LXR linux/include/linux/pid.h

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