# pid_namespaces

## NAME



           pid_namespaces - overview of Linux PID namespaces
    


## DESCRIPTION



           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 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) 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.
    
       **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



           Linux.
    


## EXAMPLES



           See user_namespaces(7).
    


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