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NAME | DESCRIPTION | SHARED SUBTREES | VERSIONS | CONFORMING TO | NOTES | EXAMPLES | SEE ALSO | COLOPHON |
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MOUNT_NAMESPACES(7) Linux Programmer's Manual MOUNT_NAMESPACES(7)
mount_namespaces - overview of Linux mount namespaces
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.)
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:
* If the namespace is created using clone(2), the mount point
list of the child's namespace is a copy of the mount point
list in the parent's namespace.
* If the namespace is created using unshare(2), the mount point
list of the new namespace is a copy of the mount point list in
the caller's previous mount namespace.
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:
* Each mount namespace has an owner user namespace. As
explained above, when a new mount namespace is created, its
mount point list is initialized as a copy of the mount point
list of another mount namespace. If the new namespace and the
namespace from which the mount point list was copied are owned
by different user namespaces, then the new mount namespace is
considered less privileged.
* 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 a more privileged mount
namespace 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). Consequently, the mount point is
removed in the mount namespace where it was a mount point.
Previously (before Linux 3.18), 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).
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 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:
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.
Mount namespaces first appeared in Linux 2.4.19.
Namespaces are a Linux-specific feature.
The propagation type assigned to a new mount point depends on the
propagation type of the parent mount. 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.
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.
See pivot_root(2).
unshare(1), clone(2), mount(2), pivot_root(2), setns(2),
umount(2), unshare(2), proc(5), namespaces(7),
user_namespaces(7), findmnt(8), mount(8), pivot_root(8),
umount(8)
Documentation/filesystems/sharedsubtree.txt in the kernel source
tree.
This page is part of release 5.10 of the Linux man-pages project.
A description of the project, information about reporting bugs,
and the latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
Linux 2020-11-01 MOUNT_NAMESPACES(7)
Pages that refer to this page: nsenter(1), unshare(1), clone(2), mount(2), pivot_root(2), umount(2), unshare(2), core(5), proc(5), systemd.exec(5), pid_namespaces(7), symlink(7), mount(8), umount(8)
Copyright and license for this manual page
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