Re: [PATCH 0/7] Permit filesystem local caching and NFS superblocksharing [try #13]

[Trond Myklebust]

On Mon, 2006-09-04 at 12:52 +0100, David Howells wrote:
> Andrew Morton <akpm@xxxxxxxx> 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@andromeda ~]# ls -lad /net/trash/usr/src
> 	drwxr-xr-x 4 root root 1024 Aug 30 10:35 /net/trash/usr/src/
> 	[root@andromeda ~]#
> 
> which isn't what I'd expect.  What I'd expect is:
> 
> 	[root@andromeda ~]# 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@andromeda ~]#

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
> --- Begin Message ---
> Signed-off-by: Trond Myklebust <Trond.Myklebust@xxxxxxxxxx>
> ---
>
>  fs/nfs/dir.c |   13 ++++++++++---
>  1 files changed, 10 insertions(+), 3 deletions(-)
>
> diff --git a/fs/nfs/dir.c b/fs/nfs/dir.c
> index 51328ae..e83a2ff 100644
> --- a/fs/nfs/dir.c
> +++ b/fs/nfs/dir.c
> @@ -904,9 +904,14 @@ static struct dentry *nfs_lookup(struct 
>  
>  	lock_kernel();
>  
> -	/* If we're doing an exclusive create, optimize away the lookup */
> -	if (nfs_is_exclusive_create(dir, nd))
> -		goto no_entry;
> +	/*
> +	 * If we're doing an exclusive create, optimize away the lookup
> +	 * but don't hash the dentry.
> +	 */
> +	if (nfs_is_exclusive_create(dir, nd)) {
> +		d_instantiate(dentry, NULL);
> +		goto out_unlock;
> +	}
>  
>  	error = NFS_PROTO(dir)->lookup(dir, &dentry->d_name, &fhandle, &fattr);
>  	if (error == -ENOENT)
> @@ -1161,6 +1166,8 @@ int nfs_instantiate(struct dentry *dentr
>  	if (IS_ERR(inode))
>  		return error;
>  	d_instantiate(dentry, inode);
> +	if (d_unhashed(dentry))
> +		d_rehash(dentry);
>  	return 0;
>  }
>  
>
> --- End Message ---