[PATCH] mnt: allow to add a mount into an existing group

[Andrei Vagin]

Now a shared group can be only inherited from a source mount.
This patch adds an ability to add a mount into an existing shared
group.

mount(source, target, NULL, MS_SET_GROUP, NULL)

mount() with the MS_SET_GROUP flag adds the "target" mount into a group
of the "source" mount. The calling process has to have the CAP_SYS_ADMIN
capability in namespaces of these mounts. The source and the target
mounts have to have the same super block.

This new functionality together with "mnt: Tuck mounts under others
instead of creating shadow/side mounts." allows CRIU to dump and restore
any set of mount namespaces.

Currently we have a lot of issues about dumping and restoring mount
namespaces. The bigest problem is that we can't construct mount trees
directly due to several reasons:
* groups can't be set, they can be only inherited
* file systems has to be mounted from the specified user namespaces
* the mount() syscall doesn't just create one mount -- the mount is
  also propagated to all members of a parent group
* umount() doesn't detach mounts from all members of a group
  (mounts with children are not umounted)
* mounts are propagated underneath of existing mounts
* mount() doesn't allow to make bind-mounts between two namespaces
* processes can have opened file descriptors to overmounted files

All these operations are non-trivial, making the task of restoring
a mount namespace practically unsolvable for reasonable time. The
proposed change allows to restore a mount namespace in a direct
manner, without any super complex logic.

Cc: Eric W. Biederman <ebiederm@xxxxxxxxxxxx>
Cc: Alexander Viro <viro@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Andrei Vagin <avagin@xxxxxxxxxx>
---
 fs/namespace.c          | 53 +++++++++++++++++++++++++++++++++++++++++++++++++
 include/uapi/linux/fs.h |  1 +
 2 files changed, 54 insertions(+)

diff --git a/fs/namespace.c b/fs/namespace.c
index b5b1259..df52fd4 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -2301,6 +2301,57 @@ static inline int tree_contains_unbindable(struct mount *mnt)
 	return 0;
 }
 
+static int do_set_group(struct path *path, const char *sibling_name)
+{
+	struct mount *sibling, *mnt;
+	struct path sibling_path;
+	int err;
+
+	if (!sibling_name || !*sibling_name)
+		return -EINVAL;
+
+	err = kern_path(sibling_name, LOOKUP_FOLLOW, &sibling_path);
+	if (err)
+		return err;
+
+	sibling = real_mount(sibling_path.mnt);
+	mnt = real_mount(path->mnt);
+
+	namespace_lock();
+
+	err = -EPERM;
+	if (!sibling->mnt_ns ||
+	    !ns_capable(sibling->mnt_ns->user_ns, CAP_SYS_ADMIN))
+		goto out_unlock;
+
+	err = -EINVAL;
+	if (sibling->mnt.mnt_sb != mnt->mnt.mnt_sb)
+		goto out_unlock;
+
+	if (IS_MNT_SHARED(mnt) || IS_MNT_SLAVE(mnt))
+		goto out_unlock;
+
+	if (IS_MNT_SLAVE(sibling)) {
+		struct mount *m = sibling->mnt_master;
+
+		list_add(&mnt->mnt_slave, &m->mnt_slave_list);
+		mnt->mnt_master = m;
+	}
+
+	if (IS_MNT_SHARED(sibling)) {
+		mnt->mnt_group_id = sibling->mnt_group_id;
+		list_add(&mnt->mnt_share, &sibling->mnt_share);
+		set_mnt_shared(mnt);
+	}
+
+	err = 0;
+out_unlock:
+	namespace_unlock();
+
+	path_put(&sibling_path);
+	return err;
+}
+
 static int do_move_mount(struct path *path, const char *old_name)
 {
 	struct path old_path, parent_path;
@@ -2779,6 +2830,8 @@ long do_mount(const char *dev_name, const char __user *dir_name,
 		retval = do_change_type(&path, flags);
 	else if (flags & MS_MOVE)
 		retval = do_move_mount(&path, dev_name);
+	else if (flags & MS_SET_GROUP)
+		retval = do_set_group(&path, dev_name);
 	else
 		retval = do_new_mount(&path, type_page, flags, mnt_flags,
 				      dev_name, data_page);
diff --git a/include/uapi/linux/fs.h b/include/uapi/linux/fs.h
index 36da93f..6e6e37d 100644
--- a/include/uapi/linux/fs.h
+++ b/include/uapi/linux/fs.h
@@ -130,6 +130,7 @@ struct inodes_stat_t {
 #define MS_I_VERSION	(1<<23) /* Update inode I_version field */
 #define MS_STRICTATIME	(1<<24) /* Always perform atime updates */
 #define MS_LAZYTIME	(1<<25) /* Update the on-disk [acm]times lazily */
+#define MS_SET_GROUP	(1<<26) /* Add a mount into a shared group */
 
 /* These sb flags are internal to the kernel */
 #define MS_NOREMOTELOCK	(1<<27)
-- 
2.7.4