Mount points/2.0

The problem

A mount namespace is a tree of mount points. In addition, mounts have another type of dependencies which is called groups. Each mount can be a member of two groups, it can be a slave in one group and a member of another group. Currently groups can’t be set, it can be only inherited from a source mount. It is always a problem when more than one type of properties have to restored for one call. This means that we have to find a sequence of steps to get a required state. In case of mount namespaces, one more problem is over-mounts. A few mounts may be over-mounted or processes can have file descriptors which are linked with over-mounted files. Another difficulty is that we are not able to create bind-mounts between namespaces, but each file system have to be mounted from a specified user namespace.

The solution

When we see all variation of commands to build a mount tree, we can understand that the final picture may be very complicated to be repeated, so we suggest to add a new flag to the mount() syscall, which allows us to add a mount into an existing group.

In this case the restore algorithm will be very simple.

• Create a temporary mount which is called “root yard”
• Create all namespaces (in specified user namespaces)
• Add root yards from all namespaces into one shared group, so a mount is created in one mntns, will be propagated into others.
• Create all mounts in separate directories in the root yards.
• Restore opened files (nothing is over-mounted at this point)
• Build mount trees in namespaces by moving mounts to right places
• Do pivot_root() in all namespaces

Let’s look at the next example:

mnt_id	parent	shared	master
1	0
2	1	1
3	2	2
4	2	3
5	1
6	0
7	6	1
8	7	2
9	7	4	3
10	6

The origin tree looks like this:



The first stage is to restore all mounts in all namespace separately. In addition, we need to create all shared groups.

 

The next step is to move mounts to proper places to restore a tree and then we restore groups for each mount.



Restore of unix sockets

Unix sockets can be bound to a file. The problem is that an address and a file are not connected between each other in term of unix sockets. For example, if you move a socket file, ss shows the origin address and you can’t find a file where the socket is bound. Another example is that an address may contain a relative path (../socket_name).

Currently socket_diag shows a device and an inode number for a socket file, but it says nothing about a path to this file and about its mount point. We introduced the SIOCUNIXFILE ioctl, which returns a file descriptor to a socket file. In this case to restore a unix socket we have to:

• create a temporary directory and mount tmpfs into it before restoring sockets
• Restore sockets
• create a socket address directory where is the last part is a symlink to a proper directory on a required mount point
• call chroot() to the temporary directory
• bind the socket to a specified address

if we restored a server socket, we can get a file descriptor for its file and use it to restore client sockets by calling connect() for /proc/self/fd/[SK_FILE_FD] umount tmpfs from the temporary directory and remove the directory after restoring all sockets

Source code

• github.com/avagin/criu/tree/mntns-2.0
• [PATCH fs: add an ioctl to get an owning userns for a superblock]