Provided by: manpages_4.13-3_all bug

NAME

       mount_namespaces - overview of Linux mount namespaces

DESCRIPTION

       For an overview of namespaces, see namespaces(7).

       Mount  namespaces  provide  isolation of the list of mount points seen by the processes in
       each namespace instance.  Thus, the processes in each of  the  mount  namespace  instances
       will see distinct single-directory hierarchies.

       The    views    provided    by    the   /proc/[pid]/mounts,   /proc/[pid]/mountinfo,   and
       /proc/[pid]/mountstats files (all described in proc(5)) correspond to the mount  namespace
       in which the process with the PID [pid] resides.  (All of the processes that reside in the
       same mount namespace will see the same view in these files.)

       When a process creates a new  mount  namespace  using  clone(2)  or  unshare(2)  with  the
       CLONE_NEWNS  flag,  the  mount  point list for the new namespace is a copy of the caller's
       mount point list.   Subsequent  modifications  to  the  mount  point  list  (mount(2)  and
       umount(2))  in  either  mount  namespace will not (by default) affect the mount point list
       seen in the other namespace (but see the following discussion of shared subtrees).

   Restrictions on mount namespaces
       Note the following points with respect to mount namespaces:

       *  A mount namespace has an owner user namespace.  A  mount  namespace  whose  owner  user
          namespace  is  different from the owner user namespace of its parent mount namespace is
          considered a less privileged mount namespace.

       *  When creating a less privileged mount namespace, shared mounts  are  reduced  to  slave
          mounts.   (Shared  and  slave  mounts are discussed below.)  This ensures that mappings
          performed in less privileged mount namespaces will not  propagate  to  more  privileged
          mount namespaces.

       *  Mounts  that  come  as a single unit from more privileged mount are locked together and
          may not be separated in a less privileged mount namespace.  (The unshare(2) CLONE_NEWNS
          operation brings across all of the mounts from the original mount namespace as a single
          unit, and recursive mounts that propagate  between  mount  namespaces  propagate  as  a
          single unit.)

       *  The  mount(2) flags MS_RDONLY, MS_NOSUID, MS_NOEXEC, and the "atime" flags (MS_NOATIME,
          MS_NODIRATIME,  MS_RELATIME)  settings  become  locked  when  propagated  from  a  more
          privileged  to  a  less  privileged mount namespace, and may not be changed in the less
          privileged mount namespace.

       *  A file or directory that is a mount point in one namespace that is not a mount point in
          another  namespace,  may  be  renamed,  unlinked,  or  removed  (rmdir(2)) in the mount
          namespace in which it is not a mount point (subject to the usual permission checks).

          Previously, attempting to unlink, rename, or remove a file  or  directory  that  was  a
          mount  point in another mount namespace would result in the error EBUSY.  That behavior
          had technical problems of enforcement (e.g., for NFS) and  permitted  denial-of-service
          attacks  against  more privileged users.  (i.e., preventing individual files from being
          updated by bind mounting on top of them).

SHARED SUBTREES

       After the implementation of mount namespaces was completed,  experience  showed  that  the
       isolation that they provided was, in some cases, too great.  For example, in order to make
       a newly loaded optical disk available in all  mount  namespaces,  a  mount  operation  was
       required in each namespace.  For this use case, and others, the shared subtree feature was
       introduced in Linux 2.6.15.  This feature allows for automatic, controlled propagation  of
       mount  and unmount events between namespaces (or, more precisely, between the members of a
       peer group that are propagating events to one another).

       Each mount point is marked (via mount(2)) as  having  one  of  the  following  propagation
       types:

       MS_SHARED
              This  mount  point  shares  events with members of a peer group.  Mount and unmount
              events immediately under this mount point will propagate to the other mount  points
              that  are members of the peer group.  Propagation here means that the same mount or
              unmount will automatically occur under all of the other mount points  in  the  peer
              group.   Conversely,  mount  and  unmount  events  that take place under peer mount
              points will propagate to this mount point.

       MS_PRIVATE
              This mount point is private; it does not have a  peer  group.   Mount  and  unmount
              events do not propagate into or out of this mount point.

       MS_SLAVE
              Mount  and  unmount  events  propagate into this mount point from a (master) shared
              peer group.  Mount and unmount events under this mount point do  not  propagate  to
              any peer.

              Note  that  a  mount point can be the slave of another peer group while at the same
              time sharing mount and unmount events with a peer group of which it  is  a  member.
              (More precisely, one peer group can be the slave of another peer group.)

       MS_UNBINDABLE
              This  is  like  a  private mount, and in addition this mount can't be bind mounted.
              Attempts to bind mount this mount (mount(2) with the MS_BIND flag) will fail.

              When a recursive bind mount  (mount(2)  with  the  MS_BIND  and  MS_REC  flags)  is
              performed  on  a  directory  subtree,  any  bind  mounts  within  the  subtree  are
              automatically pruned (i.e.,  not  replicated)  when  replicating  that  subtree  to
              produce the target subtree.

       For a discussion of the propagation type assigned to a new mount, see NOTES.

       The  propagation  type  is  a  per-mount-point setting; some mount points may be marked as
       shared (with each shared mount point being a member  of  a  distinct  peer  group),  while
       others are private (or slaved or unbindable).

       Note  that  a  mount's  propagation  type  determines whether mounts and unmounts of mount
       points immediately under the mount point are propagated.  Thus, the propagation type  does
       not  affect  propagation  of events for grandchildren and further removed descendant mount
       points.  What happens if the  mount  point  itself  is  unmounted  is  determined  by  the
       propagation type that is in effect for the parent of the mount point.

       Members are added to a peer group when a mount point is marked as shared and either:

       *  the mount point is replicated during the creation of a new mount namespace; or

       *  a new bind mount is created from the mount point.

       In  both  of  these  cases, the new mount point joins the peer group of which the existing
       mount point is a member.  A mount ceases to be a member of a peer group  when  either  the
       mount  is  explicitly unmounted, or when the mount is implicitly unmounted because a mount
       namespace is removed (because it has no more member processes).

       The propagation type of the mount points in a mount namespace can be  discovered  via  the
       "optional  fields"  exposed  in  /proc/[pid]/mountinfo.   (See proc(5) for details of this
       file.)  The following tags can appear in the optional fields for a record in that file:

       shared:X
              This mount point is shared in peer group X.  Each peer group has a unique  ID  that
              is  automatically  generated  by  the kernel, and all mount points in the same peer
              group will show the same ID.  (These IDs are assigned starting from  the  value  1,
              and may be recycled when a peer group ceases to have any members.)

       master:X
              This mount is a slave to shared peer group X.

       propagate_from:X (since Linux 2.6.26)
              This  mount is a slave and receives propagation from shared peer group X.  This tag
              will always appear in conjunction with a master:X tag.   Here,  X  is  the  closest
              dominant  peer  group  under  the  process's root directory.  If X is the immediate
              master of the mount, or if there is no dominant peer group  under  the  same  root,
              then  only  the  master:X field is present and not the propagate_from:X field.  For
              further details, see below.

       unbindable
              This is an unbindable mount.

       If none of the above tags is present, then this is a private mount.

   MS_SHARED and MS_PRIVATE example
       Suppose that on a terminal in the initial mount namespace, we  mark  one  mount  point  as
       shared and another as private, and then view the mounts in /proc/self/mountinfo:

           sh1# mount --make-shared /mntS
           sh1# mount --make-private /mntP
           sh1# cat /proc/self/mountinfo | grep '/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

       Mount namespaces first appeared in Linux 2.4.19.

CONFORMING TO

       Namespaces are a Linux-specific feature.

NOTES

       The propagation type assigned to a new mount point depends on the propagation type of  the
       parent  directory.   If  the mount point has a parent (i.e., it is a non-root mount point)
       and the propagation type of the parent is MS_SHARED, then the propagation type of the  new
       mount  is also MS_SHARED.  Otherwise, the propagation type of the new mount is MS_PRIVATE.
       But see also NOTES.

       Notwithstanding the fact that the default propagation type for new mount points is in many
       cases  MS_PRIVATE,  MS_SHARED  is  typically  more  useful.   For  this reason, systemd(1)
       automatically remounts all mount points as MS_SHARED on system  startup.   Thus,  on  most
       modern systems, the default propagation type is in practice MS_SHARED.

       Since,  when  one  uses  unshare(1)  to  create a mount namespace, the goal is commonly to
       provide full isolation of the mount points in the new namespace, unshare(1)  (since  util-
       linux version 2.27) in turn reverses the step performed by systemd(1), by making all mount
       points private in the new namespace.  That is, unshare(1) performs the equivalent  of  the
       following in the new mount namespace:

           mount --make-rprivate /

       To prevent this, one can use the --propagation unchanged option to unshare(1).

       For  a  discussion  of  propagation  types  when moving mounts (MS_MOVE) and creating bind
       mounts (MS_BIND), see Documentation/filesystems/sharedsubtree.txt.

SEE ALSO

       unshare(1), clone(2), mount(2), setns(2), umount(2), unshare(2),  proc(5),  namespaces(7),
       user_namespaces(7)

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

COLOPHON

       This  page  is  part of release 4.13 of the Linux man-pages project.  A description of the
       project, information about reporting bugs, and the latest version of  this  page,  can  be
       found at https://www.kernel.org/doc/man-pages/.