xref: /linux/kernel/pid.c (revision 1aa92cd3)
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 	.kref = KREF_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 
put_pid(struct pid * pid)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 
delayed_put_pid(struct rcu_head * rhp)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 
free_pid(struct pid * pid)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 
alloc_pid(struct pid_namespace * ns,pid_t * set_tid,size_t set_tid_size)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 
disable_pid_allocation(struct pid_namespace * ns)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 
find_pid_ns(int nr,struct pid_namespace * ns)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 
find_vpid(int nr)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 
task_pid_ptr(struct task_struct * task,enum pid_type type)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  */
attach_pid(struct task_struct * task,enum pid_type type)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 
__change_pid(struct task_struct * task,enum pid_type type,struct pid * new)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 
detach_pid(struct task_struct * task,enum pid_type type)356 void detach_pid(struct task_struct *task, enum pid_type type)
357 {
358 	__change_pid(task, type, NULL);
359 }
360 
change_pid(struct task_struct * task,enum pid_type type,struct pid * pid)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 
exchange_tids(struct task_struct * left,struct task_struct * right)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) */
transfer_pid(struct task_struct * old,struct task_struct * new,enum pid_type type)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 
pid_task(struct pid * pid,enum pid_type type)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  */
find_task_by_pid_ns(pid_t nr,struct pid_namespace * ns)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 
find_task_by_vpid(pid_t vnr)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 
find_get_task_by_vpid(pid_t nr)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 
get_task_pid(struct task_struct * task,enum pid_type type)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 
get_pid_task(struct pid * pid,enum pid_type type)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 
find_get_pid(pid_t nr)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 
pid_nr_ns(struct pid * pid,struct pid_namespace * ns)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 
pid_vnr(struct pid * pid)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 
__task_pid_nr_ns(struct task_struct * task,enum pid_type type,struct pid_namespace * ns)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 
task_active_pid_ns(struct task_struct * tsk)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  */
find_ge_pid(int nr,struct pid_namespace * ns)518 struct pid *find_ge_pid(int nr, struct pid_namespace *ns)
519 {
520 	return idr_get_next(&ns->idr, &nr);
521 }
522 
pidfd_get_pid(unsigned int fd,unsigned int * flags)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_create() - Create a new pid file descriptor.
544  *
545  * @pid:   struct pid that the pidfd will reference
546  * @flags: flags to pass
547  *
548  * This creates a new pid file descriptor with the O_CLOEXEC flag set.
549  *
550  * Note, that this function can only be called after the fd table has
551  * been unshared to avoid leaking the pidfd to the new process.
552  *
553  * Return: On success, a cloexec pidfd is returned.
554  *         On error, a negative errno number will be returned.
555  */
pidfd_create(struct pid * pid,unsigned int flags)556 static int pidfd_create(struct pid *pid, unsigned int flags)
557 {
558 	int fd;
559 
560 	fd = anon_inode_getfd("[pidfd]", &pidfd_fops, get_pid(pid),
561 			      flags | O_RDWR | O_CLOEXEC);
562 	if (fd < 0)
563 		put_pid(pid);
564 
565 	return fd;
566 }
567 
568 /**
569  * pidfd_open() - Open new pid file descriptor.
570  *
571  * @pid:   pid for which to retrieve a pidfd
572  * @flags: flags to pass
573  *
574  * This creates a new pid file descriptor with the O_CLOEXEC flag set for
575  * the process identified by @pid. Currently, the process identified by
576  * @pid must be a thread-group leader. This restriction currently exists
577  * for all aspects of pidfds including pidfd creation (CLONE_PIDFD cannot
578  * be used with CLONE_THREAD) and pidfd polling (only supports thread group
579  * leaders).
580  *
581  * Return: On success, a cloexec pidfd is returned.
582  *         On error, a negative errno number will be returned.
583  */
SYSCALL_DEFINE2(pidfd_open,pid_t,pid,unsigned int,flags)584 SYSCALL_DEFINE2(pidfd_open, pid_t, pid, unsigned int, flags)
585 {
586 	int fd;
587 	struct pid *p;
588 
589 	if (flags & ~PIDFD_NONBLOCK)
590 		return -EINVAL;
591 
592 	if (pid <= 0)
593 		return -EINVAL;
594 
595 	p = find_get_pid(pid);
596 	if (!p)
597 		return -ESRCH;
598 
599 	if (pid_has_task(p, PIDTYPE_TGID))
600 		fd = pidfd_create(p, flags);
601 	else
602 		fd = -EINVAL;
603 
604 	put_pid(p);
605 	return fd;
606 }
607 
pid_idr_init(void)608 void __init pid_idr_init(void)
609 {
610 	/* Verify no one has done anything silly: */
611 	BUILD_BUG_ON(PID_MAX_LIMIT >= PIDNS_ADDING);
612 
613 	/* bump default and minimum pid_max based on number of cpus */
614 	pid_max = min(pid_max_max, max_t(int, pid_max,
615 				PIDS_PER_CPU_DEFAULT * num_possible_cpus()));
616 	pid_max_min = max_t(int, pid_max_min,
617 				PIDS_PER_CPU_MIN * num_possible_cpus());
618 	pr_info("pid_max: default: %u minimum: %u\n", pid_max, pid_max_min);
619 
620 	idr_init(&init_pid_ns.idr);
621 
622 	init_pid_ns.pid_cachep = KMEM_CACHE(pid,
623 			SLAB_HWCACHE_ALIGN | SLAB_PANIC | SLAB_ACCOUNT);
624 }
625 
__pidfd_fget(struct task_struct * task,int fd)626 static struct file *__pidfd_fget(struct task_struct *task, int fd)
627 {
628 	struct file *file;
629 	int ret;
630 
631 	ret = mutex_lock_killable(&task->signal->exec_update_mutex);
632 	if (ret)
633 		return ERR_PTR(ret);
634 
635 	if (ptrace_may_access(task, PTRACE_MODE_ATTACH_REALCREDS))
636 		file = fget_task(task, fd);
637 	else
638 		file = ERR_PTR(-EPERM);
639 
640 	mutex_unlock(&task->signal->exec_update_mutex);
641 
642 	return file ?: ERR_PTR(-EBADF);
643 }
644 
pidfd_getfd(struct pid * pid,int fd)645 static int pidfd_getfd(struct pid *pid, int fd)
646 {
647 	struct task_struct *task;
648 	struct file *file;
649 	int ret;
650 
651 	task = get_pid_task(pid, PIDTYPE_PID);
652 	if (!task)
653 		return -ESRCH;
654 
655 	file = __pidfd_fget(task, fd);
656 	put_task_struct(task);
657 	if (IS_ERR(file))
658 		return PTR_ERR(file);
659 
660 	ret = receive_fd(file, O_CLOEXEC);
661 	fput(file);
662 
663 	return ret;
664 }
665 
666 /**
667  * sys_pidfd_getfd() - Get a file descriptor from another process
668  *
669  * @pidfd:	the pidfd file descriptor of the process
670  * @fd:		the file descriptor number to get
671  * @flags:	flags on how to get the fd (reserved)
672  *
673  * This syscall gets a copy of a file descriptor from another process
674  * based on the pidfd, and file descriptor number. It requires that
675  * the calling process has the ability to ptrace the process represented
676  * by the pidfd. The process which is having its file descriptor copied
677  * is otherwise unaffected.
678  *
679  * Return: On success, a cloexec file descriptor is returned.
680  *         On error, a negative errno number will be returned.
681  */
SYSCALL_DEFINE3(pidfd_getfd,int,pidfd,int,fd,unsigned int,flags)682 SYSCALL_DEFINE3(pidfd_getfd, int, pidfd, int, fd,
683 		unsigned int, flags)
684 {
685 	struct pid *pid;
686 	struct fd f;
687 	int ret;
688 
689 	/* flags is currently unused - make sure it's unset */
690 	if (flags)
691 		return -EINVAL;
692 
693 	f = fdget(pidfd);
694 	if (!f.file)
695 		return -EBADF;
696 
697 	pid = pidfd_pid(f.file);
698 	if (IS_ERR(pid))
699 		ret = PTR_ERR(pid);
700 	else
701 		ret = pidfd_getfd(pid, fd);
702 
703 	fdput(f);
704 	return ret;
705 }
706