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
   7 enum 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
  53 struct upid {
  54         /* Try to keep pid_chain in the same cacheline as nr for find_vpid */
  55         int nr;
  56         struct pid_namespace *ns;
  57         struct hlist_node pid_chain;
  58 };
  59
  60 struct pid
  61 {
  62         atomic_t count;
  63         unsigned int level;
  64         /* lists of tasks that use this pid */
  65         struct hlist_head tasks[PIDTYPE_MAX];
  66         struct rcu_head rcu;
  67         struct upid numbers[1];
  68 };
  69
  70 extern struct pid init_struct_pid;
  71
  72 struct pid_link
  73 {
  74         struct hlist_node node;
  75         struct pid *pid;
  76 };
  77
  78 static inline struct pid *get_pid(struct pid *pid)
  79 {
  80         if (pid)
  81                 atomic_inc(&pid->count);
  82         return pid;
  83 }
  84
  85 extern void put_pid(struct pid *pid);
  86 extern struct task_struct *pid_task(struct pid *pid, enum pid_type);
  87 extern struct task_struct *get_pid_task(struct pid *pid, enum pid_type);
  88
  89 extern struct pid *get_task_pid(struct task_struct *task, enum pid_type type);
  90
  91 /*
  92  * these helpers must be called with the tasklist_lock write-held.
  93  */
  94 extern void attach_pid(struct task_struct *task, enum pid_type);
  95 extern void detach_pid(struct task_struct *task, enum pid_type);
  96 extern void change_pid(struct task_struct *task, enum pid_type,
  97                         struct pid *pid);
  98 extern void transfer_pid(struct task_struct *old, struct task_struct *new,
  99                          enum pid_type);
 100
 101 struct pid_namespace;
 102 extern struct pid_namespace init_pid_ns;
 103
 104 /*
 105  * look up a PID in the hash table. Must be called with the tasklist_lock
 106  * or rcu_read_lock() held.
 107  *
 108  * find_pid_ns() finds the pid in the namespace specified
 109  * find_vpid() finds the pid by its virtual id, i.e. in the current namespace
 110  *
 111  * see also find_task_by_vpid() set in include/linux/sched.h
 112  */
 113 extern struct pid *find_pid_ns(int nr, struct pid_namespace *ns);
 114 extern struct pid *find_vpid(int nr);
 115
 116 /*
 117  * Lookup a PID in the hash table, and return with it's count elevated.
 118  */
 119 extern struct pid *find_get_pid(int nr);
 120 extern struct pid *find_ge_pid(int nr, struct pid_namespace *);
 121 int next_pidmap(struct pid_namespace *pid_ns, unsigned int last);
 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], pids[type].node) {
 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 */