man-pages/man7/pid_namespaces.7

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.TH PID_NAMESPACES 7 2013-01-14 "Linux" "Linux Programmer's Manual"
.SH NAME
pid_namespaces \- overview of Linux PID namespaces
.SH DESCRIPTION
For an overview of namespaces, see
.BR 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 migrate 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
.BR fork (2),
.BR vfork (2),
or
.BR 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
.B CONFIG_PID_NS
option.
.\"
.\" ============================================================
.\"
.SS The namespace "init" process
The first process created in a new namespace
(i.e., the process created using
.BR clone (2)
with the
.BR CLONE_NEWPID
flag, or the first child created by a process after a call to
.BR unshare (2)
using the
.BR CLONE_NEWPID
flag) has the PID 1, and is the "init" process for the namespace (see
.BR init (1)).
Children that are orphaned within the namespace will be reparented
to this process rather than
.BR init (1).

If the "init" process of a PID namespace terminates,
the kernel terminates all of the processes in the namespace via a
.BR 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 
.BR fork (2)
into this PID namespace (e.g., from a process that has done a
.BR setns (2)
into the namespace using an open file descriptor for a 
.I /proc/[pid]/ns/pid
file corresponding to a process that was in the namespace)
will fail with the error
.BR ENOMEM ;
it is not possible to create a new processes in a PID namespace whose "init"
process has terminated.

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\(emsubject to the usual permission checks described in
.BR 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
.I siginfo_t
.I si_pid
field described in
.BR sigaction (2)
will be zero.)
.B SIGKILL
or
.B 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).
.\"
.\" ============================================================
.\"
.SS Nesting PID namespaces
PID namespaces can be nested.
When a new PID namespace is created,
the processes in that namespace are visible
in the PID namespace of the process that created the new namespace;
analogously, if the parent PID namespace is itself
the child of another PID namespace,
then processes in the child and parent PID namespaces will both be
visible in the grandparent PID namespace.
Conversely, the processes in the "child" PID namespace do not see
the processes in the parent namespace.
More succinctly: a process can see (e.g., send signals with
.BR kill(2))
only processes contained in its own PID namespace
and the namespaces nested below that PID namespace.

A process will have one PID for each of the layers of the hierarchy
starting from the PID namespace in which it is visible,
starting from the namespace in which it resides
through to the root PID namespace.
A call to
.BR getpid (2)
always returns the PID associated with the namespace in which
the process resides.

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
.BR init (1)
process with PID 1) is necessarily in another namespace.
Likewise, the direct children of a process that uses
.BR setns (2)
to cause its children to join a PID namespace are in a different
PID namespace from the caller of
.BR setns (2).
Calls to
.BR getppid (2)
for such processes return 0.
.\"
.\" ============================================================
.\"
.SS setns(2) and unshare(2) semantics
Calls to
.BR setns (2)
that specify a PID namespace file descriptor
and calls to
.BR unshare (2)
with the
.BR 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
.BR 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.

Every thread in a process must be in the same PID namespace.
For this reason, the two following call sequences will fail:

.nf
    unshare(CLONE_NEWPID);
    clone(..., CLONE_VM, ...);    /* Fails */

    setns(fd, CLONE_NEWPID);
    clone(..., CLONE_VM, ...);    /* Fails */
.fi

Because the above
.BR unshare (2)
and
.BR setns (2)
calls only change the PID namespace for created children, the 
.BR clone (2)
calls necessarily put the new thread in a different PID namespace from
the calling thread.
.\"
.\" ============================================================
.\"
.SS Miscellaneous
After creating a new PID namespace,
it is useful for the child to change its root directory
and mount a new procfs instance at
.I /proc
so that tools such as
.BR ps (1)
work correctly.
.\" mount -t proc proc /proc
(If a new mount namespace is simultaneously created by including
.BR CLONE_NEWNS
in the
.IR flags 
argument of
.BR clone (2)
or
.BR unshare (2)),
then it isn't necessary to change the root directory:
a new procfs instance can be mounted directly over
.IR /proc .)

When a process ID is passed over a UNIX domain socket to a 
process in a different PID namespace (see the description of
.B SCM_CREDENTIALS
in
.BR unix (7)),
it is translated into the corresponding PID value in
the receiving process's PID namespace.
.\" FIXME Presumably, a similar thing happens with the UID and GID passed 
.\" via a UNIX domain socket. That needs to be confirmed and documented
.\" under the "user_namespaces" page.
.SH CONFORMING TO
Namespaces are a Linux-specific feature.
.SH SEE ALSO
.BR unshare (1),
.BR clone (2),
.BR setns (2),
.BR unshare (2),
.BR proc (5),
.BR credentials (7),
.BR capabilities (7),
.BR user_namespaces (7),
.BR switch_root (8)