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NAME
mount_namespaces - overview of Linux mount namespaces
DESCRIPTION
For an overview of namespaces, see namespaces(7).
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.)
When a process creates a new mount namespace using clone(2) or unshare(2) with the
CLONE_NEWNS flag, the mount point list for the new namespace is a copy of the caller's
mount point list. 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).
Restrictions on mount namespaces
Note the following points with respect to mount namespaces:
* A mount namespace has an owner user namespace. A mount namespace whose owner user
namespace is different from the owner user namespace of its parent mount namespace is
considered a less privileged mount namespace.
* When creating a less privileged mount namespace, shared mounts are reduced to slave
mounts. (Shared and slave mounts are discussed below.) This ensures that mappings
performed in less privileged mount namespaces will not propagate to more privileged
mount namespaces.
* Mounts that come as a single unit from more privileged mount are locked together and
may not be separated in a less privileged mount namespace. (The unshare(2) CLONE_NEWNS
operation brings across all of the mounts from the original mount namespace as a single
unit, and recursive mounts that propagate between mount namespaces propagate as a
single unit.)
* The mount(2) flags MS_RDONLY, MS_NOSUID, MS_NOEXEC, and the "atime" flags (MS_NOATIME,
MS_NODIRATIME, MS_RELATIME) settings become locked when propagated from a more
privileged to a less privileged mount namespace, and may not be changed in the less
privileged mount namespace.
* A file or directory that is a mount point in one namespace that is not a mount point in
another namespace, may be renamed, unlinked, or removed (rmdir(2)) in the mount
namespace in which it is not a mount point (subject to the usual permission checks).
Previously, attempting to unlink, rename, or remove a file or directory that was a
mount point in another mount namespace would result in the error EBUSY. That behavior
had technical problems of enforcement (e.g., for NFS) and permitted denial-of-service
attacks against more privileged users. (i.e., preventing individual files from being
updated by bind mounting on top of them).
SHARED SUBTREES
After the implementation of mount namespaces was completed, experience showed that the
isolation that they provided was, in some cases, too great. For example, in order to make
a newly loaded optical disk available in all mount namespaces, a mount operation was
required in each namespace. For this use case, and others, the shared subtree feature was
introduced in Linux 2.6.15. This feature allows for automatic, controlled propagation of
mount and unmount events between namespaces (or, more precisely, between the members of a
peer group that are propagating events to one another).
Each mount point is marked (via mount(2)) as having one of the following propagation
types:
MS_SHARED
This mount point shares events with members of a peer group. Mount and unmount
events immediately under this mount point will propagate to the other mount points
that are members of the peer group. Propagation here means that the same mount or
unmount will automatically occur under all of the other mount points in the peer
group. Conversely, mount and unmount events that take place under peer mount
points will propagate to this mount point.
MS_PRIVATE
This mount point is private; it does not have a peer group. Mount and unmount
events do not propagate into or out of this mount point.
MS_SLAVE
Mount and unmount events propagate into this mount point from a (master) shared
peer group. Mount and unmount events under this mount point do not propagate to
any peer.
Note that a mount point can be the slave of another peer group while at the same
time sharing mount and unmount events with a peer group of which it is a member.
(More precisely, one peer group can be the slave of another peer group.)
MS_UNBINDABLE
This is like a private mount, and in addition this mount can't be bind mounted.
Attempts to bind mount this mount (mount(2) with the MS_BIND flag) will fail.
When a recursive bind mount (mount(2) with the MS_BIND and MS_REC flags) is
performed on a directory subtree, any bind mounts within the subtree are
automatically pruned (i.e., not replicated) when replicating that subtree to
produce the target subtree.
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:
* the mount point is replicated during the creation of a new mount namespace; or
* a new bind mount is created from the mount point.
In both of these cases, the new mount point joins the peer group of which the existing
mount point is a member. 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:
shared:X
This mount point is shared in peer group X. Each peer group has a unique ID that
is automatically generated by the kernel, and all mount points in the same peer
group will show the same ID. (These IDs are assigned starting from the value 1,
and may be recycled when a peer group ceases to have any members.)
master:X
This mount is a slave to shared peer group X.
propagate_from:X (since Linux 2.6.26)
This mount is a slave and receives propagation from shared peer group X. This tag
will always appear in conjunction with a master:X tag. Here, X is the closest
dominant peer group under the process's root directory. If X is the immediate
master of the mount, or if there is no dominant peer group under the same root,
then only the master:X field is present and not the propagate_from:X field. For
further details, see below.
unbindable
This is an unbindable mount.
If none of the above tags is present, then this is a private mount.
MS_SHARED and MS_PRIVATE example
Suppose that on a terminal in the initial mount namespace, we mark one mount point as
shared and another as private, and then view the mounts in /proc/self/mountinfo:
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
MS_SLAVE example
Making a mount point a slave allows it to receive propagated mount and unmount events from
a master shared peer group, while preventing it from propagating events to that master.
This is useful if we want to (say) receive a mount event when an optical disk is mounted
in the master shared peer group (in another mount namespace), but want to prevent mount
and unmount events under the slave mount from having side effects in other namespaces.
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
MS_UNBINDABLE example
One of the primary purposes of unbindable mounts is to avoid the "mount point explosion"
problem when repeatedly performing bind mounts of a higher-level subtree at a lower-level
mount point. The problem is illustrated by the following shell session.
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
Propagation type transitions
The following table shows the effect that applying a new propagation type (i.e., mount
--make-xxxx) has on the existing propagation type of a mount point. The rows correspond
to existing propagation types, and the columns are the new propagation settings. For
reasons of space, "private" is abbreviated as "priv" and "unbindable" as "unbind".
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:
[1] If a shared mount is the only mount in its peer group, making it a slave automatically
makes it private.
[2] Slaving a nonshared mount has no effect on the mount.
Bind (MS_BIND) semantics
Suppose that the following command is performed:
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.
Move (MS_MOVE) semantics
Suppose that the following command is performed:
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 semantics
Suppose that we use the following command to create a mount point:
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 semantics
Suppose that we use the following command to tear down a mount point:
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.
The /proc/[pid]/mountinfo propagate_from tag
The propagate_from:X tag is shown in the optional fields of a /proc/[pid]/mountinfo record
in cases where a process can't see a slave's immediate master (i.e., the pathname of the
master is not reachable from the filesystem root directory) and so cannot determine the
chain of propagation between the mounts it can see.
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.
VERSIONS
Mount namespaces first appeared in Linux 2.4.19.
CONFORMING TO
Namespaces are a Linux-specific feature.
NOTES
The propagation type assigned to a new mount point depends on the propagation type of the
parent directory. 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 see also NOTES.
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).
For a discussion of propagation types when moving mounts (MS_MOVE) and creating bind
mounts (MS_BIND), see Documentation/filesystems/sharedsubtree.txt.
SEE ALSO
unshare(1), clone(2), mount(2), setns(2), umount(2), unshare(2), proc(5), namespaces(7),
user_namespaces(7)
Documentation/filesystems/sharedsubtree.txt in the kernel source tree.
COLOPHON
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