ecryptfs: dont call lookup_one_len to avoid NULL nameidata

I have encountered the same problem that Eric Sandeen described in
this post

 http://lkml.org/lkml/fancy/2010/4/23/467

while experimenting with stackable filesystems.

The reason seems to be that ecryptfs calls lookup_one_len() to get the
lower dentry, which in turn calls the lower parent dirs d_revalidate()
with a NULL nameidata object.
If ecryptfs is the underlaying filesystem, the NULL pointer dereference
occurs, since ecryptfs is not prepared to handle a NULL nameidata.

I know that this cant happen any more, since it is no longer allowed to
mount ecryptfs upon itself.

But maybe this patch it useful nevertheless, since the problem would still
apply for an underlaying filesystem that implements d_revalidate() and is
not prepared to handle a NULL nameidata (I dont know if there actually
is such a fs).

With this patch (against 2.6.35-rc5) ecryptfs uses the vfs_lookup_path()
function instead of lookup_one_len() which ensures that the nameidata
passed to the lower filesystems d_revalidate().
Signed-off-by: Lino Sanfilippo <LinoSanfilippo@gmx.de>
Signed-off-by: Tyler Hicks <tyhicks@linux.vnet.ibm.com>

fs/ecryptfs/inode.c

@@ -347,6 +347,76 @@ int ecryptfs_lookup_and_interpose_lower(struct dentry *ecryptfs_dentry,
 	return rc;
 }
 
+/**
+ * ecryptfs_new_lower_dentry
+ * @ename: The name of the new dentry.
+ * @lower_dir_dentry: Parent directory of the new dentry.
+ * @nd: nameidata from last lookup.
+ *
+ * Create a new dentry or get it from lower parent dir.
+ */
+static struct dentry *
+ecryptfs_new_lower_dentry(struct qstr *name, struct dentry *lower_dir_dentry,
+			  struct nameidata *nd)
+{
+	struct dentry *new_dentry;
+	struct dentry *tmp;
+	struct inode *lower_dir_inode;
+
+	lower_dir_inode = lower_dir_dentry->d_inode;
+
+	tmp = d_alloc(lower_dir_dentry, name);
+	if (!tmp)
+		return ERR_PTR(-ENOMEM);
+
+	mutex_lock(&lower_dir_inode->i_mutex);
+	new_dentry = lower_dir_inode->i_op->lookup(lower_dir_inode, tmp, nd);
+	mutex_unlock(&lower_dir_inode->i_mutex);
+
+	if (!new_dentry)
+		new_dentry = tmp;
+	else
+		dput(tmp);
+
+	return new_dentry;
+}
+
+
+/**
+ * ecryptfs_lookup_one_lower
+ * @ecryptfs_dentry: The eCryptfs dentry that we are looking up
+ * @lower_dir_dentry: lower parent directory
+ *
+ * Get the lower dentry from vfs. If lower dentry does not exist yet,
+ * create it.
+ */
+static struct dentry *
+ecryptfs_lookup_one_lower(struct dentry *ecryptfs_dentry,
+			  struct dentry *lower_dir_dentry)
+{
+	struct nameidata nd;
+	struct vfsmount *lower_mnt;
+	struct qstr *name;
+	int err;
+
+	name = &ecryptfs_dentry->d_name;
+	lower_mnt = mntget(ecryptfs_dentry_to_lower_mnt(
+				    ecryptfs_dentry->d_parent));
+	err = vfs_path_lookup(lower_dir_dentry, lower_mnt, name->name , 0, &nd);
+	mntput(lower_mnt);
+
+	if (!err) {
+		/* we dont need the mount */
+		mntput(nd.path.mnt);
+		return nd.path.dentry;
+	}
+	if (err != -ENOENT)
+		return ERR_PTR(err);
+
+	/* create a new lower dentry */
+	return ecryptfs_new_lower_dentry(name, lower_dir_dentry, &nd);
+}
+
 /**
  * ecryptfs_lookup
  * @ecryptfs_dir_inode: The eCryptfs directory inode
@@ -374,14 +444,12 @@ static struct dentry *ecryptfs_lookup(struct inode *ecryptfs_dir_inode,
 		goto out_d_drop;
 	}
 	lower_dir_dentry = ecryptfs_dentry_to_lower(ecryptfs_dentry->d_parent);
-	mutex_lock(&lower_dir_dentry->d_inode->i_mutex);
-	lower_dentry = lookup_one_len(ecryptfs_dentry->d_name.name,
-				      lower_dir_dentry,
-				      ecryptfs_dentry->d_name.len);
-	mutex_unlock(&lower_dir_dentry->d_inode->i_mutex);
+
+	lower_dentry = ecryptfs_lookup_one_lower(ecryptfs_dentry,
+						 lower_dir_dentry);
 	if (IS_ERR(lower_dentry)) {
 		rc = PTR_ERR(lower_dentry);
-		ecryptfs_printk(KERN_DEBUG, "%s: lookup_one_len() returned "
+		ecryptfs_printk(KERN_DEBUG, "%s: lookup_one_lower() returned "
 				"[%d] on lower_dentry = [%s]\n", __func__, rc,
 				encrypted_and_encoded_name);
 		goto out_d_drop;
@@ -403,14 +471,11 @@ static struct dentry *ecryptfs_lookup(struct inode *ecryptfs_dir_inode,
 		       "filename; rc = [%d]\n", __func__, rc);
 		goto out_d_drop;
 	}
-	mutex_lock(&lower_dir_dentry->d_inode->i_mutex);
-	lower_dentry = lookup_one_len(encrypted_and_encoded_name,
-				      lower_dir_dentry,
-				      encrypted_and_encoded_name_size - 1);
-	mutex_unlock(&lower_dir_dentry->d_inode->i_mutex);
+	lower_dentry = ecryptfs_lookup_one_lower(ecryptfs_dentry,
+						 lower_dir_dentry);
 	if (IS_ERR(lower_dentry)) {
 		rc = PTR_ERR(lower_dentry);
-		ecryptfs_printk(KERN_DEBUG, "%s: lookup_one_len() returned "
+		ecryptfs_printk(KERN_DEBUG, "%s: lookup_one_lower() returned "
 				"[%d] on lower_dentry = [%s]\n", __func__, rc,
 				encrypted_and_encoded_name);
 		goto out_d_drop;