linux/include/linux/pid.h
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   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
   9enum 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
  54struct upid {
  55        int nr;
  56        struct pid_namespace *ns;
  57};
  58
  59struct 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
  71extern struct pid init_struct_pid;
  72
  73extern const struct file_operations pidfd_fops;
  74
  75struct file;
  76
  77extern struct pid *pidfd_pid(const struct file *file);
  78
  79static inline struct pid *get_pid(struct pid *pid)
  80{
  81        if (pid)
  82                refcount_inc(&pid->count);
  83        return pid;
  84}
  85
  86extern void put_pid(struct pid *pid);
  87extern struct task_struct *pid_task(struct pid *pid, enum pid_type);
  88extern struct task_struct *get_pid_task(struct pid *pid, enum pid_type);
  89
  90extern 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 */
  95extern void attach_pid(struct task_struct *task, enum pid_type);
  96extern void detach_pid(struct task_struct *task, enum pid_type);
  97extern void change_pid(struct task_struct *task, enum pid_type,
  98                        struct pid *pid);
  99extern void transfer_pid(struct task_struct *old, struct task_struct *new,
 100                         enum pid_type);
 101
 102struct pid_namespace;
 103extern 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 */
 114extern struct pid *find_pid_ns(int nr, struct pid_namespace *ns);
 115extern struct pid *find_vpid(int nr);
 116
 117/*
 118 * Lookup a PID in the hash table, and return with it's count elevated.
 119 */
 120extern struct pid *find_get_pid(int nr);
 121extern struct pid *find_ge_pid(int nr, struct pid_namespace *);
 122
 123extern struct pid *alloc_pid(struct pid_namespace *ns);
 124extern void free_pid(struct pid *pid);
 125extern 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 */
 137static 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 */
 151static 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
 167static 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
 175pid_t pid_nr_ns(struct pid *pid, struct pid_namespace *ns);
 176pid_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 */
 204