Mount namespaces provide isolation of the list of mount points seen by the processes in each namespace instance. Thus, the processes in each of the mount namespace instances will see distinct single-directory hierarchies.
The views provided by the /proc/[pid]/mounts, /proc/[pid]/mountinfo, and /proc/[pid]/mountstats files (all described in proc(5)) correspond to the mount namespace in which the process with the PID [pid] resides. (All of the processes that reside in the same mount namespace will see the same view in these files.)
A new mount namespace is created using either clone(2) or unshare(2) with the CLONE_NEWNS flag. When a new mount namespace is created, its mount point list is initialized as follows:
Subsequent modifications to the mount point list (mount(2) and umount(2)) in either mount namespace will not (by default) affect the mount point list seen in the other namespace (but see the following discussion of shared subtrees).
Each mount point is marked (via mount(2)) as having one of the following propagation types:
For a discussion of the propagation type assigned to a new mount, see NOTES.
The propagation type is a per-mount-point setting; some mount points may be marked as shared (with each shared mount point being a member of a distinct peer group), while others are private (or slaved or unbindable).
Note that a mount's propagation type determines whether mounts and unmounts of mount points immediately under the mount point are propagated. Thus, the propagation type does not affect propagation of events for grandchildren and further removed descendant mount points. What happens if the mount point itself is unmounted is determined by the propagation type that is in effect for the parent of the mount point.
Members are added to a peer group when a mount point is marked as shared and either:
In both of these cases, the new mount point joins the peer group of which the existing mount point is a member.
A new peer group is also created when a child mount point is created under an existing mount point that is marked as shared. In this case, the new child mount point is also marked as shared and the resulting peer group consists of all the mount points that are replicated under the peers of parent mount.
A mount ceases to be a member of a peer group when either the mount is explicitly unmounted, or when the mount is implicitly unmounted because a mount namespace is removed (because it has no more member processes).
The propagation type of the mount points in a mount namespace can be discovered via the "optional fields" exposed in /proc/[pid]/mountinfo. (See proc(5) for details of this file.) The following tags can appear in the optional fields for a record in that file:
If none of the above tags is present, then this is a private mount.
sh1# mount --make-shared /mntS sh1# mount --make-private /mntP sh1# cat /proc/self/mountinfo | grep '/mnt' | sed 's/ - .*//' 77 61 8:17 / /mntS rw,relatime shared:1 83 61 8:15 / /mntP rw,relatime
From the /proc/self/mountinfo output, we see that /mntS is a shared mount in peer group 1, and that /mntP has no optional tags, indicating that it is a private mount. The first two fields in each record in this file are the unique ID for this mount, and the mount ID of the parent mount. We can further inspect this file to see that the parent mount point of /mntS and /mntP is the root directory, /, which is mounted as private:
sh1# cat /proc/self/mountinfo | awk '$1 == 61' | sed 's/ - .*//' 61 0 8:2 / / rw,relatime
On a second terminal, we create a new mount namespace where we run a second shell and inspect the mounts:
$ PS1='sh2# ' sudo unshare -m --propagation unchanged sh sh2# cat /proc/self/mountinfo | grep '/mnt' | sed 's/ - .*//' 222 145 8:17 / /mntS rw,relatime shared:1 225 145 8:15 / /mntP rw,relatime
The new mount namespace received a copy of the initial mount namespace's mount points. These new mount points maintain the same propagation types, but have unique mount IDs. (The --propagation unchanged option prevents unshare(1) from marking all mounts as private when creating a new mount namespace, which it does by default.)
In the second terminal, we then create submounts under each of /mntS and /mntP and inspect the set-up:
sh2# mkdir /mntS/a sh2# mount /dev/sdb6 /mntS/a sh2# mkdir /mntP/b sh2# mount /dev/sdb7 /mntP/b sh2# cat /proc/self/mountinfo | grep '/mnt' | sed 's/ - .*//' 222 145 8:17 / /mntS rw,relatime shared:1 225 145 8:15 / /mntP rw,relatime 178 222 8:22 / /mntS/a rw,relatime shared:2 230 225 8:23 / /mntP/b rw,relatime
From the above, it can be seen that /mntS/a was created as shared (inheriting this setting from its parent mount) and /mntP/b was created as a private mount.
Returning to the first terminal and inspecting the set-up, we see that the new mount created under the shared mount point /mntS propagated to its peer mount (in the initial mount namespace), but the new mount created under the private mount point /mntP did not propagate:
sh1# cat /proc/self/mountinfo | grep '/mnt' | sed 's/ - .*//' 77 61 8:17 / /mntS rw,relatime shared:1 83 61 8:15 / /mntP rw,relatime 179 77 8:22 / /mntS/a rw,relatime shared:2
We can demonstrate the effect of slaving by first marking two mount points as shared in the initial mount namespace:
sh1# mount --make-shared /mntX sh1# mount --make-shared /mntY sh1# cat /proc/self/mountinfo | grep '/mnt' | sed 's/ - .*//' 132 83 8:23 / /mntX rw,relatime shared:1 133 83 8:22 / /mntY rw,relatime shared:2
On a second terminal, we create a new mount namespace and inspect the mount points:
sh2# unshare -m --propagation unchanged sh sh2# cat /proc/self/mountinfo | grep '/mnt' | sed 's/ - .*//' 168 167 8:23 / /mntX rw,relatime shared:1 169 167 8:22 / /mntY rw,relatime shared:2
In the new mount namespace, we then mark one of the mount points as a slave:
sh2# mount --make-slave /mntY sh2# cat /proc/self/mountinfo | grep '/mnt' | sed 's/ - .*//' 168 167 8:23 / /mntX rw,relatime shared:1 169 167 8:22 / /mntY rw,relatime master:2
From the above output, we see that /mntY is now a slave mount that is receiving propagation events from the shared peer group with the ID 2.
Continuing in the new namespace, we create submounts under each of /mntX and /mntY:
sh2# mkdir /mntX/a sh2# mount /dev/sda3 /mntX/a sh2# mkdir /mntY/b sh2# mount /dev/sda5 /mntY/b
When we inspect the state of the mount points in the new mount namespace, we see that /mntX/a was created as a new shared mount (inheriting the "shared" setting from its parent mount) and /mntY/b was created as a private mount:
sh2# cat /proc/self/mountinfo | grep '/mnt' | sed 's/ - .*//' 168 167 8:23 / /mntX rw,relatime shared:1 169 167 8:22 / /mntY rw,relatime master:2 173 168 8:3 / /mntX/a rw,relatime shared:3 175 169 8:5 / /mntY/b rw,relatime
Returning to the first terminal (in the initial mount namespace), we see that the mount /mntX/a propagated to the peer (the shared /mntX), but the mount /mntY/b was not propagated:
sh1# cat /proc/self/mountinfo | grep '/mnt' | sed 's/ - .*//' 132 83 8:23 / /mntX rw,relatime shared:1 133 83 8:22 / /mntY rw,relatime shared:2 174 132 8:3 / /mntX/a rw,relatime shared:3
Now we create a new mount point under /mntY in the first shell:
sh1# mkdir /mntY/c sh1# mount /dev/sda1 /mntY/c sh1# cat /proc/self/mountinfo | grep '/mnt' | sed 's/ - .*//' 132 83 8:23 / /mntX rw,relatime shared:1 133 83 8:22 / /mntY rw,relatime shared:2 174 132 8:3 / /mntX/a rw,relatime shared:3 178 133 8:1 / /mntY/c rw,relatime shared:4
When we examine the mount points in the second mount namespace, we see that in this case the new mount has been propagated to the slave mount point, and that the new mount is itself a slave mount (to peer group 4):
sh2# cat /proc/self/mountinfo | grep '/mnt' | sed 's/ - .*//' 168 167 8:23 / /mntX rw,relatime shared:1 169 167 8:22 / /mntY rw,relatime master:2 173 168 8:3 / /mntX/a rw,relatime shared:3 175 169 8:5 / /mntY/b rw,relatime 179 169 8:1 / /mntY/c rw,relatime master:4
Suppose we have a system with the following mount points:
# mount | awk '{print $1, $2, $3}' /dev/sda1 on / /dev/sdb6 on /mntX /dev/sdb7 on /mntY
Suppose furthermore that we wish to recursively bind mount the root directory under several users' home directories. We do this for the first user, and inspect the mount points:
# mount --rbind / /home/cecilia/ # mount | awk '{print $1, $2, $3}' /dev/sda1 on / /dev/sdb6 on /mntX /dev/sdb7 on /mntY /dev/sda1 on /home/cecilia /dev/sdb6 on /home/cecilia/mntX /dev/sdb7 on /home/cecilia/mntY
When we repeat this operation for the second user, we start to see the explosion problem:
# mount --rbind / /home/henry # mount | awk '{print $1, $2, $3}' /dev/sda1 on / /dev/sdb6 on /mntX /dev/sdb7 on /mntY /dev/sda1 on /home/cecilia /dev/sdb6 on /home/cecilia/mntX /dev/sdb7 on /home/cecilia/mntY /dev/sda1 on /home/henry /dev/sdb6 on /home/henry/mntX /dev/sdb7 on /home/henry/mntY /dev/sda1 on /home/henry/home/cecilia /dev/sdb6 on /home/henry/home/cecilia/mntX /dev/sdb7 on /home/henry/home/cecilia/mntY
Under /home/henry, we have not only recursively added the /mntX and /mntY mounts, but also the recursive mounts of those directories under /home/cecilia that were created in the previous step. Upon repeating the step for a third user, it becomes obvious that the explosion is exponential in nature:
# mount --rbind / /home/otto # mount | awk '{print $1, $2, $3}' /dev/sda1 on / /dev/sdb6 on /mntX /dev/sdb7 on /mntY /dev/sda1 on /home/cecilia /dev/sdb6 on /home/cecilia/mntX /dev/sdb7 on /home/cecilia/mntY /dev/sda1 on /home/henry /dev/sdb6 on /home/henry/mntX /dev/sdb7 on /home/henry/mntY /dev/sda1 on /home/henry/home/cecilia /dev/sdb6 on /home/henry/home/cecilia/mntX /dev/sdb7 on /home/henry/home/cecilia/mntY /dev/sda1 on /home/otto /dev/sdb6 on /home/otto/mntX /dev/sdb7 on /home/otto/mntY /dev/sda1 on /home/otto/home/cecilia /dev/sdb6 on /home/otto/home/cecilia/mntX /dev/sdb7 on /home/otto/home/cecilia/mntY /dev/sda1 on /home/otto/home/henry /dev/sdb6 on /home/otto/home/henry/mntX /dev/sdb7 on /home/otto/home/henry/mntY /dev/sda1 on /home/otto/home/henry/home/cecilia /dev/sdb6 on /home/otto/home/henry/home/cecilia/mntX /dev/sdb7 on /home/otto/home/henry/home/cecilia/mntY
The mount explosion problem in the above scenario can be avoided by making each of the new mounts unbindable. The effect of doing this is that recursive mounts of the root directory will not replicate the unbindable mounts. We make such a mount for the first user:
# mount --rbind --make-unbindable / /home/cecilia
Before going further, we show that unbindable mounts are indeed unbindable:
# mkdir /mntZ
# mount --bind /home/cecilia /mntZ
mount: wrong fs type, bad option, bad superblock on /home/cecilia,
missing codepage or helper program, or other error
In some cases useful info is found in syslog - try
dmesg | tail or so.
Now we create unbindable recursive bind mounts for the other two users:
# mount --rbind --make-unbindable / /home/henry # mount --rbind --make-unbindable / /home/otto
Upon examining the list of mount points, we see there has been no explosion of mount points, because the unbindable mounts were not replicated under each user's directory:
# mount | awk '{print $1, $2, $3}' /dev/sda1 on / /dev/sdb6 on /mntX /dev/sdb7 on /mntY /dev/sda1 on /home/cecilia /dev/sdb6 on /home/cecilia/mntX /dev/sdb7 on /home/cecilia/mntY /dev/sda1 on /home/henry /dev/sdb6 on /home/henry/mntX /dev/sdb7 on /home/henry/mntY /dev/sda1 on /home/otto /dev/sdb6 on /home/otto/mntX /dev/sdb7 on /home/otto/mntY
| make-shared | make-slave | make-priv | make-unbind | ||
| shared | shared | slave/priv [1] | priv | unbind | |
| slave | slave+shared | slave [2] | priv | unbind | |
| slave+shared | slave+shared | slave | priv | unbind | |
| private | shared | priv [2] | priv | unbind | |
| unbindable | shared | unbind [2] | priv | unbind | |
Note the following details to the table:
mount --bind A/a B/b
Here, A is the source mount point, B is the destination mount point, a is a subdirectory path under the mount point A, and b is a subdirectory path under the mount point B. The propagation type of the resulting mount, B/b, depends on the propagation types of the mount points A and B, and is summarized in the following table.
| source(A) | ||||||
| shared | private | slave | unbind | |||
| dest(B) | shared | shared | shared | slave+shared | invalid | |
| nonshared | shared | private | slave | invalid | ||
Note that a recursive bind of a subtree follows the same semantics as for a bind operation on each mount in the subtree. (Unbindable mounts are automatically pruned at the target mount point.)
For further details, see Documentation/filesystems/sharedsubtree.txt in the kernel source tree.
mount --move A B/b
Here, A is the source mount point, B is the destination mount point, and b is a subdirectory path under the mount point B. The propagation type of the resulting mount, B/b, depends on the propagation types of the mount points A and B, and is summarized in the following table.
| source(A) | ||||||
| shared | private | slave | unbind | |||
| dest(B) | shared | shared | shared | slave+shared | invalid | |
| nonshared | shared | private | slave | unbindable | ||
Note: moving a mount that resides under a shared mount is invalid.
For further details, see Documentation/filesystems/sharedsubtree.txt in the kernel source tree.
mount device B/b
Here, B is the destination mount point, and b is a subdirectory path under the mount point B. The propagation type of the resulting mount, B/b, follows the same rules as for a bind mount, where the propagation type of the source mount is considered always to be private.
unmount A
Here, A is a mount point on B/b, where B is the parent mount and b is a subdirectory path under the mount point B. If B is shared, then all most-recently-mounted mounts at b on mounts that receive propagation from mount B and do not have submounts under them are unmounted.
In the following example, we first create a two-link master-slave chain between the mounts /mnt, /tmp/etc, and /mnt/tmp/etc. Then the chroot(1) command is used to make the /tmp/etc mount point unreachable from the root directory, creating a situation where the master of /mnt/tmp/etc is not reachable from the (new) root directory of the process.
First, we bind mount the root directory onto /mnt and then bind mount /proc at /mnt/proc so that after the later chroot(1) the proc(5) filesystem remains visible at the correct location in the chroot-ed environment.
# mkdir -p /mnt/proc # mount --bind / /mnt # mount --bind /proc /mnt/proc
Next, we ensure that the /mnt mount is a shared mount in a new peer group (with no peers):
# mount --make-private /mnt # Isolate from any previous peer group # mount --make-shared /mnt # cat /proc/self/mountinfo | grep '/mnt' | sed 's/ - .*//' 239 61 8:2 / /mnt ... shared:102 248 239 0:4 / /mnt/proc ... shared:5
Next, we bind mount /mnt/etc onto /tmp/etc:
# mkdir -p /tmp/etc # mount --bind /mnt/etc /tmp/etc # cat /proc/self/mountinfo | egrep '/mnt|/tmp/' | sed 's/ - .*//' 239 61 8:2 / /mnt ... shared:102 248 239 0:4 / /mnt/proc ... shared:5 267 40 8:2 /etc /tmp/etc ... shared:102
Initially, these two mount points are in the same peer group, but we then make the /tmp/etc a slave of /mnt/etc, and then make /tmp/etc shared as well, so that it can propagate events to the next slave in the chain:
# mount --make-slave /tmp/etc # mount --make-shared /tmp/etc # cat /proc/self/mountinfo | egrep '/mnt|/tmp/' | sed 's/ - .*//' 239 61 8:2 / /mnt ... shared:102 248 239 0:4 / /mnt/proc ... shared:5 267 40 8:2 /etc /tmp/etc ... shared:105 master:102
Then we bind mount /tmp/etc onto /mnt/tmp/etc. Again, the two mount points are initially in the same peer group, but we then make /mnt/tmp/etc a slave of /tmp/etc:
# mkdir -p /mnt/tmp/etc # mount --bind /tmp/etc /mnt/tmp/etc # mount --make-slave /mnt/tmp/etc # cat /proc/self/mountinfo | egrep '/mnt|/tmp/' | sed 's/ - .*//' 239 61 8:2 / /mnt ... shared:102 248 239 0:4 / /mnt/proc ... shared:5 267 40 8:2 /etc /tmp/etc ... shared:105 master:102 273 239 8:2 /etc /mnt/tmp/etc ... master:105
From the above, we see that /mnt is the master of the slave /tmp/etc, which in turn is the master of the slave /mnt/tmp/etc.
We then chroot(1) to the /mnt directory, which renders the mount with ID 267 unreachable from the (new) root directory:
# chroot /mnt
When we examine the state of the mounts inside the chroot-ed environment, we see the following:
# cat /proc/self/mountinfo | sed 's/ - .*//' 239 61 8:2 / / ... shared:102 248 239 0:4 / /proc ... shared:5 273 239 8:2 /etc /tmp/etc ... master:105 propagate_from:102
Above, we see that the mount with ID 273 is a slave whose master is the peer group 105. The mount point for that master is unreachable, and so a propagate_from tag is displayed, indicating that the closest dominant peer group (i.e., the nearest reachable mount in the slave chain) is the peer group with the ID 102 (corresponding to the /mnt mount point before the chroot(1) was performed.
Notwithstanding the fact that the default propagation type for new mount points is in many cases MS_PRIVATE, MS_SHARED is typically more useful. For this reason, systemd(1) automatically remounts all mount points as MS_SHARED on system startup. Thus, on most modern systems, the default propagation type is in practice MS_SHARED.
Since, when one uses unshare(1) to create a mount namespace, the goal is commonly to provide full isolation of the mount points in the new namespace, unshare(1) (since util-linux version 2.27) in turn reverses the step performed by systemd(1), by making all mount points private in the new namespace. That is, unshare(1) performs the equivalent of the following in the new mount namespace:
mount --make-rprivate /
To prevent this, one can use the --propagation unchanged option to unshare(1).
An application that creates a new mount namespace directly using clone(2) or unshare(2) may desire to prevent propagation of mount events to other mount namespaces (as is done by unshare(1)). This can be done by changing the propagation type of mount points in the new namespace to either MS_SLAVE or MS_PRIVATE. using a call such as the following:
mount(NULL, "/", MS_SLAVE | MS_REC, NULL);
For a discussion of propagation types when moving mounts (MS_MOVE) and creating bind mounts (MS_BIND), see Documentation/filesystems/sharedsubtree.txt.
Documentation/filesystems/sharedsubtree.txt in the kernel source tree.