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_TGID,
  11        PIDTYPE_PGID,
  12        PIDTYPE_SID,
  13        PIDTYPE_MAX,
  14};
  15
  16/*
  17 * What is struct pid?
  18 *
  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.
  25 *
  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.
  30 *
  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.
  37 *
  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
  42 * processes.
  43 */
  44
  45
  46/*
  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.
  50 */
  51
  52struct upid {
  53        int nr;
  54        struct pid_namespace *ns;
  55};
  56
  57struct pid
  58{
  59        atomic_t count;
  60        unsigned int level;
  61        /* lists of tasks that use this pid */
  62        struct hlist_head tasks[PIDTYPE_MAX];
  63        struct rcu_head rcu;
  64        struct upid numbers[1];
  65};
  66
  67extern struct pid init_struct_pid;
  68
  69static inline struct pid *get_pid(struct pid *pid)
  70{
  71        if (pid)
  72                atomic_inc(&pid->count);
  73        return pid;
  74}
  75
  76extern void put_pid(struct pid *pid);
  77extern struct task_struct *pid_task(struct pid *pid, enum pid_type);
  78extern struct task_struct *get_pid_task(struct pid *pid, enum pid_type);
  79
  80extern struct pid *get_task_pid(struct task_struct *task, enum pid_type type);
  81
  82/*
  83 * these helpers must be called with the tasklist_lock write-held.
  84 */
  85extern void attach_pid(struct task_struct *task, enum pid_type);
  86extern void detach_pid(struct task_struct *task, enum pid_type);
  87extern void change_pid(struct task_struct *task, enum pid_type,
  88                        struct pid *pid);
  89extern void transfer_pid(struct task_struct *old, struct task_struct *new,
  90                         enum pid_type);
  91
  92struct pid_namespace;
  93extern struct pid_namespace init_pid_ns;
  94
  95/*
  96 * look up a PID in the hash table. Must be called with the tasklist_lock
  97 * or rcu_read_lock() held.
  98 *
  99 * find_pid_ns() finds the pid in the namespace specified
 100 * find_vpid() finds the pid by its virtual id, i.e. in the current namespace
 101 *
 102 * see also find_task_by_vpid() set in include/linux/sched.h
 103 */
 104extern struct pid *find_pid_ns(int nr, struct pid_namespace *ns);
 105extern struct pid *find_vpid(int nr);
 106
 107/*
 108 * Lookup a PID in the hash table, and return with it's count elevated.
 109 */
 110extern struct pid *find_get_pid(int nr);
 111extern struct pid *find_ge_pid(int nr, struct pid_namespace *);
 112int next_pidmap(struct pid_namespace *pid_ns, unsigned int last);
 113
 114extern struct pid *alloc_pid(struct pid_namespace *ns);
 115extern void free_pid(struct pid *pid);
 116extern void disable_pid_allocation(struct pid_namespace *ns);
 117
 118/*
 119 * ns_of_pid() returns the pid namespace in which the specified pid was
 120 * allocated.
 121 *
 122 * NOTE:
 123 *      ns_of_pid() is expected to be called for a process (task) that has
 124 *      an attached 'struct pid' (see attach_pid(), detach_pid()) i.e @pid
 125 *      is expected to be non-NULL. If @pid is NULL, caller should handle
 126 *      the resulting NULL pid-ns.
 127 */
 128static inline struct pid_namespace *ns_of_pid(struct pid *pid)
 129{
 130        struct pid_namespace *ns = NULL;
 131        if (pid)
 132                ns = pid->numbers[pid->level].ns;
 133        return ns;
 134}
 135
 136/*
 137 * is_child_reaper returns true if the pid is the init process
 138 * of the current namespace. As this one could be checked before
 139 * pid_ns->child_reaper is assigned in copy_process, we check
 140 * with the pid number.
 141 */
 142static inline bool is_child_reaper(struct pid *pid)
 143{
 144        return pid->numbers[pid->level].nr == 1;
 145}
 146
 147/*
 148 * the helpers to get the pid's id seen from different namespaces
 149 *
 150 * pid_nr()    : global id, i.e. the id seen from the init namespace;
 151 * pid_vnr()   : virtual id, i.e. the id seen from the pid namespace of
 152 *               current.
 153 * pid_nr_ns() : id seen from the ns specified.
 154 *
 155 * see also task_xid_nr() etc in include/linux/sched.h
 156 */
 157
 158static inline pid_t pid_nr(struct pid *pid)
 159{
 160        pid_t nr = 0;
 161        if (pid)
 162                nr = pid->numbers[0].nr;
 163        return nr;
 164}
 165
 166pid_t pid_nr_ns(struct pid *pid, struct pid_namespace *ns);
 167pid_t pid_vnr(struct pid *pid);
 168
 169#define do_each_pid_task(pid, type, task)                               \
 170        do {                                                            \
 171                if ((pid) != NULL)                                      \
 172                        hlist_for_each_entry_rcu((task),                \
 173                                &(pid)->tasks[type], pid_links[type]) {
 174
 175                        /*
 176                         * Both old and new leaders may be attached to
 177                         * the same pid in the middle of de_thread().
 178                         */
 179#define while_each_pid_task(pid, type, task)                            \
 180                                if (type == PIDTYPE_PID)                \
 181                                        break;                          \
 182                        }                                               \
 183        } while (0)
 184
 185#define do_each_pid_thread(pid, type, task)                             \
 186        do_each_pid_task(pid, type, task) {                             \
 187                struct task_struct *tg___ = task;                       \
 188                for_each_thread(tg___, task) {
 189
 190#define while_each_pid_thread(pid, type, task)                          \
 191                }                                                       \
 192                task = tg___;                                           \
 193        } while_each_pid_task(pid, type, task)
 194#endif /* _LINUX_PID_H */
 195