xref: /openbmc/linux/kernel/pid.c (revision fce96cf0443083e37455eff8f78fd240c621dae3)
1  // SPDX-License-Identifier: GPL-2.0-only
2  /*
3   * Generic pidhash and scalable, time-bounded PID allocator
4   *
5   * (C) 2002-2003 Nadia Yvette Chambers, IBM
6   * (C) 2004 Nadia Yvette Chambers, Oracle
7   * (C) 2002-2004 Ingo Molnar, Red Hat
8   *
9   * pid-structures are backing objects for tasks sharing a given ID to chain
10   * against. There is very little to them aside from hashing them and
11   * parking tasks using given ID's on a list.
12   *
13   * The hash is always changed with the tasklist_lock write-acquired,
14   * and the hash is only accessed with the tasklist_lock at least
15   * read-acquired, so there's no additional SMP locking needed here.
16   *
17   * We have a list of bitmap pages, which bitmaps represent the PID space.
18   * Allocating and freeing PIDs is completely lockless. The worst-case
19   * allocation scenario when all but one out of 1 million PIDs possible are
20   * allocated already: the scanning of 32 list entries and at most PAGE_SIZE
21   * bytes. The typical fastpath is a single successful setbit. Freeing is O(1).
22   *
23   * Pid namespaces:
24   *    (C) 2007 Pavel Emelyanov <xemul@openvz.org>, OpenVZ, SWsoft Inc.
25   *    (C) 2007 Sukadev Bhattiprolu <sukadev@us.ibm.com>, IBM
26   *     Many thanks to Oleg Nesterov for comments and help
27   *
28   */
29  
30  #include <linux/mm.h>
31  #include <linux/export.h>
32  #include <linux/slab.h>
33  #include <linux/init.h>
34  #include <linux/rculist.h>
35  #include <linux/memblock.h>
36  #include <linux/pid_namespace.h>
37  #include <linux/init_task.h>
38  #include <linux/syscalls.h>
39  #include <linux/proc_ns.h>
40  #include <linux/refcount.h>
41  #include <linux/anon_inodes.h>
42  #include <linux/sched/signal.h>
43  #include <linux/sched/task.h>
44  #include <linux/idr.h>
45  #include <net/sock.h>
46  #include <uapi/linux/pidfd.h>
47  
48  struct pid init_struct_pid = {
49  	.count		= REFCOUNT_INIT(1),
50  	.tasks		= {
51  		{ .first = NULL },
52  		{ .first = NULL },
53  		{ .first = NULL },
54  	},
55  	.level		= 0,
56  	.numbers	= { {
57  		.nr		= 0,
58  		.ns		= &init_pid_ns,
59  	}, }
60  };
61  
62  int pid_max = PID_MAX_DEFAULT;
63  
64  #define RESERVED_PIDS		300
65  
66  int pid_max_min = RESERVED_PIDS + 1;
67  int pid_max_max = PID_MAX_LIMIT;
68  
69  /*
70   * PID-map pages start out as NULL, they get allocated upon
71   * first use and are never deallocated. This way a low pid_max
72   * value does not cause lots of bitmaps to be allocated, but
73   * the scheme scales to up to 4 million PIDs, runtime.
74   */
75  struct pid_namespace init_pid_ns = {
76  	.ns.count = REFCOUNT_INIT(2),
77  	.idr = IDR_INIT(init_pid_ns.idr),
78  	.pid_allocated = PIDNS_ADDING,
79  	.level = 0,
80  	.child_reaper = &init_task,
81  	.user_ns = &init_user_ns,
82  	.ns.inum = PROC_PID_INIT_INO,
83  #ifdef CONFIG_PID_NS
84  	.ns.ops = &pidns_operations,
85  #endif
86  };
87  EXPORT_SYMBOL_GPL(init_pid_ns);
88  
89  /*
90   * Note: disable interrupts while the pidmap_lock is held as an
91   * interrupt might come in and do read_lock(&tasklist_lock).
92   *
93   * If we don't disable interrupts there is a nasty deadlock between
94   * detach_pid()->free_pid() and another cpu that does
95   * spin_lock(&pidmap_lock) followed by an interrupt routine that does
96   * read_lock(&tasklist_lock);
97   *
98   * After we clean up the tasklist_lock and know there are no
99   * irq handlers that take it we can leave the interrupts enabled.
100   * For now it is easier to be safe than to prove it can't happen.
101   */
102  
103  static  __cacheline_aligned_in_smp DEFINE_SPINLOCK(pidmap_lock);
104  
105  void put_pid(struct pid *pid)
106  {
107  	struct pid_namespace *ns;
108  
109  	if (!pid)
110  		return;
111  
112  	ns = pid->numbers[pid->level].ns;
113  	if (refcount_dec_and_test(&pid->count)) {
114  		kmem_cache_free(ns->pid_cachep, pid);
115  		put_pid_ns(ns);
116  	}
117  }
118  EXPORT_SYMBOL_GPL(put_pid);
119  
120  static void delayed_put_pid(struct rcu_head *rhp)
121  {
122  	struct pid *pid = container_of(rhp, struct pid, rcu);
123  	put_pid(pid);
124  }
125  
126  void free_pid(struct pid *pid)
127  {
128  	/* We can be called with write_lock_irq(&tasklist_lock) held */
129  	int i;
130  	unsigned long flags;
131  
132  	spin_lock_irqsave(&pidmap_lock, flags);
133  	for (i = 0; i <= pid->level; i++) {
134  		struct upid *upid = pid->numbers + i;
135  		struct pid_namespace *ns = upid->ns;
136  		switch (--ns->pid_allocated) {
137  		case 2:
138  		case 1:
139  			/* When all that is left in the pid namespace
140  			 * is the reaper wake up the reaper.  The reaper
141  			 * may be sleeping in zap_pid_ns_processes().
142  			 */
143  			wake_up_process(ns->child_reaper);
144  			break;
145  		case PIDNS_ADDING:
146  			/* Handle a fork failure of the first process */
147  			WARN_ON(ns->child_reaper);
148  			ns->pid_allocated = 0;
149  			break;
150  		}
151  
152  		idr_remove(&ns->idr, upid->nr);
153  	}
154  	spin_unlock_irqrestore(&pidmap_lock, flags);
155  
156  	call_rcu(&pid->rcu, delayed_put_pid);
157  }
158  
159  struct pid *alloc_pid(struct pid_namespace *ns, pid_t *set_tid,
160  		      size_t set_tid_size)
161  {
162  	struct pid *pid;
163  	enum pid_type type;
164  	int i, nr;
165  	struct pid_namespace *tmp;
166  	struct upid *upid;
167  	int retval = -ENOMEM;
168  
169  	/*
170  	 * set_tid_size contains the size of the set_tid array. Starting at
171  	 * the most nested currently active PID namespace it tells alloc_pid()
172  	 * which PID to set for a process in that most nested PID namespace
173  	 * up to set_tid_size PID namespaces. It does not have to set the PID
174  	 * for a process in all nested PID namespaces but set_tid_size must
175  	 * never be greater than the current ns->level + 1.
176  	 */
177  	if (set_tid_size > ns->level + 1)
178  		return ERR_PTR(-EINVAL);
179  
180  	pid = kmem_cache_alloc(ns->pid_cachep, GFP_KERNEL);
181  	if (!pid)
182  		return ERR_PTR(retval);
183  
184  	tmp = ns;
185  	pid->level = ns->level;
186  
187  	for (i = ns->level; i >= 0; i--) {
188  		int tid = 0;
189  
190  		if (set_tid_size) {
191  			tid = set_tid[ns->level - i];
192  
193  			retval = -EINVAL;
194  			if (tid < 1 || tid >= pid_max)
195  				goto out_free;
196  			/*
197  			 * Also fail if a PID != 1 is requested and
198  			 * no PID 1 exists.
199  			 */
200  			if (tid != 1 && !tmp->child_reaper)
201  				goto out_free;
202  			retval = -EPERM;
203  			if (!checkpoint_restore_ns_capable(tmp->user_ns))
204  				goto out_free;
205  			set_tid_size--;
206  		}
207  
208  		idr_preload(GFP_KERNEL);
209  		spin_lock_irq(&pidmap_lock);
210  
211  		if (tid) {
212  			nr = idr_alloc(&tmp->idr, NULL, tid,
213  				       tid + 1, GFP_ATOMIC);
214  			/*
215  			 * If ENOSPC is returned it means that the PID is
216  			 * alreay in use. Return EEXIST in that case.
217  			 */
218  			if (nr == -ENOSPC)
219  				nr = -EEXIST;
220  		} else {
221  			int pid_min = 1;
222  			/*
223  			 * init really needs pid 1, but after reaching the
224  			 * maximum wrap back to RESERVED_PIDS
225  			 */
226  			if (idr_get_cursor(&tmp->idr) > RESERVED_PIDS)
227  				pid_min = RESERVED_PIDS;
228  
229  			/*
230  			 * Store a null pointer so find_pid_ns does not find
231  			 * a partially initialized PID (see below).
232  			 */
233  			nr = idr_alloc_cyclic(&tmp->idr, NULL, pid_min,
234  					      pid_max, GFP_ATOMIC);
235  		}
236  		spin_unlock_irq(&pidmap_lock);
237  		idr_preload_end();
238  
239  		if (nr < 0) {
240  			retval = (nr == -ENOSPC) ? -EAGAIN : nr;
241  			goto out_free;
242  		}
243  
244  		pid->numbers[i].nr = nr;
245  		pid->numbers[i].ns = tmp;
246  		tmp = tmp->parent;
247  	}
248  
249  	/*
250  	 * ENOMEM is not the most obvious choice especially for the case
251  	 * where the child subreaper has already exited and the pid
252  	 * namespace denies the creation of any new processes. But ENOMEM
253  	 * is what we have exposed to userspace for a long time and it is
254  	 * documented behavior for pid namespaces. So we can't easily
255  	 * change it even if there were an error code better suited.
256  	 */
257  	retval = -ENOMEM;
258  
259  	get_pid_ns(ns);
260  	refcount_set(&pid->count, 1);
261  	spin_lock_init(&pid->lock);
262  	for (type = 0; type < PIDTYPE_MAX; ++type)
263  		INIT_HLIST_HEAD(&pid->tasks[type]);
264  
265  	init_waitqueue_head(&pid->wait_pidfd);
266  	INIT_HLIST_HEAD(&pid->inodes);
267  
268  	upid = pid->numbers + ns->level;
269  	spin_lock_irq(&pidmap_lock);
270  	if (!(ns->pid_allocated & PIDNS_ADDING))
271  		goto out_unlock;
272  	for ( ; upid >= pid->numbers; --upid) {
273  		/* Make the PID visible to find_pid_ns. */
274  		idr_replace(&upid->ns->idr, pid, upid->nr);
275  		upid->ns->pid_allocated++;
276  	}
277  	spin_unlock_irq(&pidmap_lock);
278  
279  	return pid;
280  
281  out_unlock:
282  	spin_unlock_irq(&pidmap_lock);
283  	put_pid_ns(ns);
284  
285  out_free:
286  	spin_lock_irq(&pidmap_lock);
287  	while (++i <= ns->level) {
288  		upid = pid->numbers + i;
289  		idr_remove(&upid->ns->idr, upid->nr);
290  	}
291  
292  	/* On failure to allocate the first pid, reset the state */
293  	if (ns->pid_allocated == PIDNS_ADDING)
294  		idr_set_cursor(&ns->idr, 0);
295  
296  	spin_unlock_irq(&pidmap_lock);
297  
298  	kmem_cache_free(ns->pid_cachep, pid);
299  	return ERR_PTR(retval);
300  }
301  
302  void disable_pid_allocation(struct pid_namespace *ns)
303  {
304  	spin_lock_irq(&pidmap_lock);
305  	ns->pid_allocated &= ~PIDNS_ADDING;
306  	spin_unlock_irq(&pidmap_lock);
307  }
308  
309  struct pid *find_pid_ns(int nr, struct pid_namespace *ns)
310  {
311  	return idr_find(&ns->idr, nr);
312  }
313  EXPORT_SYMBOL_GPL(find_pid_ns);
314  
315  struct pid *find_vpid(int nr)
316  {
317  	return find_pid_ns(nr, task_active_pid_ns(current));
318  }
319  EXPORT_SYMBOL_GPL(find_vpid);
320  
321  static struct pid **task_pid_ptr(struct task_struct *task, enum pid_type type)
322  {
323  	return (type == PIDTYPE_PID) ?
324  		&task->thread_pid :
325  		&task->signal->pids[type];
326  }
327  
328  /*
329   * attach_pid() must be called with the tasklist_lock write-held.
330   */
331  void attach_pid(struct task_struct *task, enum pid_type type)
332  {
333  	struct pid *pid = *task_pid_ptr(task, type);
334  	hlist_add_head_rcu(&task->pid_links[type], &pid->tasks[type]);
335  }
336  
337  static void __change_pid(struct task_struct *task, enum pid_type type,
338  			struct pid *new)
339  {
340  	struct pid **pid_ptr = task_pid_ptr(task, type);
341  	struct pid *pid;
342  	int tmp;
343  
344  	pid = *pid_ptr;
345  
346  	hlist_del_rcu(&task->pid_links[type]);
347  	*pid_ptr = new;
348  
349  	for (tmp = PIDTYPE_MAX; --tmp >= 0; )
350  		if (pid_has_task(pid, tmp))
351  			return;
352  
353  	free_pid(pid);
354  }
355  
356  void detach_pid(struct task_struct *task, enum pid_type type)
357  {
358  	__change_pid(task, type, NULL);
359  }
360  
361  void change_pid(struct task_struct *task, enum pid_type type,
362  		struct pid *pid)
363  {
364  	__change_pid(task, type, pid);
365  	attach_pid(task, type);
366  }
367  
368  void exchange_tids(struct task_struct *left, struct task_struct *right)
369  {
370  	struct pid *pid1 = left->thread_pid;
371  	struct pid *pid2 = right->thread_pid;
372  	struct hlist_head *head1 = &pid1->tasks[PIDTYPE_PID];
373  	struct hlist_head *head2 = &pid2->tasks[PIDTYPE_PID];
374  
375  	/* Swap the single entry tid lists */
376  	hlists_swap_heads_rcu(head1, head2);
377  
378  	/* Swap the per task_struct pid */
379  	rcu_assign_pointer(left->thread_pid, pid2);
380  	rcu_assign_pointer(right->thread_pid, pid1);
381  
382  	/* Swap the cached value */
383  	WRITE_ONCE(left->pid, pid_nr(pid2));
384  	WRITE_ONCE(right->pid, pid_nr(pid1));
385  }
386  
387  /* transfer_pid is an optimization of attach_pid(new), detach_pid(old) */
388  void transfer_pid(struct task_struct *old, struct task_struct *new,
389  			   enum pid_type type)
390  {
391  	if (type == PIDTYPE_PID)
392  		new->thread_pid = old->thread_pid;
393  	hlist_replace_rcu(&old->pid_links[type], &new->pid_links[type]);
394  }
395  
396  struct task_struct *pid_task(struct pid *pid, enum pid_type type)
397  {
398  	struct task_struct *result = NULL;
399  	if (pid) {
400  		struct hlist_node *first;
401  		first = rcu_dereference_check(hlist_first_rcu(&pid->tasks[type]),
402  					      lockdep_tasklist_lock_is_held());
403  		if (first)
404  			result = hlist_entry(first, struct task_struct, pid_links[(type)]);
405  	}
406  	return result;
407  }
408  EXPORT_SYMBOL(pid_task);
409  
410  /*
411   * Must be called under rcu_read_lock().
412   */
413  struct task_struct *find_task_by_pid_ns(pid_t nr, struct pid_namespace *ns)
414  {
415  	RCU_LOCKDEP_WARN(!rcu_read_lock_held(),
416  			 "find_task_by_pid_ns() needs rcu_read_lock() protection");
417  	return pid_task(find_pid_ns(nr, ns), PIDTYPE_PID);
418  }
419  
420  struct task_struct *find_task_by_vpid(pid_t vnr)
421  {
422  	return find_task_by_pid_ns(vnr, task_active_pid_ns(current));
423  }
424  
425  struct task_struct *find_get_task_by_vpid(pid_t nr)
426  {
427  	struct task_struct *task;
428  
429  	rcu_read_lock();
430  	task = find_task_by_vpid(nr);
431  	if (task)
432  		get_task_struct(task);
433  	rcu_read_unlock();
434  
435  	return task;
436  }
437  
438  struct pid *get_task_pid(struct task_struct *task, enum pid_type type)
439  {
440  	struct pid *pid;
441  	rcu_read_lock();
442  	pid = get_pid(rcu_dereference(*task_pid_ptr(task, type)));
443  	rcu_read_unlock();
444  	return pid;
445  }
446  EXPORT_SYMBOL_GPL(get_task_pid);
447  
448  struct task_struct *get_pid_task(struct pid *pid, enum pid_type type)
449  {
450  	struct task_struct *result;
451  	rcu_read_lock();
452  	result = pid_task(pid, type);
453  	if (result)
454  		get_task_struct(result);
455  	rcu_read_unlock();
456  	return result;
457  }
458  EXPORT_SYMBOL_GPL(get_pid_task);
459  
460  struct pid *find_get_pid(pid_t nr)
461  {
462  	struct pid *pid;
463  
464  	rcu_read_lock();
465  	pid = get_pid(find_vpid(nr));
466  	rcu_read_unlock();
467  
468  	return pid;
469  }
470  EXPORT_SYMBOL_GPL(find_get_pid);
471  
472  pid_t pid_nr_ns(struct pid *pid, struct pid_namespace *ns)
473  {
474  	struct upid *upid;
475  	pid_t nr = 0;
476  
477  	if (pid && ns->level <= pid->level) {
478  		upid = &pid->numbers[ns->level];
479  		if (upid->ns == ns)
480  			nr = upid->nr;
481  	}
482  	return nr;
483  }
484  EXPORT_SYMBOL_GPL(pid_nr_ns);
485  
486  pid_t pid_vnr(struct pid *pid)
487  {
488  	return pid_nr_ns(pid, task_active_pid_ns(current));
489  }
490  EXPORT_SYMBOL_GPL(pid_vnr);
491  
492  pid_t __task_pid_nr_ns(struct task_struct *task, enum pid_type type,
493  			struct pid_namespace *ns)
494  {
495  	pid_t nr = 0;
496  
497  	rcu_read_lock();
498  	if (!ns)
499  		ns = task_active_pid_ns(current);
500  	nr = pid_nr_ns(rcu_dereference(*task_pid_ptr(task, type)), ns);
501  	rcu_read_unlock();
502  
503  	return nr;
504  }
505  EXPORT_SYMBOL(__task_pid_nr_ns);
506  
507  struct pid_namespace *task_active_pid_ns(struct task_struct *tsk)
508  {
509  	return ns_of_pid(task_pid(tsk));
510  }
511  EXPORT_SYMBOL_GPL(task_active_pid_ns);
512  
513  /*
514   * Used by proc to find the first pid that is greater than or equal to nr.
515   *
516   * If there is a pid at nr this function is exactly the same as find_pid_ns.
517   */
518  struct pid *find_ge_pid(int nr, struct pid_namespace *ns)
519  {
520  	return idr_get_next(&ns->idr, &nr);
521  }
522  
523  struct pid *pidfd_get_pid(unsigned int fd, unsigned int *flags)
524  {
525  	struct fd f;
526  	struct pid *pid;
527  
528  	f = fdget(fd);
529  	if (!f.file)
530  		return ERR_PTR(-EBADF);
531  
532  	pid = pidfd_pid(f.file);
533  	if (!IS_ERR(pid)) {
534  		get_pid(pid);
535  		*flags = f.file->f_flags;
536  	}
537  
538  	fdput(f);
539  	return pid;
540  }
541  
542  /**
543   * pidfd_get_task() - Get the task associated with a pidfd
544   *
545   * @pidfd: pidfd for which to get the task
546   * @flags: flags associated with this pidfd
547   *
548   * Return the task associated with @pidfd. The function takes a reference on
549   * the returned task. The caller is responsible for releasing that reference.
550   *
551   * Currently, the process identified by @pidfd is always a thread-group leader.
552   * This restriction currently exists for all aspects of pidfds including pidfd
553   * creation (CLONE_PIDFD cannot be used with CLONE_THREAD) and pidfd polling
554   * (only supports thread group leaders).
555   *
556   * Return: On success, the task_struct associated with the pidfd.
557   *	   On error, a negative errno number will be returned.
558   */
559  struct task_struct *pidfd_get_task(int pidfd, unsigned int *flags)
560  {
561  	unsigned int f_flags;
562  	struct pid *pid;
563  	struct task_struct *task;
564  
565  	pid = pidfd_get_pid(pidfd, &f_flags);
566  	if (IS_ERR(pid))
567  		return ERR_CAST(pid);
568  
569  	task = get_pid_task(pid, PIDTYPE_TGID);
570  	put_pid(pid);
571  	if (!task)
572  		return ERR_PTR(-ESRCH);
573  
574  	*flags = f_flags;
575  	return task;
576  }
577  
578  /**
579   * pidfd_create() - Create a new pid file descriptor.
580   *
581   * @pid:   struct pid that the pidfd will reference
582   * @flags: flags to pass
583   *
584   * This creates a new pid file descriptor with the O_CLOEXEC flag set.
585   *
586   * Note, that this function can only be called after the fd table has
587   * been unshared to avoid leaking the pidfd to the new process.
588   *
589   * This symbol should not be explicitly exported to loadable modules.
590   *
591   * Return: On success, a cloexec pidfd is returned.
592   *         On error, a negative errno number will be returned.
593   */
594  int pidfd_create(struct pid *pid, unsigned int flags)
595  {
596  	int fd;
597  
598  	if (!pid || !pid_has_task(pid, PIDTYPE_TGID))
599  		return -EINVAL;
600  
601  	if (flags & ~(O_NONBLOCK | O_RDWR | O_CLOEXEC))
602  		return -EINVAL;
603  
604  	fd = anon_inode_getfd("[pidfd]", &pidfd_fops, get_pid(pid),
605  			      flags | O_RDWR | O_CLOEXEC);
606  	if (fd < 0)
607  		put_pid(pid);
608  
609  	return fd;
610  }
611  
612  /**
613   * pidfd_open() - Open new pid file descriptor.
614   *
615   * @pid:   pid for which to retrieve a pidfd
616   * @flags: flags to pass
617   *
618   * This creates a new pid file descriptor with the O_CLOEXEC flag set for
619   * the process identified by @pid. Currently, the process identified by
620   * @pid must be a thread-group leader. This restriction currently exists
621   * for all aspects of pidfds including pidfd creation (CLONE_PIDFD cannot
622   * be used with CLONE_THREAD) and pidfd polling (only supports thread group
623   * leaders).
624   *
625   * Return: On success, a cloexec pidfd is returned.
626   *         On error, a negative errno number will be returned.
627   */
628  SYSCALL_DEFINE2(pidfd_open, pid_t, pid, unsigned int, flags)
629  {
630  	int fd;
631  	struct pid *p;
632  
633  	if (flags & ~PIDFD_NONBLOCK)
634  		return -EINVAL;
635  
636  	if (pid <= 0)
637  		return -EINVAL;
638  
639  	p = find_get_pid(pid);
640  	if (!p)
641  		return -ESRCH;
642  
643  	fd = pidfd_create(p, flags);
644  
645  	put_pid(p);
646  	return fd;
647  }
648  
649  void __init pid_idr_init(void)
650  {
651  	/* Verify no one has done anything silly: */
652  	BUILD_BUG_ON(PID_MAX_LIMIT >= PIDNS_ADDING);
653  
654  	/* bump default and minimum pid_max based on number of cpus */
655  	pid_max = min(pid_max_max, max_t(int, pid_max,
656  				PIDS_PER_CPU_DEFAULT * num_possible_cpus()));
657  	pid_max_min = max_t(int, pid_max_min,
658  				PIDS_PER_CPU_MIN * num_possible_cpus());
659  	pr_info("pid_max: default: %u minimum: %u\n", pid_max, pid_max_min);
660  
661  	idr_init(&init_pid_ns.idr);
662  
663  	init_pid_ns.pid_cachep = KMEM_CACHE(pid,
664  			SLAB_HWCACHE_ALIGN | SLAB_PANIC | SLAB_ACCOUNT);
665  }
666  
667  static struct file *__pidfd_fget(struct task_struct *task, int fd)
668  {
669  	struct file *file;
670  	int ret;
671  
672  	ret = down_read_killable(&task->signal->exec_update_lock);
673  	if (ret)
674  		return ERR_PTR(ret);
675  
676  	if (ptrace_may_access(task, PTRACE_MODE_ATTACH_REALCREDS))
677  		file = fget_task(task, fd);
678  	else
679  		file = ERR_PTR(-EPERM);
680  
681  	up_read(&task->signal->exec_update_lock);
682  
683  	return file ?: ERR_PTR(-EBADF);
684  }
685  
686  static int pidfd_getfd(struct pid *pid, int fd)
687  {
688  	struct task_struct *task;
689  	struct file *file;
690  	int ret;
691  
692  	task = get_pid_task(pid, PIDTYPE_PID);
693  	if (!task)
694  		return -ESRCH;
695  
696  	file = __pidfd_fget(task, fd);
697  	put_task_struct(task);
698  	if (IS_ERR(file))
699  		return PTR_ERR(file);
700  
701  	ret = receive_fd(file, O_CLOEXEC);
702  	fput(file);
703  
704  	return ret;
705  }
706  
707  /**
708   * sys_pidfd_getfd() - Get a file descriptor from another process
709   *
710   * @pidfd:	the pidfd file descriptor of the process
711   * @fd:		the file descriptor number to get
712   * @flags:	flags on how to get the fd (reserved)
713   *
714   * This syscall gets a copy of a file descriptor from another process
715   * based on the pidfd, and file descriptor number. It requires that
716   * the calling process has the ability to ptrace the process represented
717   * by the pidfd. The process which is having its file descriptor copied
718   * is otherwise unaffected.
719   *
720   * Return: On success, a cloexec file descriptor is returned.
721   *         On error, a negative errno number will be returned.
722   */
723  SYSCALL_DEFINE3(pidfd_getfd, int, pidfd, int, fd,
724  		unsigned int, flags)
725  {
726  	struct pid *pid;
727  	struct fd f;
728  	int ret;
729  
730  	/* flags is currently unused - make sure it's unset */
731  	if (flags)
732  		return -EINVAL;
733  
734  	f = fdget(pidfd);
735  	if (!f.file)
736  		return -EBADF;
737  
738  	pid = pidfd_pid(f.file);
739  	if (IS_ERR(pid))
740  		ret = PTR_ERR(pid);
741  	else
742  		ret = pidfd_getfd(pid, fd);
743  
744  	fdput(f);
745  	return ret;
746  }
747