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 #include <linux/wait.h>
   7 #include <linux/refcount.h>
   8
   9 enum 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
  54 struct upid {
  55         int nr;
  56         struct pid_namespace *ns;
  57 };
  58
  59 struct 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
  71 extern struct pid init_struct_pid;
  72
  73 extern const struct file_operations pidfd_fops;
  74
  75 struct file;
  76
  77 extern struct pid *pidfd_pid(const struct file *file);
  78
  79 static inline struct pid *get_pid(struct pid *pid)
  80 {
  81         if (pid)
  82                 refcount_inc(&pid->count);
  83         return pid;
  84 }
  85
  86 extern void put_pid(struct pid *pid);
  87 extern struct task_struct *pid_task(struct pid *pid, enum pid_type);
  88 static inline bool pid_has_task(struct pid *pid, enum pid_type type)
  89 {
  90         return !hlist_empty(&pid->tasks[type]);
  91 }
  92 extern struct task_struct *get_pid_task(struct pid *pid, enum pid_type);
  93
  94 extern struct pid *get_task_pid(struct task_struct *task, enum pid_type type);
  95
  96 /*
  97  * these helpers must be called with the tasklist_lock write-held.
  98  */
  99 extern void attach_pid(struct task_struct *task, enum pid_type);
 100 extern void detach_pid(struct task_struct *task, enum pid_type);
 101 extern void change_pid(struct task_struct *task, enum pid_type,
 102                         struct pid *pid);
 103 extern void transfer_pid(struct task_struct *old, struct task_struct *new,
 104                          enum pid_type);
 105
 106 struct pid_namespace;
 107 extern struct pid_namespace init_pid_ns;
 108
 109 /*
 110  * look up a PID in the hash table. Must be called with the tasklist_lock
 111  * or rcu_read_lock() held.
 112  *
 113  * find_pid_ns() finds the pid in the namespace specified
 114  * find_vpid() finds the pid by its virtual id, i.e. in the current namespace
 115  *
 116  * see also find_task_by_vpid() set in include/linux/sched.h
 117  */
 118 extern struct pid *find_pid_ns(int nr, struct pid_namespace *ns);
 119 extern struct pid *find_vpid(int nr);
 120
 121 /*
 122  * Lookup a PID in the hash table, and return with it's count elevated.
 123  */
 124 extern struct pid *find_get_pid(int nr);
 125 extern struct pid *find_ge_pid(int nr, struct pid_namespace *);
 126
 127 extern struct pid *alloc_pid(struct pid_namespace *ns, pid_t *set_tid,
 128                              size_t set_tid_size);
 129 extern void free_pid(struct pid *pid);
 130 extern void disable_pid_allocation(struct pid_namespace *ns);
 131
 132 /*
 133  * ns_of_pid() returns the pid namespace in which the specified pid was
 134  * allocated.
 135  *
 136  * NOTE:
 137  *      ns_of_pid() is expected to be called for a process (task) that has
 138  *      an attached 'struct pid' (see attach_pid(), detach_pid()) i.e @pid
 139  *      is expected to be non-NULL. If @pid is NULL, caller should handle
 140  *      the resulting NULL pid-ns.
 141  */
 142 static inline struct pid_namespace *ns_of_pid(struct pid *pid)
 143 {
 144         struct pid_namespace *ns = NULL;
 145         if (pid)
 146                 ns = pid->numbers[pid->level].ns;
 147         return ns;
 148 }
 149
 150 /*
 151  * is_child_reaper returns true if the pid is the init process
 152  * of the current namespace. As this one could be checked before
 153  * pid_ns->child_reaper is assigned in copy_process, we check
 154  * with the pid number.
 155  */
 156 static inline bool is_child_reaper(struct pid *pid)
 157 {
 158         return pid->numbers[pid->level].nr == 1;
 159 }
 160
 161 /*
 162  * the helpers to get the pid's id seen from different namespaces
 163  *
 164  * pid_nr()    : global id, i.e. the id seen from the init namespace;
 165  * pid_vnr()   : virtual id, i.e. the id seen from the pid namespace of
 166  *               current.
 167  * pid_nr_ns() : id seen from the ns specified.
 168  *
 169  * see also task_xid_nr() etc in include/linux/sched.h
 170  */
 171
 172 static inline pid_t pid_nr(struct pid *pid)
 173 {
 174         pid_t nr = 0;
 175         if (pid)
 176                 nr = pid->numbers[0].nr;
 177         return nr;
 178 }
 179
 180 pid_t pid_nr_ns(struct pid *pid, struct pid_namespace *ns);
 181 pid_t pid_vnr(struct pid *pid);
 182
 183 #define do_each_pid_task(pid, type, task)                               \
 184         do {                                                            \
 185                 if ((pid) != NULL)                                      \
 186                         hlist_for_each_entry_rcu((task),                \
 187                                 &(pid)->tasks[type], pid_links[type]) {
 188
 189                         /*
 190                          * Both old and new leaders may be attached to
 191                          * the same pid in the middle of de_thread().
 192                          */
 193 #define while_each_pid_task(pid, type, task)                            \
 194                                 if (type == PIDTYPE_PID)                \
 195                                         break;                          \
 196                         }                                               \
 197         } while (0)
 198
 199 #define do_each_pid_thread(pid, type, task)                             \
 200         do_each_pid_task(pid, type, task) {                             \
 201                 struct task_struct *tg___ = task;                       \
 202                 for_each_thread(tg___, task) {
 203
 204 #define while_each_pid_thread(pid, type, task)                          \
 205                 }                                                       \
 206                 task = tg___;                                           \
 207         } while_each_pid_task(pid, type, task)
 208 #endif /* _LINUX_PID_H */