Linux: Mount Namespace

.The purpose of each namespace is to wrap a particular global system resource in an abstraction that makes it appear to the processes within the namespace that they have their own isolated instance of the global resource.

Mount Namespace

They isolate the list of mount points seen by the processes in a namespace. Or, to put things another way, each mount namespace has its own list of mount points, meaning that processes in different namespaces see and are able to manipulate different views of the single directory hierarchy.

Shared subtrees

The key benefit of shared subtrees is to allow automatic, controlled propagation of mount and unmount events between namespaces. This means, for example, that mounting an optical disk in one mount namespace can trigger a mount of that disk in all other namespaces.

  • MS_SHARED: This mount point shares mount and unmount events with other mount points that are members of its “peer group” (which is described in more detail below). When a mount point is added or removed under this mount point, this change will propagate to the peer group, so that the mount or unmount will also take place under each of the peer mount points. Propagation also occurs in the reverse direction, so that mount and unmount events on a peer mount will also propagate to this mount point.
  • MS_PRIVATE: This is the converse of a shared mount point. The mount point does not propagate events to any peers, and does not receive propagation events from any peer
  • MS_SLAVE: This propagation type sits midway between shared and private. A slave mount has a master—a shared peer group whose members propagate mount and unmount events to the slave mount. However, the slave mount does not propagate events to the master peer group
  • MS_UNBINDABLE: This mount point is unbindable. Like a private mount point, this mount point does not propagate events to or from peers. In addition, this mount point can’t be the source for a bind mount operation
Peer groups

A peer group is a set of mount points that propagate mount and unmount events to one another. A peer group acquires new members when a mount point whose propagation type is shared is either replicated during the creation of a new namespace or is used as the source for a bind mount.  Conversely, a mount point ceases to be a member of a peer group when it is unmounted, either explicitly, or implicitly when a mount namespace is torn down because the last member process terminates or moves to another namespace.

Default Propagation

So what happens when we bind a mount? From the kernel’s perspective, the default when a new device mount is created is as follows:

  • If the mount point has a parent (i.e., it is a non-root mount point) and the propagation type of the parent is MS_SHARED, then the propagation type of the new mount is also MS_SHARED.
  • Otherwise, the propagation type of the new mount is MS_PRIVATE.

But you have to beware when working with unshare. When creating a new mount namespace, unshare assumes that the user wants a fully isolated namespace, and makes all mount points private by performing the equivalent of the following command (which recursively marks all mounts under the root directory as private):

    mount --make-rprivate /

To prevent this, we can use an additional option when creating the new namespace:

    unshare -m --propagation unchanged 

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