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 8 enum pid_type 9 { 10 PIDTYPE_PID, 11 PIDTYPE_TGID, 12 PIDTYPE_PGID, 13 PIDTYPE_SID, 14 PIDTYPE_MAX, 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 int nr; 55 struct pid_namespace *ns; 56 }; 57 58 struct 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 /* wait queue for pidfd notifications */ 65 wait_queue_head_t wait_pidfd; 66 struct rcu_head rcu; 67 struct upid numbers[1]; 68 }; 69 70 extern struct pid init_struct_pid; 71 72 extern const struct file_operations pidfd_fops; 73 74 static inline struct pid *get_pid(struct pid *pid) 75 { 76 if (pid) 77 atomic_inc(&pid->count); 78 return pid; 79 } 80 81 extern void put_pid(struct pid *pid); 82 extern struct task_struct *pid_task(struct pid *pid, enum pid_type); 83 extern struct task_struct *get_pid_task(struct pid *pid, enum pid_type); 84 85 extern struct pid *get_task_pid(struct task_struct *task, enum pid_type type); 86 87 /* 88 * these helpers must be called with the tasklist_lock write-held. 89 */ 90 extern void attach_pid(struct task_struct *task, enum pid_type); 91 extern void detach_pid(struct task_struct *task, enum pid_type); 92 extern void change_pid(struct task_struct *task, enum pid_type, 93 struct pid *pid); 94 extern void transfer_pid(struct task_struct *old, struct task_struct *new, 95 enum pid_type); 96 97 struct pid_namespace; 98 extern struct pid_namespace init_pid_ns; 99 100 /* 101 * look up a PID in the hash table. Must be called with the tasklist_lock 102 * or rcu_read_lock() held. 103 * 104 * find_pid_ns() finds the pid in the namespace specified 105 * find_vpid() finds the pid by its virtual id, i.e. in the current namespace 106 * 107 * see also find_task_by_vpid() set in include/linux/sched.h 108 */ 109 extern struct pid *find_pid_ns(int nr, struct pid_namespace *ns); 110 extern struct pid *find_vpid(int nr); 111 112 /* 113 * Lookup a PID in the hash table, and return with it's count elevated. 114 */ 115 extern struct pid *find_get_pid(int nr); 116 extern struct pid *find_ge_pid(int nr, struct pid_namespace *); 117 118 extern struct pid *alloc_pid(struct pid_namespace *ns); 119 extern void free_pid(struct pid *pid); 120 extern void disable_pid_allocation(struct pid_namespace *ns); 121 122 /* 123 * ns_of_pid() returns the pid namespace in which the specified pid was 124 * allocated. 125 * 126 * NOTE: 127 * ns_of_pid() is expected to be called for a process (task) that has 128 * an attached 'struct pid' (see attach_pid(), detach_pid()) i.e @pid 129 * is expected to be non-NULL. If @pid is NULL, caller should handle 130 * the resulting NULL pid-ns. 131 */ 132 static inline struct pid_namespace *ns_of_pid(struct pid *pid) 133 { 134 struct pid_namespace *ns = NULL; 135 if (pid) 136 ns = pid->numbers[pid->level].ns; 137 return ns; 138 } 139 140 /* 141 * is_child_reaper returns true if the pid is the init process 142 * of the current namespace. As this one could be checked before 143 * pid_ns->child_reaper is assigned in copy_process, we check 144 * with the pid number. 145 */ 146 static inline bool is_child_reaper(struct pid *pid) 147 { 148 return pid->numbers[pid->level].nr == 1; 149 } 150 151 /* 152 * the helpers to get the pid's id seen from different namespaces 153 * 154 * pid_nr() : global id, i.e. the id seen from the init namespace; 155 * pid_vnr() : virtual id, i.e. the id seen from the pid namespace of 156 * current. 157 * pid_nr_ns() : id seen from the ns specified. 158 * 159 * see also task_xid_nr() etc in include/linux/sched.h 160 */ 161 162 static inline pid_t pid_nr(struct pid *pid) 163 { 164 pid_t nr = 0; 165 if (pid) 166 nr = pid->numbers[0].nr; 167 return nr; 168 } 169 170 pid_t pid_nr_ns(struct pid *pid, struct pid_namespace *ns); 171 pid_t pid_vnr(struct pid *pid); 172 173 #define do_each_pid_task(pid, type, task) \ 174 do { \ 175 if ((pid) != NULL) \ 176 hlist_for_each_entry_rcu((task), \ 177 &(pid)->tasks[type], pid_links[type]) { 178 179 /* 180 * Both old and new leaders may be attached to 181 * the same pid in the middle of de_thread(). 182 */ 183 #define while_each_pid_task(pid, type, task) \ 184 if (type == PIDTYPE_PID) \ 185 break; \ 186 } \ 187 } while (0) 188 189 #define do_each_pid_thread(pid, type, task) \ 190 do_each_pid_task(pid, type, task) { \ 191 struct task_struct *tg___ = task; \ 192 for_each_thread(tg___, task) { 193 194 #define while_each_pid_thread(pid, type, task) \ 195 } \ 196 task = tg___; \ 197 } while_each_pid_task(pid, type, task) 198 #endif /* _LINUX_PID_H */ 199