From: Miklos Szeredi <mszeredi@suse.cz>

If MNT_USERMNT flag is not set in the target vfsmount, then
unprivileged mounts will be denied.

By default this flag is cleared, and can be set on new mounts, on
remounts or with the MS_SETFLAGS option.

Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
---

Index: linux/fs/namespace.c
===================================================================
--- linux.orig/fs/namespace.c	2007-04-13 13:20:12.000000000 +0200
+++ linux/fs/namespace.c	2007-04-13 13:35:40.000000000 +0200
@@ -411,6 +411,7 @@ static int show_vfsmnt(struct seq_file *
 		{ MNT_NOATIME, ",noatime" },
 		{ MNT_NODIRATIME, ",nodiratime" },
 		{ MNT_RELATIME, ",relatime" },
+		{ MNT_USERMNT, ",usermnt" },
 		{ 0, NULL }
 	};
 	struct proc_fs_info *fs_infop;
@@ -1505,9 +1506,11 @@ long do_mount(char *dev_name, char *dir_
 		mnt_flags |= MNT_NODIRATIME;
 	if (flags & MS_RELATIME)
 		mnt_flags |= MNT_RELATIME;
+	if (flags & MS_USERMNT)
+		mnt_flags |= MNT_USERMNT;
 
 	flags &= ~(MS_NOSUID | MS_NOEXEC | MS_NODEV | MS_ACTIVE |
-		   MS_NOATIME | MS_NODIRATIME | MS_RELATIME);
+		   MS_NOATIME | MS_NODIRATIME | MS_RELATIME | MS_USERMNT);
 
 	/* ... and get the mountpoint */
 	retval = path_lookup(dir_name, LOOKUP_FOLLOW, &nd);
Index: linux/include/linux/mount.h
===================================================================
--- linux.orig/include/linux/mount.h	2007-04-13 13:17:08.000000000 +0200
+++ linux/include/linux/mount.h	2007-04-13 13:22:17.000000000 +0200
@@ -28,6 +28,7 @@ struct mnt_namespace;
 #define MNT_NOATIME	0x08
 #define MNT_NODIRATIME	0x10
 #define MNT_RELATIME	0x20
+#define MNT_USERMNT	0x40
 
 #define MNT_SHRINKABLE	0x100
 #define MNT_USER	0x200
Index: linux/include/linux/fs.h
===================================================================
--- linux.orig/include/linux/fs.h	2007-04-13 13:23:05.000000000 +0200
+++ linux/include/linux/fs.h	2007-04-13 13:35:34.000000000 +0200
@@ -130,6 +130,7 @@ extern int dir_notify_enable;
 #define MS_SETFLAGS	(1<<23) /* set specified mount flags */
 #define MS_CLEARFLAGS	(1<<24) /* clear specified mount flags */
 /* MS_SETFLAGS | MS_CLEARFLAGS: change mount flags to specified */
+#define MS_USERMNT	(1<<25) /* permit unpriv. submounts under this mount */
 #define MS_ACTIVE	(1<<30)
 #define MS_NOUSER	(1<<31)
 

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