/*
   * Generic pidhash and scalable, time-bounded PID allocator
   *

   * (C) 2002-2003 Nadia Yvette Chambers, IBM
   * (C) 2004 Nadia Yvette Chambers, Oracle

   * (C) 2002-2004 Ingo Molnar, Red Hat
   *
   * pid-structures are backing objects for tasks sharing a given ID to chain
   * against. There is very little to them aside from hashing them and
   * parking tasks using given ID's on a list.
   *
   * The hash is always changed with the tasklist_lock write-acquired,
   * and the hash is only accessed with the tasklist_lock at least
   * read-acquired, so there's no additional SMP locking needed here.
   *
   * We have a list of bitmap pages, which bitmaps represent the PID space.
   * Allocating and freeing PIDs is completely lockless. The worst-case
   * allocation scenario when all but one out of 1 million PIDs possible are
   * allocated already: the scanning of 32 list entries and at most PAGE_SIZE
   * bytes. The typical fastpath is a single successful setbit. Freeing is O(1).

   *
   * Pid namespaces:
   *    (C) 2007 Pavel Emelyanov <xemul@openvz.org>, OpenVZ, SWsoft Inc.
   *    (C) 2007 Sukadev Bhattiprolu <sukadev@us.ibm.com>, IBM
   *     Many thanks to Oleg Nesterov for comments and help
   *

   */
  
  #include <linux/mm.h>

  #include <linux/export.h>

  #include <linux/slab.h>
  #include <linux/init.h>

  #include <linux/rculist.h>

  #include <linux/bootmem.h>
  #include <linux/hash.h>

  #include <linux/pid_namespace.h>

  #include <linux/init_task.h>

  #include <linux/syscalls.h>

  #include <linux/proc_ns.h>

  #include <linux/proc_fs.h>

  #include <linux/sched/task.h>

  #define pid_hashfn(nr, ns)	\
  	hash_long((unsigned long)nr + (unsigned long)ns, pidhash_shift)

  static struct hlist_head *pid_hash;

  static unsigned int pidhash_shift = 4;

  struct pid init_struct_pid = INIT_STRUCT_PID;

  
  int pid_max = PID_MAX_DEFAULT;

  
  #define RESERVED_PIDS		300
  
  int pid_max_min = RESERVED_PIDS + 1;
  int pid_max_max = PID_MAX_LIMIT;

  static inline int mk_pid(struct pid_namespace *pid_ns,
  		struct pidmap *map, int off)

  {

  	return (map - pid_ns->pidmap)*BITS_PER_PAGE + off;

  }

  #define find_next_offset(map, off)					\
  		find_next_zero_bit((map)->page, BITS_PER_PAGE, off)
  
  /*
   * PID-map pages start out as NULL, they get allocated upon
   * first use and are never deallocated. This way a low pid_max
   * value does not cause lots of bitmaps to be allocated, but
   * the scheme scales to up to 4 million PIDs, runtime.
   */

  struct pid_namespace init_pid_ns = {

  	.kref = KREF_INIT(2),

  	.pidmap = {
  		[ 0 ... PIDMAP_ENTRIES-1] = { ATOMIC_INIT(BITS_PER_PAGE), NULL }
  	},

  	.last_pid = 0,

  	.nr_hashed = PIDNS_HASH_ADDING,

  	.level = 0,
  	.child_reaper = &init_task,

  	.user_ns = &init_user_ns,

  	.ns.inum = PROC_PID_INIT_INO,

  #ifdef CONFIG_PID_NS
  	.ns.ops = &pidns_operations,
  #endif

  };

  EXPORT_SYMBOL_GPL(init_pid_ns);

  /*
   * Note: disable interrupts while the pidmap_lock is held as an
   * interrupt might come in and do read_lock(&tasklist_lock).
   *
   * If we don't disable interrupts there is a nasty deadlock between
   * detach_pid()->free_pid() and another cpu that does
   * spin_lock(&pidmap_lock) followed by an interrupt routine that does
   * read_lock(&tasklist_lock);
   *
   * After we clean up the tasklist_lock and know there are no
   * irq handlers that take it we can leave the interrupts enabled.
   * For now it is easier to be safe than to prove it can't happen.
   */

  static  __cacheline_aligned_in_smp DEFINE_SPINLOCK(pidmap_lock);

  static void free_pidmap(struct upid *upid)

  {

  	int nr = upid->nr;
  	struct pidmap *map = upid->ns->pidmap + nr / BITS_PER_PAGE;
  	int offset = nr & BITS_PER_PAGE_MASK;

  
  	clear_bit(offset, map->page);
  	atomic_inc(&map->nr_free);
  }

  /*
   * If we started walking pids at 'base', is 'a' seen before 'b'?
   */
  static int pid_before(int base, int a, int b)
  {
  	/*
  	 * This is the same as saying
  	 *
  	 * (a - base + MAXUINT) % MAXUINT < (b - base + MAXUINT) % MAXUINT
  	 * and that mapping orders 'a' and 'b' with respect to 'base'.
  	 */
  	return (unsigned)(a - base) < (unsigned)(b - base);
  }
  
  /*

   * We might be racing with someone else trying to set pid_ns->last_pid
   * at the pid allocation time (there's also a sysctl for this, but racing
   * with this one is OK, see comment in kernel/pid_namespace.c about it).

   * We want the winner to have the "later" value, because if the
   * "earlier" value prevails, then a pid may get reused immediately.
   *
   * Since pids rollover, it is not sufficient to just pick the bigger
   * value.  We have to consider where we started counting from.
   *
   * 'base' is the value of pid_ns->last_pid that we observed when
   * we started looking for a pid.
   *
   * 'pid' is the pid that we eventually found.
   */
  static void set_last_pid(struct pid_namespace *pid_ns, int base, int pid)
  {
  	int prev;
  	int last_write = base;
  	do {
  		prev = last_write;
  		last_write = cmpxchg(&pid_ns->last_pid, prev, pid);
  	} while ((prev != last_write) && (pid_before(base, last_write, pid)));
  }

  static int alloc_pidmap(struct pid_namespace *pid_ns)

  {

  	int i, offset, max_scan, pid, last = pid_ns->last_pid;

  	struct pidmap *map;

  
  	pid = last + 1;
  	if (pid >= pid_max)
  		pid = RESERVED_PIDS;
  	offset = pid & BITS_PER_PAGE_MASK;

  	map = &pid_ns->pidmap[pid/BITS_PER_PAGE];

  	/*
  	 * If last_pid points into the middle of the map->page we
  	 * want to scan this bitmap block twice, the second time
  	 * we start with offset == 0 (or RESERVED_PIDS).
  	 */
  	max_scan = DIV_ROUND_UP(pid_max, BITS_PER_PAGE) - !offset;

  	for (i = 0; i <= max_scan; ++i) {
  		if (unlikely(!map->page)) {

  			void *page = kzalloc(PAGE_SIZE, GFP_KERNEL);

  			/*
  			 * Free the page if someone raced with us
  			 * installing it:
  			 */

  			spin_lock_irq(&pidmap_lock);

  			if (!map->page) {

  				map->page = page;

  				page = NULL;
  			}

  			spin_unlock_irq(&pidmap_lock);

  			kfree(page);

  			if (unlikely(!map->page))

  				return -ENOMEM;

  		}
  		if (likely(atomic_read(&map->nr_free))) {

  			for ( ; ; ) {

  				if (!test_and_set_bit(offset, map->page)) {
  					atomic_dec(&map->nr_free);

  					set_last_pid(pid_ns, last, pid);

  					return pid;
  				}
  				offset = find_next_offset(map, offset);

  				if (offset >= BITS_PER_PAGE)
  					break;

  				pid = mk_pid(pid_ns, map, offset);

  				if (pid >= pid_max)
  					break;
  			}

  		}

  		if (map < &pid_ns->pidmap[(pid_max-1)/BITS_PER_PAGE]) {

  			++map;
  			offset = 0;
  		} else {

  			map = &pid_ns->pidmap[0];

  			offset = RESERVED_PIDS;
  			if (unlikely(last == offset))
  				break;
  		}

  		pid = mk_pid(pid_ns, map, offset);

  	}

  	return -EAGAIN;

  }

  int next_pidmap(struct pid_namespace *pid_ns, unsigned int last)

  {
  	int offset;

  	struct pidmap *map, *end;

  	if (last >= PID_MAX_LIMIT)
  		return -1;

  	offset = (last + 1) & BITS_PER_PAGE_MASK;

  	map = &pid_ns->pidmap[(last + 1)/BITS_PER_PAGE];
  	end = &pid_ns->pidmap[PIDMAP_ENTRIES];

  	for (; map < end; map++, offset = 0) {

  		if (unlikely(!map->page))
  			continue;
  		offset = find_next_bit((map)->page, BITS_PER_PAGE, offset);
  		if (offset < BITS_PER_PAGE)

  			return mk_pid(pid_ns, map, offset);

  	}
  	return -1;
  }

  void put_pid(struct pid *pid)

  {

  	struct pid_namespace *ns;

  	if (!pid)
  		return;

  	ns = pid->numbers[pid->level].ns;

  	if ((atomic_read(&pid->count) == 1) ||

  	     atomic_dec_and_test(&pid->count)) {

  		kmem_cache_free(ns->pid_cachep, pid);

  		put_pid_ns(ns);

  	}

  }

  EXPORT_SYMBOL_GPL(put_pid);

  
  static void delayed_put_pid(struct rcu_head *rhp)
  {
  	struct pid *pid = container_of(rhp, struct pid, rcu);
  	put_pid(pid);
  }

  void free_pid(struct pid *pid)

  {
  	/* We can be called with write_lock_irq(&tasklist_lock) held */

  	int i;

  	unsigned long flags;
  
  	spin_lock_irqsave(&pidmap_lock, flags);

  	for (i = 0; i <= pid->level; i++) {
  		struct upid *upid = pid->numbers + i;

  		struct pid_namespace *ns = upid->ns;

  		hlist_del_rcu(&upid->pid_chain);

  		switch(--ns->nr_hashed) {

  		case 2:

  		case 1:
  			/* When all that is left in the pid namespace
  			 * is the reaper wake up the reaper.  The reaper
  			 * may be sleeping in zap_pid_ns_processes().
  			 */
  			wake_up_process(ns->child_reaper);
  			break;

  		case PIDNS_HASH_ADDING:
  			/* Handle a fork failure of the first process */
  			WARN_ON(ns->child_reaper);
  			ns->nr_hashed = 0;
  			/* fall through */

  		case 0:

  			schedule_work(&ns->proc_work);
  			break;

  		}

  	}

  	spin_unlock_irqrestore(&pidmap_lock, flags);

  	for (i = 0; i <= pid->level; i++)

  		free_pidmap(pid->numbers + i);

  	call_rcu(&pid->rcu, delayed_put_pid);
  }

  struct pid *alloc_pid(struct pid_namespace *ns)

  {
  	struct pid *pid;
  	enum pid_type type;

  	int i, nr;
  	struct pid_namespace *tmp;

  	struct upid *upid;

  	int retval = -ENOMEM;

  	pid = kmem_cache_alloc(ns->pid_cachep, GFP_KERNEL);

  	if (!pid)

  		return ERR_PTR(retval);

  	tmp = ns;

  	pid->level = ns->level;

  	for (i = ns->level; i >= 0; i--) {
  		nr = alloc_pidmap(tmp);

  		if (nr < 0) {

  			retval = nr;

  			goto out_free;

  		}

  		pid->numbers[i].nr = nr;
  		pid->numbers[i].ns = tmp;
  		tmp = tmp->parent;
  	}

  	if (unlikely(is_child_reaper(pid))) {

  		if (pid_ns_prepare_proc(ns)) {
  			disable_pid_allocation(ns);

  			goto out_free;

  		}

  	}

  	get_pid_ns(ns);

  	atomic_set(&pid->count, 1);

  	for (type = 0; type < PIDTYPE_MAX; ++type)
  		INIT_HLIST_HEAD(&pid->tasks[type]);

  	upid = pid->numbers + ns->level;

  	spin_lock_irq(&pidmap_lock);

  	if (!(ns->nr_hashed & PIDNS_HASH_ADDING))

  		goto out_unlock;

  	for ( ; upid >= pid->numbers; --upid) {

  		hlist_add_head_rcu(&upid->pid_chain,
  				&pid_hash[pid_hashfn(upid->nr, upid->ns)]);

  		upid->ns->nr_hashed++;
  	}

  	spin_unlock_irq(&pidmap_lock);

  	return pid;

  out_unlock:

  	spin_unlock_irq(&pidmap_lock);

  	put_pid_ns(ns);

  out_free:

  	while (++i <= ns->level)
  		free_pidmap(pid->numbers + i);

  	kmem_cache_free(ns->pid_cachep, pid);

  	return ERR_PTR(retval);

  }

  void disable_pid_allocation(struct pid_namespace *ns)
  {
  	spin_lock_irq(&pidmap_lock);
  	ns->nr_hashed &= ~PIDNS_HASH_ADDING;
  	spin_unlock_irq(&pidmap_lock);
  }

  struct pid *find_pid_ns(int nr, struct pid_namespace *ns)

  {

  	struct upid *pnr;

  	hlist_for_each_entry_rcu(pnr,

  			&pid_hash[pid_hashfn(nr, ns)], pid_chain)
  		if (pnr->nr == nr && pnr->ns == ns)
  			return container_of(pnr, struct pid,
  					numbers[ns->level]);

  	return NULL;
  }

  EXPORT_SYMBOL_GPL(find_pid_ns);

  struct pid *find_vpid(int nr)
  {

  	return find_pid_ns(nr, task_active_pid_ns(current));

  }
  EXPORT_SYMBOL_GPL(find_vpid);

  /*
   * attach_pid() must be called with the tasklist_lock write-held.
   */

  void attach_pid(struct task_struct *task, enum pid_type type)

  {

  	struct pid_link *link = &task->pids[type];
  	hlist_add_head_rcu(&link->node, &link->pid->tasks[type]);

  }

  static void __change_pid(struct task_struct *task, enum pid_type type,
  			struct pid *new)

  {

  	struct pid_link *link;
  	struct pid *pid;
  	int tmp;

  	link = &task->pids[type];
  	pid = link->pid;

  	hlist_del_rcu(&link->node);

  	link->pid = new;

  	for (tmp = PIDTYPE_MAX; --tmp >= 0; )
  		if (!hlist_empty(&pid->tasks[tmp]))
  			return;

  	free_pid(pid);

  }

  void detach_pid(struct task_struct *task, enum pid_type type)
  {
  	__change_pid(task, type, NULL);
  }
  
  void change_pid(struct task_struct *task, enum pid_type type,
  		struct pid *pid)
  {
  	__change_pid(task, type, pid);

  	attach_pid(task, type);

  }

  /* transfer_pid is an optimization of attach_pid(new), detach_pid(old) */

  void transfer_pid(struct task_struct *old, struct task_struct *new,

  			   enum pid_type type)
  {
  	new->pids[type].pid = old->pids[type].pid;
  	hlist_replace_rcu(&old->pids[type].node, &new->pids[type].node);

  }

  struct task_struct *pid_task(struct pid *pid, enum pid_type type)

  {

  	struct task_struct *result = NULL;
  	if (pid) {
  		struct hlist_node *first;

  		first = rcu_dereference_check(hlist_first_rcu(&pid->tasks[type]),

  					      lockdep_tasklist_lock_is_held());

  		if (first)
  			result = hlist_entry(first, struct task_struct, pids[(type)].node);
  	}
  	return result;
  }

  EXPORT_SYMBOL(pid_task);

  /*

   * Must be called under rcu_read_lock().

   */

  struct task_struct *find_task_by_pid_ns(pid_t nr, struct pid_namespace *ns)

  {

  	RCU_LOCKDEP_WARN(!rcu_read_lock_held(),
  			 "find_task_by_pid_ns() needs rcu_read_lock() protection");

  	return pid_task(find_pid_ns(nr, ns), PIDTYPE_PID);

  }

  struct task_struct *find_task_by_vpid(pid_t vnr)
  {

  	return find_task_by_pid_ns(vnr, task_active_pid_ns(current));

  }

  struct pid *get_task_pid(struct task_struct *task, enum pid_type type)
  {
  	struct pid *pid;
  	rcu_read_lock();

  	if (type != PIDTYPE_PID)
  		task = task->group_leader;

  	pid = get_pid(rcu_dereference(task->pids[type].pid));

  	rcu_read_unlock();
  	return pid;
  }

  EXPORT_SYMBOL_GPL(get_task_pid);

  struct task_struct *get_pid_task(struct pid *pid, enum pid_type type)

  {
  	struct task_struct *result;
  	rcu_read_lock();
  	result = pid_task(pid, type);
  	if (result)
  		get_task_struct(result);
  	rcu_read_unlock();
  	return result;

  }

  EXPORT_SYMBOL_GPL(get_pid_task);

  struct pid *find_get_pid(pid_t nr)

  {
  	struct pid *pid;

  	rcu_read_lock();

  	pid = get_pid(find_vpid(nr));

  	rcu_read_unlock();

  	return pid;

  }

  EXPORT_SYMBOL_GPL(find_get_pid);

  pid_t pid_nr_ns(struct pid *pid, struct pid_namespace *ns)
  {
  	struct upid *upid;
  	pid_t nr = 0;
  
  	if (pid && ns->level <= pid->level) {
  		upid = &pid->numbers[ns->level];
  		if (upid->ns == ns)
  			nr = upid->nr;
  	}
  	return nr;
  }

  EXPORT_SYMBOL_GPL(pid_nr_ns);

  pid_t pid_vnr(struct pid *pid)
  {

  	return pid_nr_ns(pid, task_active_pid_ns(current));

  }
  EXPORT_SYMBOL_GPL(pid_vnr);

  pid_t __task_pid_nr_ns(struct task_struct *task, enum pid_type type,
  			struct pid_namespace *ns)

  {

  	pid_t nr = 0;
  
  	rcu_read_lock();
  	if (!ns)

  		ns = task_active_pid_ns(current);

  	if (likely(pid_alive(task))) {

  		if (type != PIDTYPE_PID) {
  			if (type == __PIDTYPE_TGID)
  				type = PIDTYPE_PID;

  			task = task->group_leader;

  		}

  		nr = pid_nr_ns(rcu_dereference(task->pids[type].pid), ns);

  	}
  	rcu_read_unlock();
  
  	return nr;

  }

  EXPORT_SYMBOL(__task_pid_nr_ns);

  struct pid_namespace *task_active_pid_ns(struct task_struct *tsk)
  {
  	return ns_of_pid(task_pid(tsk));
  }
  EXPORT_SYMBOL_GPL(task_active_pid_ns);

  /*

   * Used by proc to find the first pid that is greater than or equal to nr.

   *

   * If there is a pid at nr this function is exactly the same as find_pid_ns.

   */

  struct pid *find_ge_pid(int nr, struct pid_namespace *ns)

  {
  	struct pid *pid;
  
  	do {

  		pid = find_pid_ns(nr, ns);

  		if (pid)
  			break;

  		nr = next_pidmap(ns, nr);

  	} while (nr > 0);
  
  	return pid;
  }
  
  /*

   * The pid hash table is scaled according to the amount of memory in the
   * machine.  From a minimum of 16 slots up to 4096 slots at one gigabyte or
   * more.
   */
  void __init pidhash_init(void)
  {

  	pid_hash = alloc_large_system_hash("PID", sizeof(*pid_hash), 0, 18,

  					   HASH_EARLY | HASH_SMALL | HASH_ZERO,

  					   &pidhash_shift, NULL,
  					   0, 4096);

  }
  
  void __init pidmap_init(void)
  {

  	/* Verify no one has done anything silly: */

  	BUILD_BUG_ON(PID_MAX_LIMIT >= PIDNS_HASH_ADDING);

  	/* bump default and minimum pid_max based on number of cpus */
  	pid_max = min(pid_max_max, max_t(int, pid_max,
  				PIDS_PER_CPU_DEFAULT * num_possible_cpus()));
  	pid_max_min = max_t(int, pid_max_min,
  				PIDS_PER_CPU_MIN * num_possible_cpus());
  	pr_info("pid_max: default: %u minimum: %u
  ", pid_max, pid_max_min);

  	init_pid_ns.pidmap[0].page = kzalloc(PAGE_SIZE, GFP_KERNEL);

  	/* Reserve PID 0. We never call free_pidmap(0) */

  	set_bit(0, init_pid_ns.pidmap[0].page);
  	atomic_dec(&init_pid_ns.pidmap[0].nr_free);

  	init_pid_ns.pid_cachep = KMEM_CACHE(pid,

  			SLAB_HWCACHE_ALIGN | SLAB_PANIC | SLAB_ACCOUNT);

  }