I was testing using the rootdir setting in nfs.config to allow using export
paths that would normally conflict with local systems directories with NFS v3.
The idea was to support re-exporting a source NFS server such as NetApp that
might exports arbitrary paths such as /home without overwriting the /home
directory on the NFS server, and even support exporting a root directory similar
to NFS v4.
While testing I ran into an issue where the NFS server would fail to start.
During start up, the NFS server would log the error:
$ systemctl status nfs-server.service
systemd[1]: Starting NFS server and services...
exportfs[2307]: exportfs: Failed to stat /srv/nfs/usr/bin: No such
file or directory
systemd[1]: nfs-server.service: Control process exited, code=exited,
status=1/FAILURE
$ cat nfs.config
[exports]
rootdir=/srv/nfs
$ cat /etc/exports
/ 10.0.0.0/8(rw,sync,wdelay,no_root_squash,no_all_squash,no_subtree_check,sec=sys,secure,fsid=0,nohide)
/assets 10.0.0.0/8(rw,sync,wdelay,no_root_squash,no_all_squash,no_subtree_check,sec=sys,secure,fsid=10,nohide)
/bin 10.0.0.0/8(rw,sync,wdelay,no_root_squash,no_all_squash,no_subtree_check,sec=sys,secure,fsid=30,nohide)
/software 10.0.0.0/8(rw,sync,wdelay,no_root_squash,no_all_squash,no_subtree_check,sec=sys,secure,fsid=40,nohide)
If I create the directory /srv/nfs/usr/bin then the NFS server will start.
Listing the actual exports shows that a different path was exported compared to
the path from /etc/exports.
$ exportfs -s
/ 10.0.0.0/8(sync,wdelay,nohide,no_subtree_check,fsid=0,sec=sys,rw,secure,no_root_squash,no_all_squ>
/assets 10.0.0.0/8(sync,wdelay,nohide,no_subtree_check,fsid=10,sec=sys,rw,secure,no_root_squash,no_>
/usr/bin 10.0.0.0/8(sync,wdelay,nohide,no_subtree_check,fsid=30,sec=sys,rw,secure,no_root_squash,no>
/software 10.0.0.0/8(sync,wdelay,nohide,no_subtree_check,fsid=40,sec=sys,rw,secure,no_root_squash,n>
To test this further I create a symlink from /software to /usr/lib. Once again
the server failed to start because it could not find /srv/nfs/usr/lib.
Reading through the source, I think the issue is in the getexportent function in
support/nfs/exports.c.
/* resolve symlinks */
if (realpath(ee.e_path, rpath) != NULL) {
rpath[sizeof (rpath) - 1] = '\0';
strncpy(ee.e_path, rpath, sizeof (ee.e_path) - 1);
ee.e_path[sizeof (ee.e_path) - 1] = '\0';
}
It appears this function does not take into account the rootdir property when
resolving e_path.
This was tested on Ubuntu 20.04 with the 5.11.8-051108-generic kernel. nfs-utils
version is 2.5.3.
