xref: /linux/kernel/pid.c (revision b1a54551dd9ed5ef1763b97b35a0999ca002b95c)
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 #if defined(CONFIG_SYSCTL) && defined(CONFIG_MEMFD_CREATE)
87 	.memfd_noexec_scope = MEMFD_NOEXEC_SCOPE_EXEC,
88 #endif
89 };
90 EXPORT_SYMBOL_GPL(init_pid_ns);
91 
92 /*
93  * Note: disable interrupts while the pidmap_lock is held as an
94  * interrupt might come in and do read_lock(&tasklist_lock).
95  *
96  * If we don't disable interrupts there is a nasty deadlock between
97  * detach_pid()->free_pid() and another cpu that does
98  * spin_lock(&pidmap_lock) followed by an interrupt routine that does
99  * read_lock(&tasklist_lock);
100  *
101  * After we clean up the tasklist_lock and know there are no
102  * irq handlers that take it we can leave the interrupts enabled.
103  * For now it is easier to be safe than to prove it can't happen.
104  */
105 
106 static  __cacheline_aligned_in_smp DEFINE_SPINLOCK(pidmap_lock);
107 
108 void put_pid(struct pid *pid)
109 {
110 	struct pid_namespace *ns;
111 
112 	if (!pid)
113 		return;
114 
115 	ns = pid->numbers[pid->level].ns;
116 	if (refcount_dec_and_test(&pid->count)) {
117 		kmem_cache_free(ns->pid_cachep, pid);
118 		put_pid_ns(ns);
119 	}
120 }
121 EXPORT_SYMBOL_GPL(put_pid);
122 
123 static void delayed_put_pid(struct rcu_head *rhp)
124 {
125 	struct pid *pid = container_of(rhp, struct pid, rcu);
126 	put_pid(pid);
127 }
128 
129 void free_pid(struct pid *pid)
130 {
131 	/* We can be called with write_lock_irq(&tasklist_lock) held */
132 	int i;
133 	unsigned long flags;
134 
135 	spin_lock_irqsave(&pidmap_lock, flags);
136 	for (i = 0; i <= pid->level; i++) {
137 		struct upid *upid = pid->numbers + i;
138 		struct pid_namespace *ns = upid->ns;
139 		switch (--ns->pid_allocated) {
140 		case 2:
141 		case 1:
142 			/* When all that is left in the pid namespace
143 			 * is the reaper wake up the reaper.  The reaper
144 			 * may be sleeping in zap_pid_ns_processes().
145 			 */
146 			wake_up_process(ns->child_reaper);
147 			break;
148 		case PIDNS_ADDING:
149 			/* Handle a fork failure of the first process */
150 			WARN_ON(ns->child_reaper);
151 			ns->pid_allocated = 0;
152 			break;
153 		}
154 
155 		idr_remove(&ns->idr, upid->nr);
156 	}
157 	spin_unlock_irqrestore(&pidmap_lock, flags);
158 
159 	call_rcu(&pid->rcu, delayed_put_pid);
160 }
161 
162 struct pid *alloc_pid(struct pid_namespace *ns, pid_t *set_tid,
163 		      size_t set_tid_size)
164 {
165 	struct pid *pid;
166 	enum pid_type type;
167 	int i, nr;
168 	struct pid_namespace *tmp;
169 	struct upid *upid;
170 	int retval = -ENOMEM;
171 
172 	/*
173 	 * set_tid_size contains the size of the set_tid array. Starting at
174 	 * the most nested currently active PID namespace it tells alloc_pid()
175 	 * which PID to set for a process in that most nested PID namespace
176 	 * up to set_tid_size PID namespaces. It does not have to set the PID
177 	 * for a process in all nested PID namespaces but set_tid_size must
178 	 * never be greater than the current ns->level + 1.
179 	 */
180 	if (set_tid_size > ns->level + 1)
181 		return ERR_PTR(-EINVAL);
182 
183 	pid = kmem_cache_alloc(ns->pid_cachep, GFP_KERNEL);
184 	if (!pid)
185 		return ERR_PTR(retval);
186 
187 	tmp = ns;
188 	pid->level = ns->level;
189 
190 	for (i = ns->level; i >= 0; i--) {
191 		int tid = 0;
192 
193 		if (set_tid_size) {
194 			tid = set_tid[ns->level - i];
195 
196 			retval = -EINVAL;
197 			if (tid < 1 || tid >= pid_max)
198 				goto out_free;
199 			/*
200 			 * Also fail if a PID != 1 is requested and
201 			 * no PID 1 exists.
202 			 */
203 			if (tid != 1 && !tmp->child_reaper)
204 				goto out_free;
205 			retval = -EPERM;
206 			if (!checkpoint_restore_ns_capable(tmp->user_ns))
207 				goto out_free;
208 			set_tid_size--;
209 		}
210 
211 		idr_preload(GFP_KERNEL);
212 		spin_lock_irq(&pidmap_lock);
213 
214 		if (tid) {
215 			nr = idr_alloc(&tmp->idr, NULL, tid,
216 				       tid + 1, GFP_ATOMIC);
217 			/*
218 			 * If ENOSPC is returned it means that the PID is
219 			 * alreay in use. Return EEXIST in that case.
220 			 */
221 			if (nr == -ENOSPC)
222 				nr = -EEXIST;
223 		} else {
224 			int pid_min = 1;
225 			/*
226 			 * init really needs pid 1, but after reaching the
227 			 * maximum wrap back to RESERVED_PIDS
228 			 */
229 			if (idr_get_cursor(&tmp->idr) > RESERVED_PIDS)
230 				pid_min = RESERVED_PIDS;
231 
232 			/*
233 			 * Store a null pointer so find_pid_ns does not find
234 			 * a partially initialized PID (see below).
235 			 */
236 			nr = idr_alloc_cyclic(&tmp->idr, NULL, pid_min,
237 					      pid_max, GFP_ATOMIC);
238 		}
239 		spin_unlock_irq(&pidmap_lock);
240 		idr_preload_end();
241 
242 		if (nr < 0) {
243 			retval = (nr == -ENOSPC) ? -EAGAIN : nr;
244 			goto out_free;
245 		}
246 
247 		pid->numbers[i].nr = nr;
248 		pid->numbers[i].ns = tmp;
249 		tmp = tmp->parent;
250 	}
251 
252 	/*
253 	 * ENOMEM is not the most obvious choice especially for the case
254 	 * where the child subreaper has already exited and the pid
255 	 * namespace denies the creation of any new processes. But ENOMEM
256 	 * is what we have exposed to userspace for a long time and it is
257 	 * documented behavior for pid namespaces. So we can't easily
258 	 * change it even if there were an error code better suited.
259 	 */
260 	retval = -ENOMEM;
261 
262 	get_pid_ns(ns);
263 	refcount_set(&pid->count, 1);
264 	spin_lock_init(&pid->lock);
265 	for (type = 0; type < PIDTYPE_MAX; ++type)
266 		INIT_HLIST_HEAD(&pid->tasks[type]);
267 
268 	init_waitqueue_head(&pid->wait_pidfd);
269 	INIT_HLIST_HEAD(&pid->inodes);
270 
271 	upid = pid->numbers + ns->level;
272 	spin_lock_irq(&pidmap_lock);
273 	if (!(ns->pid_allocated & PIDNS_ADDING))
274 		goto out_unlock;
275 	for ( ; upid >= pid->numbers; --upid) {
276 		/* Make the PID visible to find_pid_ns. */
277 		idr_replace(&upid->ns->idr, pid, upid->nr);
278 		upid->ns->pid_allocated++;
279 	}
280 	spin_unlock_irq(&pidmap_lock);
281 
282 	return pid;
283 
284 out_unlock:
285 	spin_unlock_irq(&pidmap_lock);
286 	put_pid_ns(ns);
287 
288 out_free:
289 	spin_lock_irq(&pidmap_lock);
290 	while (++i <= ns->level) {
291 		upid = pid->numbers + i;
292 		idr_remove(&upid->ns->idr, upid->nr);
293 	}
294 
295 	/* On failure to allocate the first pid, reset the state */
296 	if (ns->pid_allocated == PIDNS_ADDING)
297 		idr_set_cursor(&ns->idr, 0);
298 
299 	spin_unlock_irq(&pidmap_lock);
300 
301 	kmem_cache_free(ns->pid_cachep, pid);
302 	return ERR_PTR(retval);
303 }
304 
305 void disable_pid_allocation(struct pid_namespace *ns)
306 {
307 	spin_lock_irq(&pidmap_lock);
308 	ns->pid_allocated &= ~PIDNS_ADDING;
309 	spin_unlock_irq(&pidmap_lock);
310 }
311 
312 struct pid *find_pid_ns(int nr, struct pid_namespace *ns)
313 {
314 	return idr_find(&ns->idr, nr);
315 }
316 EXPORT_SYMBOL_GPL(find_pid_ns);
317 
318 struct pid *find_vpid(int nr)
319 {
320 	return find_pid_ns(nr, task_active_pid_ns(current));
321 }
322 EXPORT_SYMBOL_GPL(find_vpid);
323 
324 static struct pid **task_pid_ptr(struct task_struct *task, enum pid_type type)
325 {
326 	return (type == PIDTYPE_PID) ?
327 		&task->thread_pid :
328 		&task->signal->pids[type];
329 }
330 
331 /*
332  * attach_pid() must be called with the tasklist_lock write-held.
333  */
334 void attach_pid(struct task_struct *task, enum pid_type type)
335 {
336 	struct pid *pid = *task_pid_ptr(task, type);
337 	hlist_add_head_rcu(&task->pid_links[type], &pid->tasks[type]);
338 }
339 
340 static void __change_pid(struct task_struct *task, enum pid_type type,
341 			struct pid *new)
342 {
343 	struct pid **pid_ptr = task_pid_ptr(task, type);
344 	struct pid *pid;
345 	int tmp;
346 
347 	pid = *pid_ptr;
348 
349 	hlist_del_rcu(&task->pid_links[type]);
350 	*pid_ptr = new;
351 
352 	for (tmp = PIDTYPE_MAX; --tmp >= 0; )
353 		if (pid_has_task(pid, tmp))
354 			return;
355 
356 	free_pid(pid);
357 }
358 
359 void detach_pid(struct task_struct *task, enum pid_type type)
360 {
361 	__change_pid(task, type, NULL);
362 }
363 
364 void change_pid(struct task_struct *task, enum pid_type type,
365 		struct pid *pid)
366 {
367 	__change_pid(task, type, pid);
368 	attach_pid(task, type);
369 }
370 
371 void exchange_tids(struct task_struct *left, struct task_struct *right)
372 {
373 	struct pid *pid1 = left->thread_pid;
374 	struct pid *pid2 = right->thread_pid;
375 	struct hlist_head *head1 = &pid1->tasks[PIDTYPE_PID];
376 	struct hlist_head *head2 = &pid2->tasks[PIDTYPE_PID];
377 
378 	/* Swap the single entry tid lists */
379 	hlists_swap_heads_rcu(head1, head2);
380 
381 	/* Swap the per task_struct pid */
382 	rcu_assign_pointer(left->thread_pid, pid2);
383 	rcu_assign_pointer(right->thread_pid, pid1);
384 
385 	/* Swap the cached value */
386 	WRITE_ONCE(left->pid, pid_nr(pid2));
387 	WRITE_ONCE(right->pid, pid_nr(pid1));
388 }
389 
390 /* transfer_pid is an optimization of attach_pid(new), detach_pid(old) */
391 void transfer_pid(struct task_struct *old, struct task_struct *new,
392 			   enum pid_type type)
393 {
394 	if (type == PIDTYPE_PID)
395 		new->thread_pid = old->thread_pid;
396 	hlist_replace_rcu(&old->pid_links[type], &new->pid_links[type]);
397 }
398 
399 struct task_struct *pid_task(struct pid *pid, enum pid_type type)
400 {
401 	struct task_struct *result = NULL;
402 	if (pid) {
403 		struct hlist_node *first;
404 		first = rcu_dereference_check(hlist_first_rcu(&pid->tasks[type]),
405 					      lockdep_tasklist_lock_is_held());
406 		if (first)
407 			result = hlist_entry(first, struct task_struct, pid_links[(type)]);
408 	}
409 	return result;
410 }
411 EXPORT_SYMBOL(pid_task);
412 
413 /*
414  * Must be called under rcu_read_lock().
415  */
416 struct task_struct *find_task_by_pid_ns(pid_t nr, struct pid_namespace *ns)
417 {
418 	RCU_LOCKDEP_WARN(!rcu_read_lock_held(),
419 			 "find_task_by_pid_ns() needs rcu_read_lock() protection");
420 	return pid_task(find_pid_ns(nr, ns), PIDTYPE_PID);
421 }
422 
423 struct task_struct *find_task_by_vpid(pid_t vnr)
424 {
425 	return find_task_by_pid_ns(vnr, task_active_pid_ns(current));
426 }
427 
428 struct task_struct *find_get_task_by_vpid(pid_t nr)
429 {
430 	struct task_struct *task;
431 
432 	rcu_read_lock();
433 	task = find_task_by_vpid(nr);
434 	if (task)
435 		get_task_struct(task);
436 	rcu_read_unlock();
437 
438 	return task;
439 }
440 
441 struct pid *get_task_pid(struct task_struct *task, enum pid_type type)
442 {
443 	struct pid *pid;
444 	rcu_read_lock();
445 	pid = get_pid(rcu_dereference(*task_pid_ptr(task, type)));
446 	rcu_read_unlock();
447 	return pid;
448 }
449 EXPORT_SYMBOL_GPL(get_task_pid);
450 
451 struct task_struct *get_pid_task(struct pid *pid, enum pid_type type)
452 {
453 	struct task_struct *result;
454 	rcu_read_lock();
455 	result = pid_task(pid, type);
456 	if (result)
457 		get_task_struct(result);
458 	rcu_read_unlock();
459 	return result;
460 }
461 EXPORT_SYMBOL_GPL(get_pid_task);
462 
463 struct pid *find_get_pid(pid_t nr)
464 {
465 	struct pid *pid;
466 
467 	rcu_read_lock();
468 	pid = get_pid(find_vpid(nr));
469 	rcu_read_unlock();
470 
471 	return pid;
472 }
473 EXPORT_SYMBOL_GPL(find_get_pid);
474 
475 pid_t pid_nr_ns(struct pid *pid, struct pid_namespace *ns)
476 {
477 	struct upid *upid;
478 	pid_t nr = 0;
479 
480 	if (pid && ns->level <= pid->level) {
481 		upid = &pid->numbers[ns->level];
482 		if (upid->ns == ns)
483 			nr = upid->nr;
484 	}
485 	return nr;
486 }
487 EXPORT_SYMBOL_GPL(pid_nr_ns);
488 
489 pid_t pid_vnr(struct pid *pid)
490 {
491 	return pid_nr_ns(pid, task_active_pid_ns(current));
492 }
493 EXPORT_SYMBOL_GPL(pid_vnr);
494 
495 pid_t __task_pid_nr_ns(struct task_struct *task, enum pid_type type,
496 			struct pid_namespace *ns)
497 {
498 	pid_t nr = 0;
499 
500 	rcu_read_lock();
501 	if (!ns)
502 		ns = task_active_pid_ns(current);
503 	nr = pid_nr_ns(rcu_dereference(*task_pid_ptr(task, type)), ns);
504 	rcu_read_unlock();
505 
506 	return nr;
507 }
508 EXPORT_SYMBOL(__task_pid_nr_ns);
509 
510 struct pid_namespace *task_active_pid_ns(struct task_struct *tsk)
511 {
512 	return ns_of_pid(task_pid(tsk));
513 }
514 EXPORT_SYMBOL_GPL(task_active_pid_ns);
515 
516 /*
517  * Used by proc to find the first pid that is greater than or equal to nr.
518  *
519  * If there is a pid at nr this function is exactly the same as find_pid_ns.
520  */
521 struct pid *find_ge_pid(int nr, struct pid_namespace *ns)
522 {
523 	return idr_get_next(&ns->idr, &nr);
524 }
525 EXPORT_SYMBOL_GPL(find_ge_pid);
526 
527 struct pid *pidfd_get_pid(unsigned int fd, unsigned int *flags)
528 {
529 	struct fd f;
530 	struct pid *pid;
531 
532 	f = fdget(fd);
533 	if (!f.file)
534 		return ERR_PTR(-EBADF);
535 
536 	pid = pidfd_pid(f.file);
537 	if (!IS_ERR(pid)) {
538 		get_pid(pid);
539 		*flags = f.file->f_flags;
540 	}
541 
542 	fdput(f);
543 	return pid;
544 }
545 
546 /**
547  * pidfd_get_task() - Get the task associated with a pidfd
548  *
549  * @pidfd: pidfd for which to get the task
550  * @flags: flags associated with this pidfd
551  *
552  * Return the task associated with @pidfd. The function takes a reference on
553  * the returned task. The caller is responsible for releasing that reference.
554  *
555  * Currently, the process identified by @pidfd is always a thread-group leader.
556  * This restriction currently exists for all aspects of pidfds including pidfd
557  * creation (CLONE_PIDFD cannot be used with CLONE_THREAD) and pidfd polling
558  * (only supports thread group leaders).
559  *
560  * Return: On success, the task_struct associated with the pidfd.
561  *	   On error, a negative errno number will be returned.
562  */
563 struct task_struct *pidfd_get_task(int pidfd, unsigned int *flags)
564 {
565 	unsigned int f_flags;
566 	struct pid *pid;
567 	struct task_struct *task;
568 
569 	pid = pidfd_get_pid(pidfd, &f_flags);
570 	if (IS_ERR(pid))
571 		return ERR_CAST(pid);
572 
573 	task = get_pid_task(pid, PIDTYPE_TGID);
574 	put_pid(pid);
575 	if (!task)
576 		return ERR_PTR(-ESRCH);
577 
578 	*flags = f_flags;
579 	return task;
580 }
581 
582 /**
583  * pidfd_create() - Create a new pid file descriptor.
584  *
585  * @pid:   struct pid that the pidfd will reference
586  * @flags: flags to pass
587  *
588  * This creates a new pid file descriptor with the O_CLOEXEC flag set.
589  *
590  * Note, that this function can only be called after the fd table has
591  * been unshared to avoid leaking the pidfd to the new process.
592  *
593  * This symbol should not be explicitly exported to loadable modules.
594  *
595  * Return: On success, a cloexec pidfd is returned.
596  *         On error, a negative errno number will be returned.
597  */
598 int pidfd_create(struct pid *pid, unsigned int flags)
599 {
600 	int pidfd;
601 	struct file *pidfd_file;
602 
603 	pidfd = pidfd_prepare(pid, flags, &pidfd_file);
604 	if (pidfd < 0)
605 		return pidfd;
606 
607 	fd_install(pidfd, pidfd_file);
608 	return pidfd;
609 }
610 
611 /**
612  * sys_pidfd_open() - Open new pid file descriptor.
613  *
614  * @pid:   pid for which to retrieve a pidfd
615  * @flags: flags to pass
616  *
617  * This creates a new pid file descriptor with the O_CLOEXEC flag set for
618  * the process identified by @pid. Currently, the process identified by
619  * @pid must be a thread-group leader. This restriction currently exists
620  * for all aspects of pidfds including pidfd creation (CLONE_PIDFD cannot
621  * be used with CLONE_THREAD) and pidfd polling (only supports thread group
622  * leaders).
623  *
624  * Return: On success, a cloexec pidfd is returned.
625  *         On error, a negative errno number will be returned.
626  */
627 SYSCALL_DEFINE2(pidfd_open, pid_t, pid, unsigned int, flags)
628 {
629 	int fd;
630 	struct pid *p;
631 
632 	if (flags & ~PIDFD_NONBLOCK)
633 		return -EINVAL;
634 
635 	if (pid <= 0)
636 		return -EINVAL;
637 
638 	p = find_get_pid(pid);
639 	if (!p)
640 		return -ESRCH;
641 
642 	fd = pidfd_create(p, flags);
643 
644 	put_pid(p);
645 	return fd;
646 }
647 
648 void __init pid_idr_init(void)
649 {
650 	/* Verify no one has done anything silly: */
651 	BUILD_BUG_ON(PID_MAX_LIMIT >= PIDNS_ADDING);
652 
653 	/* bump default and minimum pid_max based on number of cpus */
654 	pid_max = min(pid_max_max, max_t(int, pid_max,
655 				PIDS_PER_CPU_DEFAULT * num_possible_cpus()));
656 	pid_max_min = max_t(int, pid_max_min,
657 				PIDS_PER_CPU_MIN * num_possible_cpus());
658 	pr_info("pid_max: default: %u minimum: %u\n", pid_max, pid_max_min);
659 
660 	idr_init(&init_pid_ns.idr);
661 
662 	init_pid_ns.pid_cachep = kmem_cache_create("pid",
663 			struct_size_t(struct pid, numbers, 1),
664 			__alignof__(struct pid),
665 			SLAB_HWCACHE_ALIGN | SLAB_PANIC | SLAB_ACCOUNT,
666 			NULL);
667 }
668 
669 static struct file *__pidfd_fget(struct task_struct *task, int fd)
670 {
671 	struct file *file;
672 	int ret;
673 
674 	ret = down_read_killable(&task->signal->exec_update_lock);
675 	if (ret)
676 		return ERR_PTR(ret);
677 
678 	if (ptrace_may_access(task, PTRACE_MODE_ATTACH_REALCREDS))
679 		file = fget_task(task, fd);
680 	else
681 		file = ERR_PTR(-EPERM);
682 
683 	up_read(&task->signal->exec_update_lock);
684 
685 	return file ?: ERR_PTR(-EBADF);
686 }
687 
688 static int pidfd_getfd(struct pid *pid, int fd)
689 {
690 	struct task_struct *task;
691 	struct file *file;
692 	int ret;
693 
694 	task = get_pid_task(pid, PIDTYPE_PID);
695 	if (!task)
696 		return -ESRCH;
697 
698 	file = __pidfd_fget(task, fd);
699 	put_task_struct(task);
700 	if (IS_ERR(file))
701 		return PTR_ERR(file);
702 
703 	ret = receive_fd(file, NULL, O_CLOEXEC);
704 	fput(file);
705 
706 	return ret;
707 }
708 
709 /**
710  * sys_pidfd_getfd() - Get a file descriptor from another process
711  *
712  * @pidfd:	the pidfd file descriptor of the process
713  * @fd:		the file descriptor number to get
714  * @flags:	flags on how to get the fd (reserved)
715  *
716  * This syscall gets a copy of a file descriptor from another process
717  * based on the pidfd, and file descriptor number. It requires that
718  * the calling process has the ability to ptrace the process represented
719  * by the pidfd. The process which is having its file descriptor copied
720  * is otherwise unaffected.
721  *
722  * Return: On success, a cloexec file descriptor is returned.
723  *         On error, a negative errno number will be returned.
724  */
725 SYSCALL_DEFINE3(pidfd_getfd, int, pidfd, int, fd,
726 		unsigned int, flags)
727 {
728 	struct pid *pid;
729 	struct fd f;
730 	int ret;
731 
732 	/* flags is currently unused - make sure it's unset */
733 	if (flags)
734 		return -EINVAL;
735 
736 	f = fdget(pidfd);
737 	if (!f.file)
738 		return -EBADF;
739 
740 	pid = pidfd_pid(f.file);
741 	if (IS_ERR(pid))
742 		ret = PTR_ERR(pid);
743 	else
744 		ret = pidfd_getfd(pid, fd);
745 
746 	fdput(f);
747 	return ret;
748 }
749