NFS: don't try to cross a mountpount when there isn't one there.

consider the sequence of commands:
 mkdir -p /import/nfs /import/bind /import/etc
 mount --bind / /import/bind
 mount --make-private /import/bind
 mount --bind /import/etc /import/bind/etc

 exportfs -o rw,no_root_squash,crossmnt,async,no_subtree_check localhost:/
 mount -o vers=4 localhost:/ /import/nfs
 ls -l /import/nfs/etc

You would not expect this to report a stale file handle.
Yet it does.

The manipulations under /import/bind cause the dentry for
/etc to get the DCACHE_MOUNTED flag set, even though nothing
is mounted on /etc.  This causes nfsd to call
nfsd_cross_mnt() even though there is no mountpoint.  So an
upcall to mountd for "/etc" is performed.

The 'crossmnt' flag on the export of / causes mountd to
report that /etc is exported as it is a descendant of /.  It
assumes the kernel wouldn't ask about something that wasn't
a mountpoint.  The filehandle returned identifies the
filesystem and the inode number of /etc.

When this filehandle is presented to rpc.mountd, via
"nfsd.fh", the inode cannot be found associated with any
name in /etc/exports, or with any mountpoint listed by
getmntent().  So rpc.mountd says the filehandle doesn't
exist. Hence ESTALE.

This is fixed by teaching nfsd not to trust DCACHE_MOUNTED
too much.  It is just a hint, not a guarantee.
Change nfsd_mountpoint() to return '1' for a certain mountpoint,
'2' for a possible mountpoint, and 0 otherwise.

Then change nfsd_crossmnt() to check if follow_down()
actually found a mountpount and, if not, to avoid performing
a lookup if the location is not known to certainly require
an export-point.

Signed-off-by: NeilBrown <neilb@suse.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

1 files changed, 20 insertions, 4 deletions

diff --git a/fs/nfsd/vfs.c b/fs/nfsd/vfs.c
index 19d50f600e8d..04cafaa94bf7 100644
--- a/fs/nfsd/vfs.c
+++ b/fs/nfsd/vfs.c
@@ -94,6 +94,12 @@ nfsd_cross_mnt(struct svc_rqst *rqstp, struct dentry **dpp,
         err = follow_down(&path);
         if (err < 0)
                 goto out;
+        if (path.mnt == exp->ex_path.mnt && path.dentry == dentry &&
+            nfsd_mountpoint(dentry, exp) == 2) {
+                /* This is only a mountpoint in some other namespace */
+                path_put(&path);
+                goto out;
+        }
 
         exp2 = rqst_exp_get_by_name(rqstp, &path);
         if (IS_ERR(exp2)) {
@@ -167,16 +173,26 @@ static int nfsd_lookup_parent(struct svc_rqst *rqstp, struct dentry *dparent, st
 /*
  * For nfsd purposes, we treat V4ROOT exports as though there was an
  * export at *every* directory.
+ * We return:
+ * '1' if this dentry *must* be an export point,
+ * '2' if it might be, if there is really a mount here, and
+ * '0' if there is no chance of an export point here.
  */
 int nfsd_mountpoint(struct dentry *dentry, struct svc_export *exp)
 {
-        if (d_mountpoint(dentry))
+        if (!d_inode(dentry))
+                return 0;
+        if (exp->ex_flags & NFSEXP_V4ROOT)
                 return 1;
         if (nfsd4_is_junction(dentry))
                 return 1;
-        if (!(exp->ex_flags & NFSEXP_V4ROOT))
-                return 0;
-        return d_inode(dentry) != NULL;
+        if (d_mountpoint(dentry))
+                /*
+                 * Might only be a mountpoint in a different namespace,
+                 * but we need to check.
+                 */
+                return 2;
+        return 0;
 }
 
 __be32