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27 .TH PID_NAMESPACES 7 2016-12-12 "Linux" "Linux Programmer's Manual"
29 pid_namespaces \- overview of Linux PID namespaces
31 For an overview of namespaces, see
34 PID namespaces isolate the process ID number space,
35 meaning that processes in different PID namespaces can have the same PID.
36 PID namespaces allow containers to provide functionality
37 such as suspending/resuming the set of processes in the container and
38 migrating the container to a new host
39 while the processes inside the container maintain the same PIDs.
41 PIDs in a new PID namespace start at 1,
42 somewhat like a standalone system, and calls to
47 will produce processes with PIDs that are unique within the namespace.
49 Use of PID namespaces requires a kernel that is configured with the
53 .\" ============================================================
55 .SS The namespace "init" process
56 The first process created in a new namespace
57 (i.e., the process created using
61 flag, or the first child created by a process after a call to
65 flag) has the PID 1, and is the "init" process for the namespace (see
67 A child process that is orphaned within the namespace will be reparented
68 to this process rather than
70 (unless one of the ancestors of the child
71 in the same PID namespace employed the
73 .B PR_SET_CHILD_SUBREAPER
74 command to mark itself as the reaper of orphaned descendant processes).
76 If the "init" process of a PID namespace terminates,
77 the kernel terminates all of the processes in the namespace via a
80 This behavior reflects the fact that the "init" process
81 is essential for the correct operation of a PID namespace.
82 In this case, a subsequent
84 into this PID namespace will fail with the error
86 it is not possible to create a new processes in a PID namespace whose "init"
87 process has terminated.
88 Such scenarios can occur when, for example,
89 a process uses an open file descriptor for a
91 file corresponding to a process that was in a namespace to
93 into that namespace after the "init" process has terminated.
94 Another possible scenario can occur after a call to
96 if the first child subsequently created by a
98 terminates, then subsequent calls to
103 Only signals for which the "init" process has established a signal handler
104 can be sent to the "init" process by other members of the PID namespace.
105 This restriction applies even to privileged processes,
106 and prevents other members of the PID namespace from
107 accidentally killing the "init" process.
109 Likewise, a process in an ancestor namespace
110 can\(emsubject to the usual permission checks described in
112 signals to the "init" process of a child PID namespace only
113 if the "init" process has established a handler for that signal.
114 (Within the handler, the
123 are treated exceptionally:
124 these signals are forcibly delivered when sent from an ancestor PID namespace.
125 Neither of these signals can be caught by the "init" process,
126 and so will result in the usual actions associated with those signals
127 (respectively, terminating and stopping the process).
129 Starting with Linux 3.4, the
131 system call causes a signal to be sent to the namespace "init" process.
136 .\" ============================================================
138 .SS Nesting PID namespaces
139 PID namespaces can be nested:
140 each PID namespace has a parent,
141 except for the initial ("root") PID namespace.
142 The parent of a PID namespace is the PID namespace of the process that
143 created the namespace using
147 PID namespaces thus form a tree,
148 with all namespaces ultimately tracing their ancestry to the root namespace.
150 .\" commit f2302505775fd13ba93f034206f1e2a587017929
151 .\" The kernel constant MAX_PID_NS_LEVEL
152 the kernel limits the maximum nesting depth for PID namespaces to 32.
154 A process is visible to other processes in its PID namespace,
155 and to the processes in each direct ancestor PID namespace
156 going back to the root PID namespace.
157 In this context, "visible" means that one process
158 can be the target of operations by another process using
159 system calls that specify a process ID.
160 Conversely, the processes in a child PID namespace can't see
161 processes in the parent and further removed ancestor namespaces.
162 More succinctly: a process can see (e.g., send signals with
166 etc.) only processes contained in its own PID namespace
167 and in descendants of that namespace.
169 A process has one process ID in each of the layers of the PID
170 namespace hierarchy in which is visible,
171 and walking back though each direct ancestor namespace
172 through to the root PID namespace.
173 System calls that operate on process IDs always
174 operate using the process ID that is visible in the
175 PID namespace of the caller.
178 always returns the PID associated with the namespace in which
179 the process was created.
181 Some processes in a PID namespace may have parents
182 that are outside of the namespace.
183 For example, the parent of the initial process in the namespace
186 process with PID 1) is necessarily in another namespace.
187 Likewise, the direct children of a process that uses
189 to cause its children to join a PID namespace are in a different
190 PID namespace from the caller of
194 for such processes return 0.
196 While processes may freely descend into child PID namespaces
201 they may not move in the other direction.
202 That is to say, processes may not enter any ancestor namespaces
203 (parent, grandparent, etc.).
204 Changing PID namespaces is a one-way operation.
209 operation can be used to discover the parental relationship
210 between PID namespaces; see
213 .\" ============================================================
215 .SS setns(2) and unshare(2) semantics
218 that specify a PID namespace file descriptor
223 flag cause children subsequently created
224 by the caller to be placed in a different PID namespace from the caller.
225 These calls do not, however,
226 change the PID namespace of the calling process,
227 because doing so would change the caller's idea of its own PID
230 which would break many applications and libraries.
232 To put things another way:
233 a process's PID namespace membership is determined when the process is created
234 and cannot be changed thereafter.
235 Among other things, this means that the parental relationship
236 between processes mirrors the parental relationship between PID namespaces:
237 the parent of a process is either in the same namespace
238 or resides in the immediate parent PID namespace.
239 .SS Compatibility of CLONE_NEWPID with other CLONE_* flags
240 In current versions of Linux,
242 can't be combined with
244 Threads are required to be in the same PID namespace such that
245 the threads in a process can send signals to each other.
246 Similarly, it must be possible to see all of the threads
247 of a processes in the
250 Additionally, if two threads were in different PID
251 namespaces, the process ID of the process sending a signal
252 could not be meaningfully encoded when a signal is sent
253 (see the description of the
257 Since this is computed when a signal is enqueued,
258 a signal queue shared by processes in multiple PID namespaces
261 .\" Note these restrictions were all introduced in
262 .\" 8382fcac1b813ad0a4e68a838fc7ae93fa39eda0
263 .\" when CLONE_NEWPID|CLONE_VM was disallowed
264 In earlier versions of Linux,
266 was additionally disallowed (failing with the error
270 .\" (restriction lifted in faf00da544045fdc1454f3b9e6d7f65c841de302)
271 (before Linux 4.3) as well as
272 .\" (restriction lifted in e79f525e99b04390ca4d2366309545a836c03bf1)
275 The changes that lifted these restrictions have also been ported to
276 earlier stable kernels.
278 .\" ============================================================
280 .SS /proc and PID namespaces
283 filesystem shows (in the
285 directories) only processes visible in the PID namespace
286 of the process that performed the mount, even if the
288 filesystem is viewed from processes in other namespaces.
290 After creating a new PID namespace,
291 it is useful for the child to change its root directory
292 and mount a new procfs instance at
294 so that tools such as
297 If a new mount namespace is simultaneously created by including
305 then it isn't necessary to change the root directory:
306 a new procfs instance can be mounted directly over
309 From a shell, the command to mount
313 $ mount -t proc proc /proc
319 yields the process ID of the caller in the PID namespace of the procfs mount
320 (i.e., the PID namespace of the process that mounted the procfs).
321 This can be useful for introspection purposes,
322 when a process wants to discover its PID in other namespaces.
324 .\" ============================================================
327 When a process ID is passed over a UNIX domain socket to a
328 process in a different PID namespace (see the description of
332 it is translated into the corresponding PID value in
333 the receiving process's PID namespace.
335 Namespaces are a Linux-specific feature.
338 .BR user_namespaces (7).
344 .BR capabilities (7),
347 .BR user_namespaces (7),