Add an AT_NO_AUTOMOUNT flag to suppress terminal automount

Add an AT_NO_AUTOMOUNT flag to suppress terminal automounting of automount
point directories.  This can be used by fstatat() users to permit the
gathering of attributes on an automount point and also prevent
mass-automounting of a directory of automount points by ls.

Signed-off-by: David Howells <dhowells@redhat.com>
Acked-by: Ian Kent <raven@themaw.net>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/fs/namei.c b/fs/namei.c
index 9d3033d..dc50bfb 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -908,6 +908,12 @@
 	if (!path->dentry->d_op || !path->dentry->d_op->d_automount)
 		return -EREMOTE;
 
+	/* We don't want to mount if someone supplied AT_NO_AUTOMOUNT
+	 * and this is the terminal part of the path.
+	 */
+	if ((flags & LOOKUP_NO_AUTOMOUNT) && !(flags & LOOKUP_CONTINUE))
+		return -EISDIR; /* we actually want to stop here */
+
 	/* We want to mount if someone is trying to open/create a file of any
 	 * type under the mountpoint, wants to traverse through the mountpoint
 	 * or wants to open the mounted directory.
diff --git a/fs/stat.c b/fs/stat.c
index 12e90e2..d5c61cf 100644
--- a/fs/stat.c
+++ b/fs/stat.c
@@ -75,11 +75,13 @@
 	int error = -EINVAL;
 	int lookup_flags = 0;
 
-	if ((flag & ~AT_SYMLINK_NOFOLLOW) != 0)
+	if ((flag & ~(AT_SYMLINK_NOFOLLOW | AT_NO_AUTOMOUNT)) != 0)
 		goto out;
 
 	if (!(flag & AT_SYMLINK_NOFOLLOW))
 		lookup_flags |= LOOKUP_FOLLOW;
+	if (flag & AT_NO_AUTOMOUNT)
+		lookup_flags |= LOOKUP_NO_AUTOMOUNT;
 
 	error = user_path_at(dfd, filename, lookup_flags, &path);
 	if (error)