MOUNT_NAMESPACES
Section: Linux Programmer's Manual (7)
Updated: 2017-09-15
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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 aq/mntaq | sed aqs/ - .*//aq
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 aq$1 == 61aq | sed aqs/ - .*//aq
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=aqsh2# aq sudo unshare -m --propagation unchanged sh
sh2# cat /proc/self/mountinfo | grep aq/mntaq | sed aqs/ - .*//aq
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 aq/mntaq | sed aqs/ - .*//aq
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 aq/mntaq | sed aqs/ - .*//aq
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 aq/mntaq | sed aqs/ - .*//aq
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 aq/mntaq | sed aqs/ - .*//aq
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 aq/mntaq | sed aqs/ - .*//aq
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 aq/mntaq | sed aqs/ - .*//aq
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 aq/mntaq | sed aqs/ - .*//aq
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 aq/mntaq | sed aqs/ - .*//aq
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 aq{print $1, $2, $3}aq
/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 aq{print $1, $2, $3}aq
/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 aq{print $1, $2, $3}aq
/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 aq{print $1, $2, $3}aq
/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 aq{print $1, $2, $3}aq
/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 aq/mntaq | sed aqs/ - .*//aq
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 aq/mnt|/tmp/aq | sed aqs/ - .*//aq
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 aq/mnt|/tmp/aq | sed aqs/ - .*//aq
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 aq/mnt|/tmp/aq | sed aqs/ - .*//aq
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 aqs/ - .*//aq
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
This page is part of release 4.13 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/.
Index
- NAME
-
- DESCRIPTION
-
- Restrictions on mount namespaces
-
- SHARED SUBTREES
-
- MS_SHARED and MS_PRIVATE example
-
- MS_SLAVE example
-
- MS_UNBINDABLE example
-
- Propagation type transitions
-
- Bind (MS_BIND) semantics
-
- Move (MS_MOVE) semantics
-
- Mount semantics
-
- Unmount semantics
-
- The /proc/[pid]/mountinfo propagate_from tag
-
- VERSIONS
-
- CONFORMING TO
-
- NOTES
-
- SEE ALSO
-
- COLOPHON
-
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