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_ns(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.
