pid_namespaces - overview of Linux PID namespaces
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 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.
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_ns(2).
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.
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.
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.
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 /procCalling 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.
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.
Linux.
See user_namespaces(7).
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)