mount_namespaces(7) — Linux manual page

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MOUNT_NAMESPACES(7)     Linux Programmer's Manual    MOUNT_NAMESPACES(7)

NAME         top

       mount_namespaces - overview of Linux mount namespaces

DESCRIPTION         top

       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).

SHARED SUBTREES         top

       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.

VERSIONS         top

       Mount namespaces first appeared in Linux 2.4.19.

CONFORMING TO         top

       Namespaces are a Linux-specific feature.

NOTES         top

       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.

EXAMPLES         top

       See pivot_root(2).

SEE ALSO         top

       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), pam_namespace(8),
       pivot_root(8), umount(8)

       Documentation/filesystems/sharedsubtree.txt in the kernel source
       tree.

COLOPHON         top

       This page is part of release 5.12 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                          2021-03-22            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)