svcadm(1M)을 검색하려면 섹션에서 1M 을 선택하고, 맨 페이지 이름에 svcadm을 입력하고 검색을 누른다.
mount_namespaces(7)
MOUNT_NAMESPACES(7) Linux Programmer's Manual MOUNT_NAMESPACES(7)
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 modi‐
fications 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 sub‐
trees).
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 names‐
pace 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 names‐
paces.
* 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 propa‐
gate 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 tech‐
nical 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 fea‐
ture was introduced in Linux 2.6.15. This feature allows for auto‐
matic, 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 follow‐
ing 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 unbind‐
able).
Note that a mount's propagation type determines whether mounts and
unmounts of mount points immediately under the mount point are propa‐
gated. 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 mas‐
ter: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 propa‐
gate_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 names‐
pace's mount points. These new mount points maintain the same propaga‐
tion types, but have unique mount IDs. (The --propagation unchanged
option prevents unshare(1) from marking all mounts as private when cre‐
ating 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 cre‐
ated 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 mas‐
ter 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 names‐
pace, 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 unbind‐
able:
# 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".
lb2 lb2 lb2 lb2 lb1 lb l l l l l. 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 pri‐
vate 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.
lb2 lb1 lb2 lb2 lb2 lb0 lb2 lb1 lb2 lb2 lb2 lb0 lb lb l l l l l.
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.
lb2 lb1 lb2 lb2 lb2 lb0 lb2 lb1 lb2 lb2 lb2 lb0 lb lb l l l l l.
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 environ‐
ment, 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 prop‐
agation 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_PRI‐
VATE.
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/shared‐
subtree.txt.
SEE ALSO
unshare(1), clone(2), mount(2), pivot_root(2), setns(2), umount(2),
unshare(2), proc(5), namespaces(7), user_namespaces(7), findmnt(8),
pivot_root(8)
Documentation/filesystems/sharedsubtree.txt in the kernel source tree.
COLOPHON
This page is part of release 5.02 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 2019-08-02 MOUNT_NAMESPACES(7)