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NAME | DESCRIPTION | SHARED SUBTREES | STANDARDS | HISTORY | NOTES | EXAMPLES | SEE ALSO | COLOPHON |
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mount_namespaces(7) Miscellaneous Information 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 mounts 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 list is initialized as follows:
• If the namespace is created using clone(2), the mount list of
the child's namespace is a copy of the mount list in the
parent process's mount namespace.
• If the namespace is created using unshare(2), the mount list
of the new namespace is a copy of the mount list in the
caller's previous mount namespace.
Subsequent modifications to the mount list (mount(2) and
umount(2)) in either mount namespace will not (by default) affect
the mount list seen in the other namespace (but see the following
discussion of 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(2) and umount(2) events between namespaces
(or, more precisely, between the mounts that are members of a
peer group that are propagating events to one another).
Each mount is marked (via mount(2)) as having one of the
following propagation types:
MS_SHARED
This mount shares events with members of a peer group.
mount(2) and umount(2) events immediately under this mount
will propagate to the other mounts that are members of the
peer group. Propagation here means that the same mount(2)
or umount(2) will automatically occur under all of the
other mounts in the peer group. Conversely, mount(2) and
umount(2) events that take place under peer mounts will
propagate to this mount.
MS_PRIVATE
This mount is private; it does not have a peer group.
mount(2) and umount(2) events do not propagate into or out
of this mount.
MS_SLAVE
mount(2) and umount(2) events propagate into this mount
from a (master) shared peer group. mount(2) and umount(2)
events under this mount do not propagate to any peer.
Note that a mount can be the slave of another peer group
while at the same time sharing mount(2) and umount(2)
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 mounts
may be marked as shared (with each shared mount 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 mount(2)
and umount(2) of mounts immediately under the mount are
propagated. Thus, the propagation type does not affect
propagation of events for grandchildren and further removed
descendant mounts. What happens if the mount itself is unmounted
is determined by the propagation type that is in effect for the
parent of the mount.
Members are added to a peer group when a mount is marked as
shared and either:
(a) the mount is replicated during the creation of a new mount
namespace; or
(b) a new bind mount is created from the mount.
In both of these cases, the new mount joins the peer group of
which the existing mount is a member.
A new peer group is also created when a child mount is created
under an existing mount that is marked as shared. In this case,
the new child mount is also marked as shared and the resulting
peer group consists of all the mounts that are replicated under
the peers of parent mounts.
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 mounts 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 is shared in peer group X. Each peer group has
a unique ID that is automatically generated by the kernel,
and all mounts 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 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 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 mounts. These new mounts 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 /mntS
propagated to its peer mount (in the initial mount namespace),
but the new mount created under the private mount /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 a slave allows it to receive propagated mount(2)
and umount(2) 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(2) and umount(2) events
under the slave mount from having side effects in other
namespaces.
We can demonstrate the effect of slaving by first marking two
mounts 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 mounts:
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 mounts 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 mounts 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 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 mounts in the second mount namespace, we see
that in this case the new mount has been propagated to the slave
mount, 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 explosion" problem when repeatedly performing bind mounts
of a higher-level subtree at a lower-level mount. The problem is
illustrated by the following shell session.
Suppose we have a system with the following mounts:
# 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 mounts:
# 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 mounts, we see there has been no
explosion of mounts, 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. 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, B is the destination mount, 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 mounts 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.rst 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, B is the destination mount, 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 mounts 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.rst in the kernel source
tree.
Mount semantics
Suppose that we use the following command to create a mount:
mount device B/b
Here, B is the destination mount, 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:
umount A
Here, A is a mount 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 mounts 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
mounts 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).
Linux.
Linux 2.4.19.
The propagation type assigned to a new mount depends on the
propagation type of the parent mount. If the mount 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 is in many cases MS_PRIVATE, MS_SHARED is typically
more useful. For this reason, systemd(1) automatically remounts
all mounts 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 mounts in the
new namespace, unshare(1) (since util-linux 2.27) in turn
reverses the step performed by systemd(1), by making all mounts
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 mounts 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.rst.
Restrictions on mount namespaces
Note the following points with respect to mount namespaces:
[1] Each mount namespace has an owner user namespace. As
explained above, when a new mount namespace is created, its
mount list is initialized as a copy of the mount list of
another mount namespace. If the new namespace and the
namespace from which the mount list was copied are owned by
different user namespaces, then the new mount namespace is
considered less privileged.
[2] When creating a less privileged mount namespace, shared
mounts are reduced to slave mounts. This ensures that
mappings performed in less privileged mount namespaces will
not propagate to more privileged mount namespaces.
[3] 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.)
In this context, "may not be separated" means that the
mounts are locked so that they may not be individually
unmounted. Consider the following example:
$ sudo sh
# mount --bind /dev/null /etc/shadow
# cat /etc/shadow # Produces no output
The above steps, performed in a more privileged mount
namespace, have created a bind mount that obscures the
contents of the shadow password file, /etc/shadow. For
security reasons, it should not be possible to umount(2)
that mount in a less privileged mount namespace, since that
would reveal the contents of /etc/shadow.
Suppose we now create a new mount namespace owned by a new
user namespace. The new mount namespace will inherit copies
of all of the mounts from the previous mount namespace.
However, those mounts will be locked because the new mount
namespace is less privileged. Consequently, an attempt to
umount(2) the mount fails as show in the following step:
# unshare --user --map-root-user --mount \
strace -o /tmp/log \
umount /mnt/dir
umount: /etc/shadow: not mounted.
# grep '^umount' /tmp/log
umount2("/etc/shadow", 0) = -1 EINVAL (Invalid argument)
The error message from mount(8) is a little confusing, but
the strace(1) output reveals that the underlying umount2(2)
system call failed with the error EINVAL, which is the error
that the kernel returns to indicate that the mount is
locked.
Note, however, that it is possible to stack (and unstack) a
mount on top of one of the inherited locked mounts in a less
privileged mount namespace:
# echo 'aaaaa' > /tmp/a # File to mount onto /etc/shadow
# unshare --user --map-root-user --mount \
sh -c 'mount --bind /tmp/a /etc/shadow; cat /etc/shadow'
aaaaa
# umount /etc/shadow
The final umount(8) command above, which is performed in the
initial mount namespace, makes the original /etc/shadow file
once more visible in that namespace.
[4] Following on from point [3], note that it is possible to
umount(2) an entire subtree of mounts that propagated as a
unit into a less privileged mount namespace, as illustrated
in the following example.
First, we create new user and mount namespaces using
unshare(1). In the new mount namespace, the propagation
type of all mounts is set to private. We then create a
shared bind mount at /mnt, and a small hierarchy of mounts
underneath that mount.
$ PS1='ns1# ' sudo unshare --user --map-root-user \
--mount --propagation private bash
ns1# echo $$ # We need the PID of this shell later
778501
ns1# mount --make-shared --bind /mnt /mnt
ns1# mkdir /mnt/x
ns1# mount --make-private -t tmpfs none /mnt/x
ns1# mkdir /mnt/x/y
ns1# mount --make-private -t tmpfs none /mnt/x/y
ns1# grep /mnt /proc/self/mountinfo | sed 's/ - .*//'
986 83 8:5 /mnt /mnt rw,relatime shared:344
989 986 0:56 / /mnt/x rw,relatime
990 989 0:57 / /mnt/x/y rw,relatime
Continuing in the same shell session, we then create a
second shell in a new user namespace and a new (less
privileged) mount namespace and check the state of the
propagated mounts rooted at /mnt.
ns1# PS1='ns2# ' unshare --user --map-root-user \
--mount --propagation unchanged bash
ns2# grep /mnt /proc/self/mountinfo | sed 's/ - .*//'
1239 1204 8:5 /mnt /mnt rw,relatime master:344
1240 1239 0:56 / /mnt/x rw,relatime
1241 1240 0:57 / /mnt/x/y rw,relatime
Of note in the above output is that the propagation type of
the mount /mnt has been reduced to slave, as explained in
point [2]. This means that submount events will propagate
from the master /mnt in "ns1", but propagation will not
occur in the opposite direction.
From a separate terminal window, we then use nsenter(1) to
enter the mount and user namespaces corresponding to "ns1".
In that terminal window, we then recursively bind mount
/mnt/x at the location /mnt/ppp.
$ PS1='ns3# ' sudo nsenter -t 778501 --user --mount
ns3# mount --rbind --make-private /mnt/x /mnt/ppp
ns3# grep /mnt /proc/self/mountinfo | sed 's/ - .*//'
986 83 8:5 /mnt /mnt rw,relatime shared:344
989 986 0:56 / /mnt/x rw,relatime
990 989 0:57 / /mnt/x/y rw,relatime
1242 986 0:56 / /mnt/ppp rw,relatime
1243 1242 0:57 / /mnt/ppp/y rw,relatime shared:518
Because the propagation type of the parent mount, /mnt, was
shared, the recursive bind mount propagated a small subtree
of mounts under the slave mount /mnt into "ns2", as can be
verified by executing the following command in that shell
session:
ns2# grep /mnt /proc/self/mountinfo | sed 's/ - .*//'
1239 1204 8:5 /mnt /mnt rw,relatime master:344
1240 1239 0:56 / /mnt/x rw,relatime
1241 1240 0:57 / /mnt/x/y rw,relatime
1244 1239 0:56 / /mnt/ppp rw,relatime
1245 1244 0:57 / /mnt/ppp/y rw,relatime master:518
While it is not possible to umount(2) a part of the
propagated subtree (/mnt/ppp/y) in "ns2", it is possible to
umount(2) the entire subtree, as shown by the following
commands:
ns2# umount /mnt/ppp/y
umount: /mnt/ppp/y: not mounted.
ns2# umount -l /mnt/ppp | sed 's/ - .*//' # Succeeds...
ns2# grep /mnt /proc/self/mountinfo
1239 1204 8:5 /mnt /mnt rw,relatime master:344
1240 1239 0:56 / /mnt/x rw,relatime
1241 1240 0:57 / /mnt/x/y rw,relatime
[5] 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.
This point is illustrated in the following example where, in
a more privileged mount namespace, we create a bind mount
that is marked as read-only. For security reasons, it
should not be possible to make the mount writable in a less
privileged mount namespace, and indeed the kernel prevents
this:
$ sudo mkdir /mnt/dir
$ sudo mount --bind -o ro /some/path /mnt/dir
$ sudo unshare --user --map-root-user --mount \
mount -o remount,rw /mnt/dir
mount: /mnt/dir: permission denied.
[6] 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).
See pivot_root(2).
unshare(1), clone(2), mount(2), mount_setattr(2), pivot_root(2),
setns(2), umount(2), unshare(2), proc(5), namespaces(7),
user_namespaces(7), findmnt(8), mount(8), pam_namespace(8),
pivot_root(8), umount(8)
Documentation/filesystems/sharedsubtree.rst in the kernel source
tree.
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
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Linux man-pages 6.9.1 2024-06-15 mount_namespaces(7)
Pages that refer to this page: fuser(1), nsenter(1), unshare(1), clone(2), mount(2), mount_setattr(2), pivot_root(2), umount(2), unshare(2), lttng-ust(3), core(5), proc_pid_mountinfo(5), proc_pid_mounts(5), proc_pid_mountstats(5), systemd.exec(5), landlock(7), pid_namespaces(7), symlink(7), mount(8), umount(8)
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