1 /* SPDX-License-Identifier: GPL-2.0 */
5 #include <linux/rculist.h>
19 * A struct pid is the kernel's internal notion of a process identifier.
20 * It refers to individual tasks, process groups, and sessions. While
21 * there are processes attached to it the struct pid lives in a hash
22 * table, so it and then the processes that it refers to can be found
23 * quickly from the numeric pid value. The attached processes may be
24 * quickly accessed by following pointers from struct pid.
26 * Storing pid_t values in the kernel and referring to them later has a
27 * problem. The process originally with that pid may have exited and the
28 * pid allocator wrapped, and another process could have come along
29 * and been assigned that pid.
31 * Referring to user space processes by holding a reference to struct
32 * task_struct has a problem. When the user space process exits
33 * the now useless task_struct is still kept. A task_struct plus a
34 * stack consumes around 10K of low kernel memory. More precisely
35 * this is THREAD_SIZE + sizeof(struct task_struct). By comparison
36 * a struct pid is about 64 bytes.
38 * Holding a reference to struct pid solves both of these problems.
39 * It is small so holding a reference does not consume a lot of
40 * resources, and since a new struct pid is allocated when the numeric pid
41 * value is reused (when pids wrap around) we don't mistakenly refer to new
47 * struct upid is used to get the id of the struct pid, as it is
48 * seen in particular namespace. Later the struct pid is found with
49 * find_pid_ns() using the int nr and struct pid_namespace *ns.
54 struct pid_namespace *ns;
61 /* lists of tasks that use this pid */
62 struct hlist_head tasks[PIDTYPE_MAX];
64 struct upid numbers[1];
67 extern struct pid init_struct_pid;
69 extern const struct file_operations pidfd_fops;
71 static inline struct pid *get_pid(struct pid *pid)
74 atomic_inc(&pid->count);
78 extern void put_pid(struct pid *pid);
79 extern struct task_struct *pid_task(struct pid *pid, enum pid_type);
80 extern struct task_struct *get_pid_task(struct pid *pid, enum pid_type);
82 extern struct pid *get_task_pid(struct task_struct *task, enum pid_type type);
85 * these helpers must be called with the tasklist_lock write-held.
87 extern void attach_pid(struct task_struct *task, enum pid_type);
88 extern void detach_pid(struct task_struct *task, enum pid_type);
89 extern void change_pid(struct task_struct *task, enum pid_type,
91 extern void transfer_pid(struct task_struct *old, struct task_struct *new,
95 extern struct pid_namespace init_pid_ns;
98 * look up a PID in the hash table. Must be called with the tasklist_lock
99 * or rcu_read_lock() held.
101 * find_pid_ns() finds the pid in the namespace specified
102 * find_vpid() finds the pid by its virtual id, i.e. in the current namespace
104 * see also find_task_by_vpid() set in include/linux/sched.h
106 extern struct pid *find_pid_ns(int nr, struct pid_namespace *ns);
107 extern struct pid *find_vpid(int nr);
110 * Lookup a PID in the hash table, and return with it's count elevated.
112 extern struct pid *find_get_pid(int nr);
113 extern struct pid *find_ge_pid(int nr, struct pid_namespace *);
115 extern struct pid *alloc_pid(struct pid_namespace *ns);
116 extern void free_pid(struct pid *pid);
117 extern void disable_pid_allocation(struct pid_namespace *ns);
120 * ns_of_pid() returns the pid namespace in which the specified pid was
124 * ns_of_pid() is expected to be called for a process (task) that has
125 * an attached 'struct pid' (see attach_pid(), detach_pid()) i.e @pid
126 * is expected to be non-NULL. If @pid is NULL, caller should handle
127 * the resulting NULL pid-ns.
129 static inline struct pid_namespace *ns_of_pid(struct pid *pid)
131 struct pid_namespace *ns = NULL;
133 ns = pid->numbers[pid->level].ns;
138 * is_child_reaper returns true if the pid is the init process
139 * of the current namespace. As this one could be checked before
140 * pid_ns->child_reaper is assigned in copy_process, we check
141 * with the pid number.
143 static inline bool is_child_reaper(struct pid *pid)
145 return pid->numbers[pid->level].nr == 1;
149 * the helpers to get the pid's id seen from different namespaces
151 * pid_nr() : global id, i.e. the id seen from the init namespace;
152 * pid_vnr() : virtual id, i.e. the id seen from the pid namespace of
154 * pid_nr_ns() : id seen from the ns specified.
156 * see also task_xid_nr() etc in include/linux/sched.h
159 static inline pid_t pid_nr(struct pid *pid)
163 nr = pid->numbers[0].nr;
167 pid_t pid_nr_ns(struct pid *pid, struct pid_namespace *ns);
168 pid_t pid_vnr(struct pid *pid);
170 #define do_each_pid_task(pid, type, task) \
173 hlist_for_each_entry_rcu((task), \
174 &(pid)->tasks[type], pid_links[type]) {
177 * Both old and new leaders may be attached to
178 * the same pid in the middle of de_thread().
180 #define while_each_pid_task(pid, type, task) \
181 if (type == PIDTYPE_PID) \
186 #define do_each_pid_thread(pid, type, task) \
187 do_each_pid_task(pid, type, task) { \
188 struct task_struct *tg___ = task; \
189 for_each_thread(tg___, task) {
191 #define while_each_pid_thread(pid, type, task) \
194 } while_each_pid_task(pid, type, task)
195 #endif /* _LINUX_PID_H */