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_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
  52 struct upid {
  53         int nr;
  54         struct pid_namespace *ns;
  55 };
  56
  57 struct 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
  67 extern struct pid init_struct_pid;
  68
  69 static inline struct pid *get_pid(struct pid *pid)
  70 {
  71         if (pid)
  72                 atomic_inc(&pid->count);
  73         return pid;
  74 }
  75
  76 extern void put_pid(struct pid *pid);
  77 extern struct task_struct *pid_task(struct pid *pid, enum pid_type);
  78 extern struct task_struct *get_pid_task(struct pid *pid, enum pid_type);
  79
  80 extern 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  */
  85 extern void attach_pid(struct task_struct *task, enum pid_type);
  86 extern void detach_pid(struct task_struct *task, enum pid_type);
  87 extern void change_pid(struct task_struct *task, enum pid_type,
  88                         struct pid *pid);
  89 extern void transfer_pid(struct task_struct *old, struct task_struct *new,
  90                          enum pid_type);
  91
  92 struct pid_namespace;
  93 extern 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  */
 104 extern struct pid *find_pid_ns(int nr, struct pid_namespace *ns);
 105 extern struct pid *find_vpid(int nr);
 106
 107 /*
 108  * Lookup a PID in the hash table, and return with it's count elevated.
 109  */
 110 extern struct pid *find_get_pid(int nr);
 111 extern struct pid *find_ge_pid(int nr, struct pid_namespace *);
 112 int next_pidmap(struct pid_namespace *pid_ns, unsigned int last);
 113
 114 extern struct pid *alloc_pid(struct pid_namespace *ns);
 115 extern void free_pid(struct pid *pid);
 116 extern 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  */
 128 static 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  */
 142 static 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
 158 static 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
 166 pid_t pid_nr_ns(struct pid *pid, struct pid_namespace *ns);
 167 pid_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 */