pid_namespaces(7) - Linux manual page (original) (raw)

pidnamespaces(7) Miscellaneous Information Manual pidnamespaces(7)

NAME top

   pid_namespaces - overview of Linux PID namespaces

DESCRIPTION top

   For an overview of namespaces, see [namespaces(7)](../man7/namespaces.7.html).

   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)](../man2/fork.2.html), [vfork(2)](../man2/vfork.2.html), or [clone(2)](../man2/clone.2.html) will produce
   processes with PIDs that are unique within the namespace.

   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 details).

   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)](../man2/fork.2.html) 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)](../man2/setns.2.html) into that namespace after the
   "init" process has terminated.  Another possible scenario can
   occur after a call to [unshare(2)](../man2/unshare.2.html): if the first child subsequently
   created by a [fork(2)](../man2/fork.2.html) terminates, then subsequent calls to [fork(2)](../man2/fork.2.html)
   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)](../man2/kill.2.html)—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 _siginfot sipid_ field described in [sigaction(2)](../man2/sigaction.2.html) 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)](../man2/reboot.2.html) system call causes a signal
   to be sent to the namespace "init" process.  See [reboot(2)](../man2/reboot.2.html) 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)](../man2/kill.2.html), set nice
   values with [setpriority(2)](../man2/setpriority.2.html), 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)](../man2/getpid.2.html) 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)](../man1/init.1.html) process with PID 1) is
   necessarily in another namespace.  Likewise, the direct children
   of a process that uses [setns(2)](../man2/setns.2.html) to cause its children to join a
   PID namespace are in a different PID namespace from the caller of
   [setns(2)](../man2/setns.2.html).  Calls to [getppid(2)](../man2/getppid.2.html) for such processes return 0.

   While processes may freely descend into child PID namespaces
   (e.g., using [setns(2)](../man2/setns.2.html) 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)](../man2/ioctl%5Fnsfs.2.html).

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/pidforchildren 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)](../man2/unshare.2.html) with the **CLONE_NEWPID** flag only
   once.  After it has performed this operation, its
   _/proc/_pid_/ns/pidforchildren_ 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 siginfot 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)](../man1/ps.1.html) work correctly.  If a new mount
   namespace is simultaneously created by including **CLONE_NEWNS** in
   the _flags_ argument of [clone(2)](../man2/clone.2.html) or [unshare(2)](../man2/unshare.2.html), 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)](../man2/readlink.2.html) 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.

Miscellaneous 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

   Linux.

EXAMPLES top

   See [user_namespaces(7)](../man7/user%5Fnamespaces.7.html).

COLOPHON top

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Linux man-pages 6.10 2024-06-13 pidnamespaces(7)