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.
Use of PID namespaces requires a kernel that is configured
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)). A child process
that is orphaned within the namespace will be reparented to
this process rather than init(1) (unless one of the
ancestors of the child in the same PID namespace employed
PR_SET_CHILD_SUBREAPER command to mark
itself as the reaper of orphaned descendant processes).
If the "init" process of a PID namespace terminates, the
kernel terminates all of the processes in the namespace via
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 will fail with the error ENOMEM; it is not possible to create a
new processes 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) will fail with
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)\(emsend 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
si_pid field described in
sigaction(2) will be
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).
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.
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
CLONE_NEWPID), 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.
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. 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.
CLONE_NEWPID can't be
combined with some other
requires being in the same PID namespace in order
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 processes in the proc(5)
requires being in the same PID namespace; otherwise
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
signal queue shared by processes in multiple PID
namespaces will defeat that.
all of the threads to be in the same PID namespace,
because, from the point of view of a core dump, if
two processes share the same address space then they
are threads and will be core dumped together. When a
core dump is written, the PID of each thread is
written into the core dump. Writing the process IDs
could not meaningfully succeed if some of the process
IDs were in a parent PID namespace.
To summarize: there is a technical requirement for each
CLONE_VM to share a PID namespace. (Note
furthermore that in clone(2) requires
CLONE_VM to be specified if
CLONE_SIGHAND is specified.) Thus, call
sequences such as the following will fail (with the error
unshare(CLONE_NEWPID); clone(..., CLONE_VM, ...); /* Fails */ setns(fd, CLONE_NEWPID); clone(..., CLONE_VM, ...); /* Fails */ clone(..., CLONE_VM, ...); setns(fd, CLONE_NEWPID); /* Fails */ clone(..., CLONE_VM, ...); unshare(CLONE_NEWPID); /* Fails */
/proc filesystem shows
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
/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
From a shell, the command to mount
$ 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
When a process ID is passed over a UNIX domain socket to
a process in a different PID namespace (see the description
unix(7)), it is
translated into the corresponding PID value in the
receiving process's PID namespace.
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