Mount points/2.0

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Problem[edit]

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.

Solution[edit]

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:

Mntns-2.0-tree.svg

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

Mntns-2.0-tree-2.svg

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

Mntns-2.0-tree-3.svg

Restore of unix sockets[edit]

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[edit]