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Created page with "= 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 gr..."
= 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 you need to restore this sort of properties. This means that we have to find a sequence of steps to get a required step.
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 these 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:
{| class="wikitable"
!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:
[[File: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.
[[File: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.
[[File:mntns-2.0-tree-3.svg]]
= 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 can introduce a new ioctl, which will return 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
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 you need to restore this sort of properties. This means that we have to find a sequence of steps to get a required step.
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 these 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:
{| class="wikitable"
!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:
[[File: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.
[[File: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.
[[File:mntns-2.0-tree-3.svg]]
= 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 can introduce a new ioctl, which will return 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