pid_namespaces(7) — Linux manual page


pid_namespaces(7)   Miscellaneous Information Manual   pid_namespaces(7)

NAME         top

       pid_namespaces - overview of Linux PID namespaces

DESCRIPTION         top

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

       PID namespaces isolate the process ID number space, meaning that
       processes in different PID namespaces can have the same PID.  PID
       namespaces allow containers to provide functionality such as
       suspending/resuming the set of processes in the container and
       migrating the container to a new host while the processes inside
       the container maintain the same PIDs.

       PIDs in a new PID namespace start at 1, somewhat like a
       standalone system, and calls to fork(2), vfork(2), or clone(2)
       will produce processes with PIDs that are unique within the

       Use of PID namespaces requires a kernel that is configured with
       the CONFIG_PID_NS option.

   The namespace init process
       The first process created in a new namespace (i.e., the process
       created using clone(2) with the CLONE_NEWPID flag, or the first
       child created by a process after a call to unshare(2) using the
       CLONE_NEWPID flag) has the PID 1, and is the "init" process for
       the namespace (see init(1)).  This process becomes the parent of
       any child processes that are orphaned because a process that
       resides in this PID namespace terminated (see below for further

       If the "init" process of a PID namespace terminates, the kernel
       terminates all of the processes in the namespace via a SIGKILL
       signal.  This behavior reflects the fact that the "init" process
       is essential for the correct operation of a PID namespace.  In
       this case, a subsequent fork(2) into this PID namespace fail with
       the error ENOMEM; it is not possible to create a new process in a
       PID namespace whose "init" process has terminated.  Such
       scenarios can occur when, for example, a process uses an open
       file descriptor for a /proc/pid/ns/pid file corresponding to a
       process that was in a namespace to setns(2) into that namespace
       after the "init" process has terminated.  Another possible
       scenario can occur after a call to unshare(2): if the first child
       subsequently created by a fork(2) terminates, then subsequent
       calls to fork(2) fail with ENOMEM.

       Only signals for which the "init" process has established a
       signal handler can be sent to the "init" process by other members
       of the PID namespace.  This restriction applies even to
       privileged processes, and prevents other members of the PID
       namespace from accidentally killing the "init" process.

       Likewise, a process in an ancestor namespace can—subject to the
       usual permission checks described in kill(2)—send signals to the
       "init" process of a child PID namespace only if the "init"
       process has established a handler for that signal.  (Within the
       handler, the siginfo_t si_pid field described in sigaction(2)
       will be zero.)  SIGKILL or SIGSTOP are treated exceptionally:
       these signals are forcibly delivered when sent from an ancestor
       PID namespace.  Neither of these signals can be caught by the
       "init" process, and so will result in the usual actions
       associated with those signals (respectively, terminating and
       stopping the process).

       Starting with Linux 3.4, the reboot(2) system call causes a
       signal to be sent to the namespace "init" process.  See reboot(2)
       for more details.

   Nesting PID namespaces
       PID namespaces can be nested: each PID namespace has a parent,
       except for the initial ("root") PID namespace.  The parent of a
       PID namespace is the PID namespace of the process that created
       the namespace using clone(2) or unshare(2).  PID namespaces thus
       form a tree, with all namespaces ultimately tracing their
       ancestry to the root namespace.  Since Linux 3.7, the kernel
       limits the maximum nesting depth for PID namespaces to 32.

       A process is visible to other processes in its PID namespace, and
       to the processes in each direct ancestor PID namespace going back
       to the root PID namespace.  In this context, "visible" means that
       one process can be the target of operations by another process
       using system calls that specify a process ID.  Conversely, the
       processes in a child PID namespace can't see processes in the
       parent and further removed ancestor namespaces.  More succinctly:
       a process can see (e.g., send signals with kill(2), set nice
       values with setpriority(2), etc.) only processes contained in its
       own PID namespace and in descendants of that namespace.

       A process has one process ID in each of the layers of the PID
       namespace hierarchy in which is visible, and walking back though
       each direct ancestor namespace through to the root PID namespace.
       System calls that operate on process IDs always operate using the
       process ID that is visible in the PID namespace of the caller.  A
       call to getpid(2) always returns the PID associated with the
       namespace in which the process was created.

       Some processes in a PID namespace may have parents that are
       outside of the namespace.  For example, the parent of the initial
       process in the namespace (i.e., the init(1) process with PID 1)
       is necessarily in another namespace.  Likewise, the direct
       children of a process that uses setns(2) to cause its children to
       join a PID namespace are in a different PID namespace from the
       caller of setns(2).  Calls to getppid(2) for such processes
       return 0.

       While processes may freely descend into child PID namespaces
       (e.g., using setns(2) with a PID namespace file descriptor), they
       may not move in the other direction.  That is to say, processes
       may not enter any ancestor namespaces (parent, grandparent,
       etc.).  Changing PID namespaces is a one-way operation.

       The NS_GET_PARENT ioctl(2) operation can be used to discover the
       parental relationship between PID namespaces; see ioctl_nsfs(2).

   setns(2) and unshare(2) semantics
       Calls to setns(2) that specify a PID namespace file descriptor
       and calls to unshare(2) with the CLONE_NEWPID flag cause children
       subsequently created by the caller to be placed in a different
       PID namespace from the caller.  (Since Linux 4.12, that PID
       namespace is shown via the /proc/pid/ns/pid_for_children file, as
       described in namespaces(7).)  These calls do not, however, change
       the PID namespace of the calling process, because doing so would
       change the caller's idea of its own PID (as reported by
       getpid()), which would break many applications and libraries.

       To put things another way: a process's PID namespace membership
       is determined when the process is created and cannot be changed
       thereafter.  Among other things, this means that the parental
       relationship between processes mirrors the parental relationship
       between PID namespaces: the parent of a process is either in the
       same namespace or resides in the immediate parent PID namespace.

       A process may call unshare(2) with the CLONE_NEWPID flag only
       once.  After it has performed this operation, its
       /proc/pid/ns/pid_for_children symbolic link will be empty until
       the first child is created in the namespace.

   Adoption of orphaned children
       When a child process becomes orphaned, it is reparented to the
       "init" process in the PID namespace of its parent (unless one of
       the nearer ancestors of the parent employed the prctl(2)
       PR_SET_CHILD_SUBREAPER command to mark itself as the reaper of
       orphaned descendant processes).  Note that because of the
       setns(2) and unshare(2) semantics described above, this may be
       the "init" process in the PID namespace that is the parent of the
       child's PID namespace, rather than the "init" process in the
       child's own PID namespace.

   Compatibility of CLONE_NEWPID with other CLONE_* flags
       In current versions of Linux, CLONE_NEWPID can't be combined with
       CLONE_THREAD.  Threads are required to be in the same PID
       namespace such that the threads in a process can send signals to
       each other.  Similarly, it must be possible to see all of the
       threads of a process in the proc(5) filesystem.  Additionally, if
       two threads were in different PID namespaces, the process ID of
       the process sending a signal could not be meaningfully encoded
       when a signal is sent (see the description of the siginfo_t type
       in sigaction(2)).  Since this is computed when a signal is
       enqueued, a signal queue shared by processes in multiple PID
       namespaces would defeat that.

       In earlier versions of Linux, CLONE_NEWPID was additionally
       disallowed (failing with the error EINVAL) in combination with
       CLONE_SIGHAND (before Linux 4.3) as well as CLONE_VM (before
       Linux 3.12).  The changes that lifted these restrictions have
       also been ported to earlier stable kernels.

   /proc and PID namespaces
       A /proc filesystem shows (in the /proc/pid directories) only
       processes visible in the PID namespace of the process that
       performed the mount, even if the /proc filesystem is viewed from
       processes in other namespaces.

       After creating a new PID namespace, it is useful for the child to
       change its root directory and mount a new procfs instance at
       /proc so that tools such as ps(1) work correctly.  If a new mount
       namespace is simultaneously created by including CLONE_NEWNS in
       the flags argument of clone(2) or unshare(2), then it isn't
       necessary to change the root directory: a new procfs instance can
       be mounted directly over /proc.

       From a shell, the command to mount /proc is:

           $ mount -t proc proc /proc

       Calling readlink(2) on the path /proc/self yields the process ID
       of the caller in the PID namespace of the procfs mount (i.e., the
       PID namespace of the process that mounted the procfs).  This can
       be useful for introspection purposes, when a process wants to
       discover its PID in other namespaces.

   /proc files
       /proc/sys/kernel/ns_last_pid (since Linux 3.3)
              This file (which is virtualized per PID namespace)
              displays the last PID that was allocated in this PID
              namespace.  When the next PID is allocated, the kernel
              will search for the lowest unallocated PID that is greater
              than this value, and when this file is subsequently read
              it will show that PID.

              This file is writable by a process that has the
              CAP_SYS_ADMIN or (since Linux 5.9) CAP_CHECKPOINT_RESTORE
              capability inside the user namespace that owns the PID
              namespace.  This makes it possible to determine the PID
              that is allocated to the next process that is created
              inside this PID namespace.

       When a process ID is passed over a UNIX domain socket to a
       process in a different PID namespace (see the description of
       SCM_CREDENTIALS in unix(7)), it is translated into the
       corresponding PID value in the receiving process's PID namespace.

STANDARDS         top


EXAMPLES         top

       See user_namespaces(7).

SEE ALSO         top

       clone(2), reboot(2), setns(2), unshare(2), proc(5),
       capabilities(7), credentials(7), mount_namespaces(7),
       namespaces(7), user_namespaces(7), switch_root(8)

Linux man-pages (unreleased)   2024-06-13              pid_namespaces(7)

Pages that refer to this page: nsenter(1)unshare(1)clone(2)fork(2)getpid(2)NS_GET_USERNS(2const)pidfd_send_signal(2)reboot(2)setns(2)unshare(2)cap_get_proc(3)lttng-ust(3)proc_locks(5)proc_sys_kernel(5)capabilities(7)credentials(7)namespaces(7)user_namespaces(7)