cachepc-linux

Fork of AMDESE/linux with modifications for CachePC side-channel attack
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pid.c (18611B)


      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
     48struct 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
     62int pid_max = PID_MAX_DEFAULT;
     63
     64#define RESERVED_PIDS		300
     65
     66int pid_max_min = RESERVED_PIDS + 1;
     67int 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 */
     75struct 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};
     87EXPORT_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
    103static  __cacheline_aligned_in_smp DEFINE_SPINLOCK(pidmap_lock);
    104
    105void 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}
    118EXPORT_SYMBOL_GPL(put_pid);
    119
    120static 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
    126void 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
    159struct 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
    281out_unlock:
    282	spin_unlock_irq(&pidmap_lock);
    283	put_pid_ns(ns);
    284
    285out_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
    302void 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
    309struct pid *find_pid_ns(int nr, struct pid_namespace *ns)
    310{
    311	return idr_find(&ns->idr, nr);
    312}
    313EXPORT_SYMBOL_GPL(find_pid_ns);
    314
    315struct pid *find_vpid(int nr)
    316{
    317	return find_pid_ns(nr, task_active_pid_ns(current));
    318}
    319EXPORT_SYMBOL_GPL(find_vpid);
    320
    321static 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 */
    331void 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
    337static 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
    356void detach_pid(struct task_struct *task, enum pid_type type)
    357{
    358	__change_pid(task, type, NULL);
    359}
    360
    361void 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
    368void 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) */
    388void 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
    396struct 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}
    408EXPORT_SYMBOL(pid_task);
    409
    410/*
    411 * Must be called under rcu_read_lock().
    412 */
    413struct 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
    420struct 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
    425struct 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
    438struct 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}
    446EXPORT_SYMBOL_GPL(get_task_pid);
    447
    448struct 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}
    458EXPORT_SYMBOL_GPL(get_pid_task);
    459
    460struct 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}
    470EXPORT_SYMBOL_GPL(find_get_pid);
    471
    472pid_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}
    484EXPORT_SYMBOL_GPL(pid_nr_ns);
    485
    486pid_t pid_vnr(struct pid *pid)
    487{
    488	return pid_nr_ns(pid, task_active_pid_ns(current));
    489}
    490EXPORT_SYMBOL_GPL(pid_vnr);
    491
    492pid_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}
    505EXPORT_SYMBOL(__task_pid_nr_ns);
    506
    507struct pid_namespace *task_active_pid_ns(struct task_struct *tsk)
    508{
    509	return ns_of_pid(task_pid(tsk));
    510}
    511EXPORT_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 */
    518struct pid *find_ge_pid(int nr, struct pid_namespace *ns)
    519{
    520	return idr_get_next(&ns->idr, &nr);
    521}
    522
    523struct 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 */
    559struct 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 */
    594int 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 */
    628SYSCALL_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
    649void __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
    667static 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
    686static 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 */
    723SYSCALL_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}