (3), but that's not the only circumstance in which
> this can occur.
> 
> 
> If, for example, I do "chmod a-w /" on the NFS server, I can see the same
> effects on the client without the need for SELinux to put its foot in the door.
> Automount does:
> 
> [pid  3838] mkdir("/net", 0555)         = -1 EEXIST (File exists)
> [pid  3838] stat64("/net", {st_mode=S_IFDIR|0755, st_size=0, ...}) = 0
> [pid  3838] mkdir("/net/trash", 0555)   = -1 EEXIST (File exists)
> [pid  3838] stat64("/net/trash", {st_mode=S_IFDIR|0555, st_size=1024, ...}) = 0
> [pid  3838] mkdir("/net/trash/mnt", 0555) = -1 EACCES (Permission denied)
> 
> And where I was listing the disputed directory, I see:
> 
> 	[root@...romeda ~]# ls -lad /net/trash/usr/src
> 	drwxr-xr-x 4 root root 1024 Aug 30 10:35 /net/trash/usr/src/
> 	[root@...romeda ~]#
> 
> which isn't what I'd expect.  What I'd expect is:
> 
> 	[root@...romeda ~]# ls -l /net/trash/usr/src
> 	total 15
> 	drwxr-xr-x 3 root root  1024 Aug 30 10:35 debug/
> 	-rw-r--r-- 1 root root     0 Aug 16 10:01 hello
> 	drwx------ 2 root root 12288 Aug 16 10:00 lost+found/
> 	[root@...romeda ~]#

One way to fix this is to simply not hash the dentry when we're doing
the O_EXCL intent optimisation, but rather to only hash it _after_ we've
successfully created the file on the server. Something like the attached
patch ought to do it.

Note, though, that this will not fix the autofs problem: autofs is
trying to perform a totally unnecessary mkdir(), and is giving up when
it is told that SELinux won't authorise that particular operation. This
is clearly an autofs bug...

Cheers,
  Trond

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