.\" %%%LICENSE_END
.\"
.\"
-.TH PID_NAMESPACES 7 2013-01-14 "Linux" "Linux Programmer's Manual"
+.TH PID_NAMESPACES 7 2019-03-06 "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).
-
+.PP
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.
-
+.PP
PIDs in a new PID namespace start at 1,
somewhat like a standalone system, and calls to
.BR fork (2),
or
.BR clone (2)
will produce processes with PIDs that are unique within the namespace.
-
+.PP
Use of PID namespaces requires a kernel that is configured with the
.B CONFIG_PID_NS
option.
.BR CLONE_NEWPID
flag) has the PID 1, and is the "init" process for the namespace (see
.BR init (1)).
-A child process that is orphaned within the namespace will be reparented
-to this process rather than
-.BR init (1)
-(unless one of the ancestors of the child
-in the same PID namespace employed the
-.BR prctl (2)
-.B PR_GET_CHILD_SUBREAPER
-command to mark itself as the reaper of orphaned descendant processes).
-
+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).
+.PP
If the "init" process of a PID namespace terminates,
the kernel terminates all of the processes in the namespace via a
.BR SIGKILL
is essential for the correct operation of a PID namespace.
In this case, a subsequent
.BR fork (2)
-into this PID namespace will fail with the error
+into this PID namespace fail with the error
.BR ENOMEM ;
-it is not possible to create a new processes in a PID namespace whose "init"
+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
.BR fork (2)
terminates, then subsequent calls to
.BR fork (2)
-will fail with
+fail with
.BR ENOMEM .
-
+.PP
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.
-
+.PP
Likewise, a process in an ancestor namespace
can\(emsubject to the usual permission checks described in
.BR kill (2)\(emsend
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).
-
+.PP
Starting with Linux 3.4, the
.BR reboot (2)
-system causes a signal to be sent to the namespace "init" process.
+system call causes a signal to be sent to the namespace "init" process.
See
.BR reboot (2)
for more details.
.BR unshare (2).
PID namespaces thus form a tree,
with all namespaces ultimately tracing their ancestry to the root namespace.
-
+Since Linux 3.7,
+.\" commit f2302505775fd13ba93f034206f1e2a587017929
+.\" The kernel constant MAX_PID_NS_LEVEL
+the kernel limits the maximum nesting depth for PID namespaces to 32.
+.PP
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.
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 namespace.
+processes in the parent and further removed ancestor namespaces.
More succinctly: a process can see (e.g., send signals with
.BR kill (2),
set nice values with
.BR setpriority (2),
etc.) only processes contained in its own PID namespace
and in descendants of that namespace.
-
+.PP
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
.BR getpid (2)
always returns the PID associated with the namespace in which
the process was created.
-
+.PP
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
Calls to
.BR getppid (2)
for such processes return 0.
+.PP
+While processes may freely descend into child PID namespaces
+(e.g., using
+.BR 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.
+.PP
+The
+.BR NS_GET_PARENT
+.BR ioctl (2)
+operation can be used to discover the parental relationship
+between PID namespaces; see
+.BR ioctl_ns (2).
.\"
.\" ============================================================
.\"
.BR 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
+.IR /proc/[pid]/ns/pid_for_children
+file, as described in
+.BR 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
.BR getpid ()),
which would break many applications and libraries.
-
+.PP
To put things another way:
a process's PID namespace membership is determined when the process is created
and cannot be changed thereafter.
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.
+.PP
+A process may call
+.BR unshare (2)
+with the
+.B CLONE_NEWPID
+flag only once.
+After it has performed this operation, its
+.IR /proc/PID/ns/pid_for_children
+symbolic link will be empty until the first child is created in the namespace.
+.\"
+.\" ============================================================
+.\"
+.SS 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
+.BR prctl (2)
+.B PR_SET_CHILD_SUBREAPER
+command to mark itself as the reaper of orphaned descendant processes).
+Note that because of the
+.BR setns (2)
+and
+.BR unshare (2)
+semantics described above, this may be the "init" process in the PID
+namespace that is the
+.I parent
+of the child's PID namespace,
+rather than the "init" process in the child's own PID namespace.
+\" Furthermore, by definition, the parent of the "init" process
+.\" of a PID namespace resides in the parent PID namespace.
+.\"
+.\" ============================================================
+.\"
.SS Compatibility of CLONE_NEWPID with other CLONE_* flags
+In current versions of Linux,
.BR CLONE_NEWPID
-can't be combined with some other
-.BR CLONE_*
-flags:
-.IP * 3
-.B CLONE_THREAD
-requires being in the same PID namespace in order that that
+can't be combined with
+.BR 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 processes in the
.BR proc (5)
filesystem.
-.IP *
-.BR CLONE_SIGHAND
-requires being in the same PID namespace;
-otherwise the process ID of the process sending a signal
+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
.I siginfo_t
type in
.BR sigaction (2)).
-A signal queue shared by processes in multiple PID namespaces
-will defeat that.
-.IP *
-.BR CLONE_VM
-requires 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 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.
+Since this is computed when a signal is enqueued,
+a signal queue shared by processes in multiple PID namespaces
+would defeat that.
.PP
-To summarize: there is a technical requirement for each of
-.BR CLONE_THREAD ,
-.BR CLONE_SIGHAND ,
-and
-.BR CLONE_VM
-to share a PID namespace.
-(Note furthermore that in
-.BR clone (2)
-requires
-.BR CLONE_VM
-to be specified if
-.BR CLONE_THREAD
-or
+.\" Note these restrictions were all introduced in
+.\" 8382fcac1b813ad0a4e68a838fc7ae93fa39eda0
+.\" when CLONE_NEWPID|CLONE_VM was disallowed
+In earlier versions of Linux,
+.BR CLONE_NEWPID
+was additionally disallowed (failing with the error
+.BR EINVAL )
+in combination with
.BR CLONE_SIGHAND
-is specified.)
-Thus, call sequences such as the following will fail (with the error
-.BR EINVAL ):
-
-.nf
- 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 */
-.fi
+.\" (restriction lifted in faf00da544045fdc1454f3b9e6d7f65c841de302)
+(before Linux 4.3) as well as
+.\" (restriction lifted in e79f525e99b04390ca4d2366309545a836c03bf1)
+.BR CLONE_VM
+(before Linux 3.12).
+The changes that lifted these restrictions have also been ported to
+earlier stable kernels.
.\"
.\" ============================================================
.\"
A
.I /proc
filesystem shows (in the
-.I /proc/PID
+.I /proc/[pid]
directories) only processes visible in the PID namespace
of the process that performed the mount, even if the
.I /proc
filesystem is viewed from processes in other namespaces.
-
+.PP
After creating a new PID namespace,
it is useful for the child to change its root directory
and mount a new procfs instance at
then it isn't necessary to change the root directory:
a new procfs instance can be mounted directly over
.IR /proc .
-
+.PP
From a shell, the command to mount
.I /proc
is:
-
- $ mount -t proc proc /proc
-
+.PP
+.in +4n
+.EX
+$ mount -t proc proc /proc
+.EE
+.in
+.PP
Calling
.BR readlink (2)
on the path
.\"
.\" ============================================================
.\"
+.SS /proc files
+.TP
+.BR /proc/sys/kernel/ns_last_pid " (since Linux 3.3)"
+.\" commit b8f566b04d3cddd192cfd2418ae6d54ac6353792
+This file 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.
+.IP
+This file is writable by a process that has the
+.B CAP_SYS_ADMIN
+capability inside its user namespace.
+.\" This ability is necessary to support checkpoint restore in user-space
+This makes it possible to determine the PID that is allocated
+to the next process that is created inside this PID namespace.
+.\"
+.\" ============================================================
+.\"
.SS Miscellaneous
When a process ID is passed over a UNIX domain socket to a
process in a different PID namespace (see the description of
.BR user_namespaces (7).
.SH SEE ALSO
.BR clone (2),
+.BR reboot (2),
.BR setns (2),
.BR unshare (2),
.BR proc (5),
-.BR credentials (7),
.BR capabilities (7),
+.BR credentials (7),
+.BR mount_namespaces (7),
+.BR namespaces (7),
.BR user_namespaces (7),
.BR switch_root (8)
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