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         spinlock_t lock;
  64         /* lists of tasks that use this pid */
  65         struct hlist_head tasks[PIDTYPE_MAX];
  66         struct hlist_head inodes;
  67         /* wait queue for pidfd notifications */
  68         wait_queue_head_t wait_pidfd;
  69         struct rcu_head rcu;
  70         struct upid numbers[1];
  71 };
  72
  73 extern struct pid init_struct_pid;
  74
  75 extern const struct file_operations pidfd_fops;
  76
  77 struct file;
  78
  79 extern struct pid *pidfd_pid(const struct file *file);
  80 struct pid *pidfd_get_pid(unsigned int fd, unsigned int *flags);
  81 struct task_struct *pidfd_get_task(int pidfd, unsigned int *flags);
  82 int pidfd_create(struct pid *pid, unsigned int flags);
  83 int pidfd_prepare(struct pid *pid, unsigned int flags, struct file **ret);
  84
  85 static inline struct pid *get_pid(struct pid *pid)
  86 {
  87         if (pid)
  88                 refcount_inc(&pid->count);
  89         return pid;
  90 }
  91
  92 extern void put_pid(struct pid *pid);
  93 extern struct task_struct *pid_task(struct pid *pid, enum pid_type);
  94 static inline bool pid_has_task(struct pid *pid, enum pid_type type)
  95 {
  96         return !hlist_empty(&pid->tasks[type]);
  97 }
  98 extern struct task_struct *get_pid_task(struct pid *pid, enum pid_type);
  99
 100 extern struct pid *get_task_pid(struct task_struct *task, enum pid_type type);
 101
 102 /*
 103  * these helpers must be called with the tasklist_lock write-held.
 104  */
 105 extern void attach_pid(struct task_struct *task, enum pid_type);
 106 extern void detach_pid(struct task_struct *task, enum pid_type);
 107 extern void change_pid(struct task_struct *task, enum pid_type,
 108                         struct pid *pid);
 109 extern void exchange_tids(struct task_struct *task, struct task_struct *old);
 110 extern void transfer_pid(struct task_struct *old, struct task_struct *new,
 111                          enum pid_type);
 112
 113 struct pid_namespace;
 114 extern struct pid_namespace init_pid_ns;
 115
 116 extern int pid_max;
 117 extern int pid_max_min, pid_max_max;
 118
 119 /*
 120  * look up a PID in the hash table. Must be called with the tasklist_lock
 121  * or rcu_read_lock() held.
 122  *
 123  * find_pid_ns() finds the pid in the namespace specified
 124  * find_vpid() finds the pid by its virtual id, i.e. in the current namespace
 125  *
 126  * see also find_task_by_vpid() set in include/linux/sched.h
 127  */
 128 extern struct pid *find_pid_ns(int nr, struct pid_namespace *ns);
 129 extern struct pid *find_vpid(int nr);
 130
 131 /*
 132  * Lookup a PID in the hash table, and return with it's count elevated.
 133  */
 134 extern struct pid *find_get_pid(int nr);
 135 extern struct pid *find_ge_pid(int nr, struct pid_namespace *);
 136
 137 extern struct pid *alloc_pid(struct pid_namespace *ns, pid_t *set_tid,
 138                              size_t set_tid_size);
 139 extern void free_pid(struct pid *pid);
 140 extern void disable_pid_allocation(struct pid_namespace *ns);
 141
 142 /*
 143  * ns_of_pid() returns the pid namespace in which the specified pid was
 144  * allocated.
 145  *
 146  * NOTE:
 147  *      ns_of_pid() is expected to be called for a process (task) that has
 148  *      an attached 'struct pid' (see attach_pid(), detach_pid()) i.e @pid
 149  *      is expected to be non-NULL. If @pid is NULL, caller should handle
 150  *      the resulting NULL pid-ns.
 151  */
 152 static inline struct pid_namespace *ns_of_pid(struct pid *pid)
 153 {
 154         struct pid_namespace *ns = NULL;
 155         if (pid)
 156                 ns = pid->numbers[pid->level].ns;
 157         return ns;
 158 }
 159
 160 /*
 161  * is_child_reaper returns true if the pid is the init process
 162  * of the current namespace. As this one could be checked before
 163  * pid_ns->child_reaper is assigned in copy_process, we check
 164  * with the pid number.
 165  */
 166 static inline bool is_child_reaper(struct pid *pid)
 167 {
 168         return pid->numbers[pid->level].nr == 1;
 169 }
 170
 171 /*
 172  * the helpers to get the pid's id seen from different namespaces
 173  *
 174  * pid_nr()    : global id, i.e. the id seen from the init namespace;
 175  * pid_vnr()   : virtual id, i.e. the id seen from the pid namespace of
 176  *               current.
 177  * pid_nr_ns() : id seen from the ns specified.
 178  *
 179  * see also task_xid_nr() etc in include/linux/sched.h
 180  */
 181
 182 static inline pid_t pid_nr(struct pid *pid)
 183 {
 184         pid_t nr = 0;
 185         if (pid)
 186                 nr = pid->numbers[0].nr;
 187         return nr;
 188 }
 189
 190 pid_t pid_nr_ns(struct pid *pid, struct pid_namespace *ns);
 191 pid_t pid_vnr(struct pid *pid);
 192
 193 #define do_each_pid_task(pid, type, task)                               \
 194         do {                                                            \
 195                 if ((pid) != NULL)                                      \
 196                         hlist_for_each_entry_rcu((task),                \
 197                                 &(pid)->tasks[type], pid_links[type]) {
 198
 199                         /*
 200                          * Both old and new leaders may be attached to
 201                          * the same pid in the middle of de_thread().
 202                          */
 203 #define while_each_pid_task(pid, type, task)                            \
 204                                 if (type == PIDTYPE_PID)                \
 205                                         break;                          \
 206                         }                                               \
 207         } while (0)
 208
 209 #define do_each_pid_thread(pid, type, task)                             \
 210         do_each_pid_task(pid, type, task) {                             \
 211                 struct task_struct *tg___ = task;                       \
 212                 for_each_thread(tg___, task) {
 213
 214 #define while_each_pid_thread(pid, type, task)                          \
 215                 }                                                       \
 216                 task = tg___;                                           \
 217         } while_each_pid_task(pid, type, task)
 218 #endif /* _LINUX_PID_H */