1 /* SPDX-License-Identifier: GPL-2.0 */
2 #ifndef _LINUX_PID_H
3 #define _LINUX_PID_H
4
5 #include <linux/rculist.h>
6 #include <linux/wait.h>
7 #include <linux/refcount.h>
8
9 enum pid_type
10 {
11 PIDTYPE_PID,
12 PIDTYPE_TGID,
13 PIDTYPE_PGID,
14 PIDTYPE_SID,
15 PIDTYPE_MAX,
16 };
17
18 /*
19 * What is struct pid?
20 *
21 * A struct pid is the kernel's internal notion of a process identifier.
22 * It refers to individual tasks, process groups, and sessions. While
23 * there are processes attached to it the struct pid lives in a hash
24 * table, so it and then the processes that it refers to can be found
25 * quickly from the numeric pid value. The attached processes may be
26 * quickly accessed by following pointers from struct pid.
27 *
28 * Storing pid_t values in the kernel and referring to them later has a
29 * problem. The process originally with that pid may have exited and the
30 * pid allocator wrapped, and another process could have come along
31 * and been assigned that pid.
32 *
33 * Referring to user space processes by holding a reference to struct
34 * task_struct has a problem. When the user space process exits
35 * the now useless task_struct is still kept. A task_struct plus a
36 * stack consumes around 10K of low kernel memory. More precisely
37 * this is THREAD_SIZE + sizeof(struct task_struct). By comparison
38 * a struct pid is about 64 bytes.
39 *
40 * Holding a reference to struct pid solves both of these problems.
41 * It is small so holding a reference does not consume a lot of
42 * resources, and since a new struct pid is allocated when the numeric pid
43 * value is reused (when pids wrap around) we don't mistakenly refer to new
44 * processes.
45 */
46
47
48 /*
49 * struct upid is used to get the id of the struct pid, as it is
50 * seen in particular namespace. Later the struct pid is found with
51 * find_pid_ns() using the int nr and struct pid_namespace *ns.
52 */
53
54 struct upid {
55 int nr;
56 struct pid_namespace *ns;
57 };
58
59 struct pid
60 {
61 refcount_t count;
62 unsigned int level;
63 /* lists of tasks that use this pid */
64 struct hlist_head tasks[PIDTYPE_MAX];
65 /* wait queue for pidfd notifications */
66 wait_queue_head_t wait_pidfd;
67 struct rcu_head rcu;
68 struct upid numbers[1];
69 };
70
71 extern struct pid init_struct_pid;
72
73 extern const struct file_operations pidfd_fops;
74
75 struct file;
76
77 extern struct pid *pidfd_pid(const struct file *file);
78
79 static inline struct pid *get_pid(struct pid *pid)
80 {
81 if (pid)
82 refcount_inc(&pid->count);
83 return pid;
84 }
85
86 extern void put_pid(struct pid *pid);
87 extern struct task_struct *pid_task(struct pid *pid, enum pid_type);
88 extern struct task_struct *get_pid_task(struct pid *pid, enum pid_type);
89
90 extern struct pid *get_task_pid(struct task_struct *task, enum pid_type type);
91
92 /*
93 * these helpers must be called with the tasklist_lock write-held.
94 */
95 extern void attach_pid(struct task_struct *task, enum pid_type);
96 extern void detach_pid(struct task_struct *task, enum pid_type);
97 extern void change_pid(struct task_struct *task, enum pid_type,
98 struct pid *pid);
99 extern void transfer_pid(struct task_struct *old, struct task_struct *new,
100 enum pid_type);
101
102 struct pid_namespace;
103 extern struct pid_namespace init_pid_ns;
104
105 /*
106 * look up a PID in the hash table. Must be called with the tasklist_lock
107 * or rcu_read_lock() held.
108 *
109 * find_pid_ns() finds the pid in the namespace specified
110 * find_vpid() finds the pid by its virtual id, i.e. in the current namespace
111 *
112 * see also find_task_by_vpid() set in include/linux/sched.h
113 */
114 extern struct pid *find_pid_ns(int nr, struct pid_namespace *ns);
115 extern struct pid *find_vpid(int nr);
116
117 /*
118 * Lookup a PID in the hash table, and return with it's count elevated.
119 */
120 extern struct pid *find_get_pid(int nr);
121 extern struct pid *find_ge_pid(int nr, struct pid_namespace *);
122
123 extern struct pid *alloc_pid(struct pid_namespace *ns);
124 extern void free_pid(struct pid *pid);
125 extern void disable_pid_allocation(struct pid_namespace *ns);
126
127 /*
128 * ns_of_pid() returns the pid namespace in which the specified pid was
129 * allocated.
130 *
131 * NOTE:
132 * ns_of_pid() is expected to be called for a process (task) that has
133 * an attached 'struct pid' (see attach_pid(), detach_pid()) i.e @pid
134 * is expected to be non-NULL. If @pid is NULL, caller should handle
135 * the resulting NULL pid-ns.
136 */
137 static inline struct pid_namespace *ns_of_pid(struct pid *pid)
138 {
139 struct pid_namespace *ns = NULL;
140 if (pid)
141 ns = pid->numbers[pid->level].ns;
142 return ns;
143 }
144
145 /*
146 * is_child_reaper returns true if the pid is the init process
147 * of the current namespace. As this one could be checked before
148 * pid_ns->child_reaper is assigned in copy_process, we check
149 * with the pid number.
150 */
151 static inline bool is_child_reaper(struct pid *pid)
152 {
153 return pid->numbers[pid->level].nr == 1;
154 }
155
156 /*
157 * the helpers to get the pid's id seen from different namespaces
158 *
159 * pid_nr() : global id, i.e. the id seen from the init namespace;
160 * pid_vnr() : virtual id, i.e. the id seen from the pid namespace of
161 * current.
162 * pid_nr_ns() : id seen from the ns specified.
163 *
164 * see also task_xid_nr() etc in include/linux/sched.h
165 */
166
167 static inline pid_t pid_nr(struct pid *pid)
168 {
169 pid_t nr = 0;
170 if (pid)
171 nr = pid->numbers[0].nr;
172 return nr;
173 }
174
175 pid_t pid_nr_ns(struct pid *pid, struct pid_namespace *ns);
176 pid_t pid_vnr(struct pid *pid);
177
178 #define do_each_pid_task(pid, type, task) \
179 do { \
180 if ((pid) != NULL) \
181 hlist_for_each_entry_rcu((task), \
182 &(pid)->tasks[type], pid_links[type]) {
183
184 /*
185 * Both old and new leaders may be attached to
186 * the same pid in the middle of de_thread().
187 */
188 #define while_each_pid_task(pid, type, task) \
189 if (type == PIDTYPE_PID) \
190 break; \
191 } \
192 } while (0)
193
194 #define do_each_pid_thread(pid, type, task) \
195 do_each_pid_task(pid, type, task) { \
196 struct task_struct *tg___ = task; \
197 for_each_thread(tg___, task) {
198
199 #define while_each_pid_thread(pid, type, task) \
200 } \
201 task = tg___; \
202 } while_each_pid_task(pid, type, task)
203 #endif /* _LINUX_PID_H */