[RFC,PATCH 0/4] Dynamic Pseudo Root

[Steve Dickson]

The following patch series gives rpc.mountd the ability to allocate
a dynamic pseudo root, so the 'fsid=0' export option is no longer 
required. This allows v2, v3 and v4 clients mounts without any 
changes to the server's exports list.

One anomaly of the Linux NFS server is that it requires a pseudo root
to be defined. Currently the only way a pseudo root can be defined is by 
setting the fsid to zero (i.e. fsid=0). So if we wanted to make v4
the default mounting version and have things just work like v2/v3
all of the existing exports configurations would have to change 
(i.e. a 'fsid=0' would have to be added) to support a v4 mounts,
which, imho, is unacceptable. So this patch series address
this problem.

How it works:

Working with Bruce Fields, it was decide to implement this 
functionality in userland (i.e. in rpc.mountd) by using existing 
utilities like tmpfs.Please remember this is an RFC, so this may not 
be the final solution, but it is a first step...

The new '-R' command line argument, tells rpc.mountd to create a pseudo root
if and only if, one is not already defined. Meaning if there is an
export entry in /etc/exports with an 'fsid=0' option, that entry
will be the pseudo root and rpc.mountd will not create a root.

Now when a pseudo root is not defined, rpc.mountd will creates a pseudo
root in '/var/lib/nfs/v4root'. By create I mean, the /var/lib/nfs/v4root
directory will be created and then be mounted as a tmpfs filesystem:

    mount -t tmpfs tmpfs /var/lib/nfs/v4root

Making /var/lib/nfs/v4root a true mount point.

Next, all the exported directories in the /etc/export file will 
be 'mount --bind' mounted under /var/lib/nfs/v4root. For example, for 
the following export entires:

    /usr *(rw,nohide)
    /home/tmp *(rw)

will cause the following 'mount --bind's to occur.
   
   mount --bind /usr /var/lib/nfs/v4root/usr
   mount --bind /home/tmp /var/lib/nfs/v4root/home/tmp

Now when the PUTROOTFH comes in from the client mount
the /var/lib/nfs/v4root directory is returned as the 
root file handle. This means subsequent GETFHs and LOOKUPs 
will use file handle of /var/lib/nfs/v4root as the rootfh. 

Things do get a little tricky if the real root (i.e. '/') 
is exported. In this case, the real root becomes the pseudo 
root, regardless of how the fsid is or is not set. This became
the easiest thing to do.

This patches can be found on my experimental git tree:
    git://linux-nfs.org/~steved/nfs-utils-exp nfs-utils.exp
on the 'pseudo-v4root' branch.

comments?

steved.