[PATCH] autofs4: avoid panic on bind mount of autofs owned directory

While this is not a solution to bind and move mounts on autofs owned
directories it is necessary to fix the trady error handling.

At least it avoids the kernel panic I observed checking out bug #4589.
Signed-off-by: Ian Kent <raven@themaw.net>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

fs/autofs4/autofs_i.h

@@ -185,6 +185,19 @@ int autofs4_wait(struct autofs_sb_info *,struct dentry *, enum autofs_notify);
 int autofs4_wait_release(struct autofs_sb_info *,autofs_wqt_t,int);
 void autofs4_catatonic_mode(struct autofs_sb_info *);
 
+static inline int autofs4_follow_mount(struct vfsmount **mnt, struct dentry **dentry)
+{
+	int res = 0;
+
+	while (d_mountpoint(*dentry)) {
+		int followed = follow_down(mnt, dentry);
+		if (!followed)
+			break;
+		res = 1;
+	}
+	return res;
+}
+
 static inline int simple_positive(struct dentry *dentry)
 {
 	return dentry->d_inode && !d_unhashed(dentry);

fs/autofs4/expire.c

@@ -56,12 +56,9 @@ static int autofs4_check_mount(struct vfsmount *mnt, struct dentry *dentry)
 	mntget(mnt);
 	dget(dentry);
 
-	if (!follow_down(&mnt, &dentry))
+	if (!autofs4_follow_mount(&mnt, &dentry))
 		goto done;
 
-	while (d_mountpoint(dentry) && follow_down(&mnt, &dentry))
-		;
-
 	/* This is an autofs submount, we can't expire it */
 	if (is_autofs4_dentry(dentry))
 		goto done;

fs/autofs4/root.c

@@ -205,7 +205,11 @@ static int autofs4_dir_open(struct inode *inode, struct file *file)
 		struct vfsmount *fp_mnt = mntget(mnt);
 		struct dentry *fp_dentry = dget(dentry);
 
-		while (follow_down(&fp_mnt, &fp_dentry) && d_mountpoint(fp_dentry));
+		if (!autofs4_follow_mount(&fp_mnt, &fp_dentry)) {
+			dput(fp_dentry);
+			mntput(fp_mnt);
+			return -ENOENT;
+		}
 
 		fp = dentry_open(fp_dentry, fp_mnt, file->f_flags);
 		status = PTR_ERR(fp);