1pid_namespaces(7) Miscellaneous Information Manual pid_namespaces(7)
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6 pid_namespaces - overview of Linux PID namespaces
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9 For an overview of namespaces, see namespaces(7).
10 PID namespaces isolate the process ID number space, meaning that pro‐
12 cesses in different PID namespaces can have the same PID. PID name‐
13 spaces allow containers to provide functionality such as suspending/re‐
14 suming the set of processes in the container and migrating the con‐
15 tainer to a new host while the processes inside the container maintain
16 the same PIDs.
17 PIDs in a new PID namespace start at 1, somewhat like a standalone sys‐
19 tem, and calls to fork(2), vfork(2), or clone(2) will produce processes
20 with PIDs that are unique within the namespace.
21 Use of PID namespaces requires a kernel that is configured with the
23 CONFIG_PID_NS option.
24 The namespace init process
26 The first process created in a new namespace (i.e., the process created
27 using clone(2) with the CLONE_NEWPID flag, or the first child created
28 by a process after a call to unshare(2) using the CLONE_NEWPID flag)
29 has the PID 1, and is the "init" process for the namespace (see
30 init(1)). This process becomes the parent of any child processes that
31 are orphaned because a process that resides in this PID namespace ter‐
32 minated (see below for further details).
33 If the "init" process of a PID namespace terminates, the kernel termi‐
35 nates all of the processes in the namespace via a SIGKILL signal. This
36 behavior reflects the fact that the "init" process is essential for the
37 correct operation of a PID namespace. In this case, a subsequent
38 fork(2) into this PID namespace fail with the error ENOMEM; it is not
39 possible to create a new process in a PID namespace whose "init"
40 process has terminated. Such scenarios can occur when, for example, a
41 process uses an open file descriptor for a /proc/pid/ns/pid file corre‐
42 sponding to a process that was in a namespace to setns(2) into that
43 namespace after the "init" process has terminated. Another possible
44 scenario can occur after a call to unshare(2): if the first child sub‐
45 sequently created by a fork(2) terminates, then subsequent calls to
46 fork(2) fail with ENOMEM.
47 Only signals for which the "init" process has established a signal han‐
49 dler can be sent to the "init" process by other members of the PID
50 namespace. This restriction applies even to privileged processes, and
51 prevents other members of the PID namespace from accidentally killing
52 the "init" process.
53 Likewise, a process in an ancestor namespace can—subject to the usual
55 permission checks described in kill(2)—send signals to the "init"
56 process of a child PID namespace only if the "init" process has estab‐
57 lished a handler for that signal. (Within the handler, the siginfo_t
58 si_pid field described in sigaction(2) will be zero.) SIGKILL or
59 SIGSTOP are treated exceptionally: these signals are forcibly delivered
60 when sent from an ancestor PID namespace. Neither of these signals can
61 be caught by the "init" process, and so will result in the usual ac‐
62 tions associated with those signals (respectively, terminating and
63 stopping the process).
64 Starting with Linux 3.4, the reboot(2) system call causes a signal to
66 be sent to the namespace "init" process. See reboot(2) for more de‐
67 tails.
68 Nesting PID namespaces
70 PID namespaces can be nested: each PID namespace has a parent, except
71 for the initial ("root") PID namespace. The parent of a PID namespace
72 is the PID namespace of the process that created the namespace using
73 clone(2) or unshare(2). PID namespaces thus form a tree, with all
74 namespaces ultimately tracing their ancestry to the root namespace.
75 Since Linux 3.7, the kernel limits the maximum nesting depth for PID
76 namespaces to 32.
77 A process is visible to other processes in its PID namespace, and to
79 the processes in each direct ancestor PID namespace going back to the
80 root PID namespace. In this context, "visible" means that one process
81 can be the target of operations by another process using system calls
82 that specify a process ID. Conversely, the processes in a child PID
83 namespace can't see processes in the parent and further removed ances‐
84 tor namespaces. More succinctly: a process can see (e.g., send signals
85 with kill(2), set nice values with setpriority(2), etc.) only processes
86 contained in its own PID namespace and in descendants of that name‐
87 space.
88 A process has one process ID in each of the layers of the PID namespace
90 hierarchy in which is visible, and walking back though each direct an‐
91 cestor namespace through to the root PID namespace. System calls that
92 operate on process IDs always operate using the process ID that is vis‐
93 ible in the PID namespace of the caller. A call to getpid(2) always
94 returns the PID associated with the namespace in which the process was
95 created.
96 Some processes in a PID namespace may have parents that are outside of
98 the namespace. For example, the parent of the initial process in the
99 namespace (i.e., the init(1) process with PID 1) is necessarily in an‐
100 other namespace. Likewise, the direct children of a process that uses
101 setns(2) to cause its children to join a PID namespace are in a differ‐
102 ent PID namespace from the caller of setns(2). Calls to getppid(2) for
103 such processes return 0.
104 While processes may freely descend into child PID namespaces (e.g., us‐
106 ing setns(2) with a PID namespace file descriptor), they may not move
107 in the other direction. That is to say, processes may not enter any
108 ancestor namespaces (parent, grandparent, etc.). Changing PID name‐
109 spaces is a one-way operation.
110 The NS_GET_PARENT ioctl(2) operation can be used to discover the
112 parental relationship between PID namespaces; see ioctl_ns(2).
113 setns(2) and unshare(2) semantics
115 Calls to setns(2) that specify a PID namespace file descriptor and
116 calls to unshare(2) with the CLONE_NEWPID flag cause children subse‐
117 quently created by the caller to be placed in a different PID namespace
118 from the caller. (Since Linux 4.12, that PID namespace is shown via
119 the /proc/pid/ns/pid_for_children file, as described in namespaces(7).)
120 These calls do not, however, change the PID namespace of the calling
121 process, because doing so would change the caller's idea of its own PID
122 (as reported by getpid()), which would break many applications and li‐
123 braries.
124 To put things another way: a process's PID namespace membership is de‐
126 termined when the process is created and cannot be changed thereafter.
127 Among other things, this means that the parental relationship between
128 processes mirrors the parental relationship between PID namespaces: the
129 parent of a process is either in the same namespace or resides in the
130 immediate parent PID namespace.
131 A process may call unshare(2) with the CLONE_NEWPID flag only once.
133 After it has performed this operation, its /proc/pid/ns/pid_for_chil‐
134 dren symbolic link will be empty until the first child is created in
135 the namespace.
136 Adoption of orphaned children
138 When a child process becomes orphaned, it is reparented to the "init"
139 process in the PID namespace of its parent (unless one of the nearer
140 ancestors of the parent employed the prctl(2) PR_SET_CHILD_SUBREAPER
141 command to mark itself as the reaper of orphaned descendant processes).
142 Note that because of the setns(2) and unshare(2) semantics described
143 above, this may be the "init" process in the PID namespace that is the
144 parent of the child's PID namespace, rather than the "init" process in
145 the child's own PID namespace.
146 Compatibility of CLONE_NEWPID with other CLONE_* flags
148 In current versions of Linux, CLONE_NEWPID can't be combined with
149 CLONE_THREAD. Threads are required to be in the same PID namespace
150 such that the threads in a process can send signals to each other.
151 Similarly, it must be possible to see all of the threads of a process
152 in the proc(5) filesystem. Additionally, if two threads were in dif‐
153 ferent PID namespaces, the process ID of the process sending a signal
154 could not be meaningfully encoded when a signal is sent (see the de‐
155 scription of the siginfo_t type in sigaction(2)). Since this is com‐
156 puted when a signal is enqueued, a signal queue shared by processes in
157 multiple PID namespaces would defeat that.
158 In earlier versions of Linux, CLONE_NEWPID was additionally disallowed
160 (failing with the error EINVAL) in combination with CLONE_SIGHAND (be‐
161 fore Linux 4.3) as well as CLONE_VM (before Linux 3.12). The changes
162 that lifted these restrictions have also been ported to earlier stable
163 kernels.
164 /proc and PID namespaces
166 A /proc filesystem shows (in the /proc/pid directories) only processes
167 visible in the PID namespace of the process that performed the mount,
168 even if the /proc filesystem is viewed from processes in other name‐
169 spaces.
170 After creating a new PID namespace, it is useful for the child to
172 change its root directory and mount a new procfs instance at /proc so
173 that tools such as ps(1) work correctly. If a new mount namespace is
174 simultaneously created by including CLONE_NEWNS in the flags argument
175 of clone(2) or unshare(2), then it isn't necessary to change the root
176 directory: a new procfs instance can be mounted directly over /proc.
177 From a shell, the command to mount /proc is:
179 $ mount -t proc proc /proc
181 Calling readlink(2) on the path /proc/self yields the process ID of the
183 caller in the PID namespace of the procfs mount (i.e., the PID name‐
184 space of the process that mounted the procfs). This can be useful for
185 introspection purposes, when a process wants to discover its PID in
186 other namespaces.
187 /proc files
189 /proc/sys/kernel/ns_last_pid (since Linux 3.3)
190 This file (which is virtualized per PID namespace) displays the
191 last PID that was allocated in this PID namespace. When the
192 next PID is allocated, the kernel will search for the lowest un‐
193 allocated PID that is greater than this value, and when this
194 file is subsequently read it will show that PID.
195 This file is writable by a process that has the CAP_SYS_ADMIN or
197 (since Linux 5.9) CAP_CHECKPOINT_RESTORE capability inside the
198 user namespace that owns the PID namespace. This makes it pos‐
199 sible to determine the PID that is allocated to the next process
200 that is created inside this PID namespace.
201 Miscellaneous
203 When a process ID is passed over a UNIX domain socket to a process in a
204 different PID namespace (see the description of SCM_CREDENTIALS in
205 unix(7)), it is translated into the corresponding PID value in the re‐
206 ceiving process's PID namespace.
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209 Linux.
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212 See user_namespaces(7).
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215 clone(2), reboot(2), setns(2), unshare(2), proc(5), capabilities(7),
216 credentials(7), mount_namespaces(7), namespaces(7), user_namespaces(7),
217 switch_root(8)
218Linux man-pages 6.05 2023-03-30 pid_namespaces(7)