[PATCH] knfsd: ignore ref_fh when crossing a mountpoint

nfsd tries to return to a client the same sort of filehandle as was used by
the client.  This removes some filehandle aliasing issues and means that a
server upgrade followed by a downgrade will not confused clients not restarted
during that time.

However when crossing a mountpoint, the filehandle used for one filesystem
doesn't provide any useful information on what sort of filehandle should be
used on the other, and can provide misleading information.  So if the
reference filehandle is on a different filesystem to the one being generated,
ignore it.
Signed-off-by: Neil Brown <neilb@suse.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

fs/nfsd/nfsfh.c

@@ -312,8 +312,8 @@ int
 fh_compose(struct svc_fh *fhp, struct svc_export *exp, struct dentry *dentry, struct svc_fh *ref_fh)
 {
 	/* ref_fh is a reference file handle.
-	 * if it is non-null, then we should compose a filehandle which is
-	 * of the same version, where possible.
+	 * if it is non-null and for the same filesystem, then we should compose
+	 * a filehandle which is of the same version, where possible.
 	 * Currently, that means that if ref_fh->fh_handle.fh_version == 0xca
 	 * Then create a 32byte filehandle using nfs_fhbase_old
 	 *
@@ -332,7 +332,7 @@ fh_compose(struct svc_fh *fhp, struct svc_export *exp, struct dentry *dentry, st
 		parent->d_name.name, dentry->d_name.name,
 		(inode ? inode->i_ino : 0));
 
-	if (ref_fh) {
+	if (ref_fh && ref_fh->fh_export == exp) {
 		ref_fh_version = ref_fh->fh_handle.fh_version;
 		if (ref_fh_version == 0xca)
 			ref_fh_fsid_type = 0;