1 // SPDX-License-Identifier: GPL-2.0
2 /*
3  *  linux/fs/file.c
4  *
5  *  Copyright (C) 1998-1999, Stephen Tweedie and Bill Hawes
6  *
7  *  Manage the dynamic fd arrays in the process files_struct.
8  */
9 
10 #include <linux/syscalls.h>
11 #include <linux/export.h>
12 #include <linux/fs.h>
13 #include <linux/kernel.h>
14 #include <linux/mm.h>
15 #include <linux/sched/signal.h>
16 #include <linux/slab.h>
17 #include <linux/file.h>
18 #include <linux/fdtable.h>
19 #include <linux/bitops.h>
20 #include <linux/spinlock.h>
21 #include <linux/rcupdate.h>
22 #include <linux/close_range.h>
23 #include <net/sock.h>
24 
25 #include "internal.h"
26 
27 unsigned int sysctl_nr_open __read_mostly = 1024*1024;
28 unsigned int sysctl_nr_open_min = BITS_PER_LONG;
29 /* our min() is unusable in constant expressions ;-/ */
30 #define __const_min(x, y) ((x) < (y) ? (x) : (y))
31 unsigned int sysctl_nr_open_max =
32 	__const_min(INT_MAX, ~(size_t)0/sizeof(void *)) & -BITS_PER_LONG;
33 
__free_fdtable(struct fdtable * fdt)34 static void __free_fdtable(struct fdtable *fdt)
35 {
36 	kvfree(fdt->fd);
37 	kvfree(fdt->open_fds);
38 	kfree(fdt);
39 }
40 
free_fdtable_rcu(struct rcu_head * rcu)41 static void free_fdtable_rcu(struct rcu_head *rcu)
42 {
43 	__free_fdtable(container_of(rcu, struct fdtable, rcu));
44 }
45 
46 #define BITBIT_NR(nr)	BITS_TO_LONGS(BITS_TO_LONGS(nr))
47 #define BITBIT_SIZE(nr)	(BITBIT_NR(nr) * sizeof(long))
48 
49 /*
50  * Copy 'count' fd bits from the old table to the new table and clear the extra
51  * space if any.  This does not copy the file pointers.  Called with the files
52  * spinlock held for write.
53  */
copy_fd_bitmaps(struct fdtable * nfdt,struct fdtable * ofdt,unsigned int count)54 static void copy_fd_bitmaps(struct fdtable *nfdt, struct fdtable *ofdt,
55 			    unsigned int count)
56 {
57 	unsigned int cpy, set;
58 
59 	cpy = count / BITS_PER_BYTE;
60 	set = (nfdt->max_fds - count) / BITS_PER_BYTE;
61 	memcpy(nfdt->open_fds, ofdt->open_fds, cpy);
62 	memset((char *)nfdt->open_fds + cpy, 0, set);
63 	memcpy(nfdt->close_on_exec, ofdt->close_on_exec, cpy);
64 	memset((char *)nfdt->close_on_exec + cpy, 0, set);
65 
66 	cpy = BITBIT_SIZE(count);
67 	set = BITBIT_SIZE(nfdt->max_fds) - cpy;
68 	memcpy(nfdt->full_fds_bits, ofdt->full_fds_bits, cpy);
69 	memset((char *)nfdt->full_fds_bits + cpy, 0, set);
70 }
71 
72 /*
73  * Copy all file descriptors from the old table to the new, expanded table and
74  * clear the extra space.  Called with the files spinlock held for write.
75  */
copy_fdtable(struct fdtable * nfdt,struct fdtable * ofdt)76 static void copy_fdtable(struct fdtable *nfdt, struct fdtable *ofdt)
77 {
78 	size_t cpy, set;
79 
80 	BUG_ON(nfdt->max_fds < ofdt->max_fds);
81 
82 	cpy = ofdt->max_fds * sizeof(struct file *);
83 	set = (nfdt->max_fds - ofdt->max_fds) * sizeof(struct file *);
84 	memcpy(nfdt->fd, ofdt->fd, cpy);
85 	memset((char *)nfdt->fd + cpy, 0, set);
86 
87 	copy_fd_bitmaps(nfdt, ofdt, ofdt->max_fds);
88 }
89 
90 /*
91  * Note how the fdtable bitmap allocations very much have to be a multiple of
92  * BITS_PER_LONG. This is not only because we walk those things in chunks of
93  * 'unsigned long' in some places, but simply because that is how the Linux
94  * kernel bitmaps are defined to work: they are not "bits in an array of bytes",
95  * they are very much "bits in an array of unsigned long".
96  *
97  * The ALIGN(nr, BITS_PER_LONG) here is for clarity: since we just multiplied
98  * by that "1024/sizeof(ptr)" before, we already know there are sufficient
99  * clear low bits. Clang seems to realize that, gcc ends up being confused.
100  *
101  * On a 128-bit machine, the ALIGN() would actually matter. In the meantime,
102  * let's consider it documentation (and maybe a test-case for gcc to improve
103  * its code generation ;)
104  */
alloc_fdtable(unsigned int nr)105 static struct fdtable * alloc_fdtable(unsigned int nr)
106 {
107 	struct fdtable *fdt;
108 	void *data;
109 
110 	/*
111 	 * Figure out how many fds we actually want to support in this fdtable.
112 	 * Allocation steps are keyed to the size of the fdarray, since it
113 	 * grows far faster than any of the other dynamic data. We try to fit
114 	 * the fdarray into comfortable page-tuned chunks: starting at 1024B
115 	 * and growing in powers of two from there on.
116 	 */
117 	nr /= (1024 / sizeof(struct file *));
118 	nr = roundup_pow_of_two(nr + 1);
119 	nr *= (1024 / sizeof(struct file *));
120 	nr = ALIGN(nr, BITS_PER_LONG);
121 	/*
122 	 * Note that this can drive nr *below* what we had passed if sysctl_nr_open
123 	 * had been set lower between the check in expand_files() and here.  Deal
124 	 * with that in caller, it's cheaper that way.
125 	 *
126 	 * We make sure that nr remains a multiple of BITS_PER_LONG - otherwise
127 	 * bitmaps handling below becomes unpleasant, to put it mildly...
128 	 */
129 	if (unlikely(nr > sysctl_nr_open))
130 		nr = ((sysctl_nr_open - 1) | (BITS_PER_LONG - 1)) + 1;
131 
132 	fdt = kmalloc(sizeof(struct fdtable), GFP_KERNEL_ACCOUNT);
133 	if (!fdt)
134 		goto out;
135 	fdt->max_fds = nr;
136 	data = kvmalloc_array(nr, sizeof(struct file *), GFP_KERNEL_ACCOUNT);
137 	if (!data)
138 		goto out_fdt;
139 	fdt->fd = data;
140 
141 	data = kvmalloc(max_t(size_t,
142 				 2 * nr / BITS_PER_BYTE + BITBIT_SIZE(nr), L1_CACHE_BYTES),
143 				 GFP_KERNEL_ACCOUNT);
144 	if (!data)
145 		goto out_arr;
146 	fdt->open_fds = data;
147 	data += nr / BITS_PER_BYTE;
148 	fdt->close_on_exec = data;
149 	data += nr / BITS_PER_BYTE;
150 	fdt->full_fds_bits = data;
151 
152 	return fdt;
153 
154 out_arr:
155 	kvfree(fdt->fd);
156 out_fdt:
157 	kfree(fdt);
158 out:
159 	return NULL;
160 }
161 
162 /*
163  * Expand the file descriptor table.
164  * This function will allocate a new fdtable and both fd array and fdset, of
165  * the given size.
166  * Return <0 error code on error; 1 on successful completion.
167  * The files->file_lock should be held on entry, and will be held on exit.
168  */
expand_fdtable(struct files_struct * files,unsigned int nr)169 static int expand_fdtable(struct files_struct *files, unsigned int nr)
170 	__releases(files->file_lock)
171 	__acquires(files->file_lock)
172 {
173 	struct fdtable *new_fdt, *cur_fdt;
174 
175 	spin_unlock(&files->file_lock);
176 	new_fdt = alloc_fdtable(nr);
177 
178 	/* make sure all fd_install() have seen resize_in_progress
179 	 * or have finished their rcu_read_lock_sched() section.
180 	 */
181 	if (atomic_read(&files->count) > 1)
182 		synchronize_rcu();
183 
184 	spin_lock(&files->file_lock);
185 	if (!new_fdt)
186 		return -ENOMEM;
187 	/*
188 	 * extremely unlikely race - sysctl_nr_open decreased between the check in
189 	 * caller and alloc_fdtable().  Cheaper to catch it here...
190 	 */
191 	if (unlikely(new_fdt->max_fds <= nr)) {
192 		__free_fdtable(new_fdt);
193 		return -EMFILE;
194 	}
195 	cur_fdt = files_fdtable(files);
196 	BUG_ON(nr < cur_fdt->max_fds);
197 	copy_fdtable(new_fdt, cur_fdt);
198 	rcu_assign_pointer(files->fdt, new_fdt);
199 	if (cur_fdt != &files->fdtab)
200 		call_rcu(&cur_fdt->rcu, free_fdtable_rcu);
201 	/* coupled with smp_rmb() in fd_install() */
202 	smp_wmb();
203 	return 1;
204 }
205 
206 /*
207  * Expand files.
208  * This function will expand the file structures, if the requested size exceeds
209  * the current capacity and there is room for expansion.
210  * Return <0 error code on error; 0 when nothing done; 1 when files were
211  * expanded and execution may have blocked.
212  * The files->file_lock should be held on entry, and will be held on exit.
213  */
expand_files(struct files_struct * files,unsigned int nr)214 static int expand_files(struct files_struct *files, unsigned int nr)
215 	__releases(files->file_lock)
216 	__acquires(files->file_lock)
217 {
218 	struct fdtable *fdt;
219 	int expanded = 0;
220 
221 repeat:
222 	fdt = files_fdtable(files);
223 
224 	/* Do we need to expand? */
225 	if (nr < fdt->max_fds)
226 		return expanded;
227 
228 	/* Can we expand? */
229 	if (nr >= sysctl_nr_open)
230 		return -EMFILE;
231 
232 	if (unlikely(files->resize_in_progress)) {
233 		spin_unlock(&files->file_lock);
234 		expanded = 1;
235 		wait_event(files->resize_wait, !files->resize_in_progress);
236 		spin_lock(&files->file_lock);
237 		goto repeat;
238 	}
239 
240 	/* All good, so we try */
241 	files->resize_in_progress = true;
242 	expanded = expand_fdtable(files, nr);
243 	files->resize_in_progress = false;
244 
245 	wake_up_all(&files->resize_wait);
246 	return expanded;
247 }
248 
__set_close_on_exec(unsigned int fd,struct fdtable * fdt)249 static inline void __set_close_on_exec(unsigned int fd, struct fdtable *fdt)
250 {
251 	__set_bit(fd, fdt->close_on_exec);
252 }
253 
__clear_close_on_exec(unsigned int fd,struct fdtable * fdt)254 static inline void __clear_close_on_exec(unsigned int fd, struct fdtable *fdt)
255 {
256 	if (test_bit(fd, fdt->close_on_exec))
257 		__clear_bit(fd, fdt->close_on_exec);
258 }
259 
__set_open_fd(unsigned int fd,struct fdtable * fdt)260 static inline void __set_open_fd(unsigned int fd, struct fdtable *fdt)
261 {
262 	__set_bit(fd, fdt->open_fds);
263 	fd /= BITS_PER_LONG;
264 	if (!~fdt->open_fds[fd])
265 		__set_bit(fd, fdt->full_fds_bits);
266 }
267 
__clear_open_fd(unsigned int fd,struct fdtable * fdt)268 static inline void __clear_open_fd(unsigned int fd, struct fdtable *fdt)
269 {
270 	__clear_bit(fd, fdt->open_fds);
271 	__clear_bit(fd / BITS_PER_LONG, fdt->full_fds_bits);
272 }
273 
count_open_files(struct fdtable * fdt)274 static unsigned int count_open_files(struct fdtable *fdt)
275 {
276 	unsigned int size = fdt->max_fds;
277 	unsigned int i;
278 
279 	/* Find the last open fd */
280 	for (i = size / BITS_PER_LONG; i > 0; ) {
281 		if (fdt->open_fds[--i])
282 			break;
283 	}
284 	i = (i + 1) * BITS_PER_LONG;
285 	return i;
286 }
287 
288 /*
289  * Note that a sane fdtable size always has to be a multiple of
290  * BITS_PER_LONG, since we have bitmaps that are sized by this.
291  *
292  * 'max_fds' will normally already be properly aligned, but it
293  * turns out that in the close_range() -> __close_range() ->
294  * unshare_fd() -> dup_fd() -> sane_fdtable_size() we can end
295  * up having a 'max_fds' value that isn't already aligned.
296  *
297  * Rather than make close_range() have to worry about this,
298  * just make that BITS_PER_LONG alignment be part of a sane
299  * fdtable size. Becuase that's really what it is.
300  */
sane_fdtable_size(struct fdtable * fdt,unsigned int max_fds)301 static unsigned int sane_fdtable_size(struct fdtable *fdt, unsigned int max_fds)
302 {
303 	unsigned int count;
304 
305 	count = count_open_files(fdt);
306 	if (max_fds < NR_OPEN_DEFAULT)
307 		max_fds = NR_OPEN_DEFAULT;
308 	return ALIGN(min(count, max_fds), BITS_PER_LONG);
309 }
310 
311 /*
312  * Allocate a new files structure and copy contents from the
313  * passed in files structure.
314  * errorp will be valid only when the returned files_struct is NULL.
315  */
dup_fd(struct files_struct * oldf,unsigned int max_fds,int * errorp)316 struct files_struct *dup_fd(struct files_struct *oldf, unsigned int max_fds, int *errorp)
317 {
318 	struct files_struct *newf;
319 	struct file **old_fds, **new_fds;
320 	unsigned int open_files, i;
321 	struct fdtable *old_fdt, *new_fdt;
322 
323 	*errorp = -ENOMEM;
324 	newf = kmem_cache_alloc(files_cachep, GFP_KERNEL);
325 	if (!newf)
326 		goto out;
327 
328 	atomic_set(&newf->count, 1);
329 
330 	spin_lock_init(&newf->file_lock);
331 	newf->resize_in_progress = false;
332 	init_waitqueue_head(&newf->resize_wait);
333 	newf->next_fd = 0;
334 	new_fdt = &newf->fdtab;
335 	new_fdt->max_fds = NR_OPEN_DEFAULT;
336 	new_fdt->close_on_exec = newf->close_on_exec_init;
337 	new_fdt->open_fds = newf->open_fds_init;
338 	new_fdt->full_fds_bits = newf->full_fds_bits_init;
339 	new_fdt->fd = &newf->fd_array[0];
340 
341 	spin_lock(&oldf->file_lock);
342 	old_fdt = files_fdtable(oldf);
343 	open_files = sane_fdtable_size(old_fdt, max_fds);
344 
345 	/*
346 	 * Check whether we need to allocate a larger fd array and fd set.
347 	 */
348 	while (unlikely(open_files > new_fdt->max_fds)) {
349 		spin_unlock(&oldf->file_lock);
350 
351 		if (new_fdt != &newf->fdtab)
352 			__free_fdtable(new_fdt);
353 
354 		new_fdt = alloc_fdtable(open_files - 1);
355 		if (!new_fdt) {
356 			*errorp = -ENOMEM;
357 			goto out_release;
358 		}
359 
360 		/* beyond sysctl_nr_open; nothing to do */
361 		if (unlikely(new_fdt->max_fds < open_files)) {
362 			__free_fdtable(new_fdt);
363 			*errorp = -EMFILE;
364 			goto out_release;
365 		}
366 
367 		/*
368 		 * Reacquire the oldf lock and a pointer to its fd table
369 		 * who knows it may have a new bigger fd table. We need
370 		 * the latest pointer.
371 		 */
372 		spin_lock(&oldf->file_lock);
373 		old_fdt = files_fdtable(oldf);
374 		open_files = sane_fdtable_size(old_fdt, max_fds);
375 	}
376 
377 	copy_fd_bitmaps(new_fdt, old_fdt, open_files);
378 
379 	old_fds = old_fdt->fd;
380 	new_fds = new_fdt->fd;
381 
382 	for (i = open_files; i != 0; i--) {
383 		struct file *f = *old_fds++;
384 		if (f) {
385 			get_file(f);
386 		} else {
387 			/*
388 			 * The fd may be claimed in the fd bitmap but not yet
389 			 * instantiated in the files array if a sibling thread
390 			 * is partway through open().  So make sure that this
391 			 * fd is available to the new process.
392 			 */
393 			__clear_open_fd(open_files - i, new_fdt);
394 		}
395 		rcu_assign_pointer(*new_fds++, f);
396 	}
397 	spin_unlock(&oldf->file_lock);
398 
399 	/* clear the remainder */
400 	memset(new_fds, 0, (new_fdt->max_fds - open_files) * sizeof(struct file *));
401 
402 	rcu_assign_pointer(newf->fdt, new_fdt);
403 
404 	return newf;
405 
406 out_release:
407 	kmem_cache_free(files_cachep, newf);
408 out:
409 	return NULL;
410 }
411 
close_files(struct files_struct * files)412 static struct fdtable *close_files(struct files_struct * files)
413 {
414 	/*
415 	 * It is safe to dereference the fd table without RCU or
416 	 * ->file_lock because this is the last reference to the
417 	 * files structure.
418 	 */
419 	struct fdtable *fdt = rcu_dereference_raw(files->fdt);
420 	unsigned int i, j = 0;
421 
422 	for (;;) {
423 		unsigned long set;
424 		i = j * BITS_PER_LONG;
425 		if (i >= fdt->max_fds)
426 			break;
427 		set = fdt->open_fds[j++];
428 		while (set) {
429 			if (set & 1) {
430 				struct file * file = xchg(&fdt->fd[i], NULL);
431 				if (file) {
432 					filp_close(file, files);
433 					cond_resched();
434 				}
435 			}
436 			i++;
437 			set >>= 1;
438 		}
439 	}
440 
441 	return fdt;
442 }
443 
put_files_struct(struct files_struct * files)444 void put_files_struct(struct files_struct *files)
445 {
446 	if (atomic_dec_and_test(&files->count)) {
447 		struct fdtable *fdt = close_files(files);
448 
449 		/* free the arrays if they are not embedded */
450 		if (fdt != &files->fdtab)
451 			__free_fdtable(fdt);
452 		kmem_cache_free(files_cachep, files);
453 	}
454 }
455 
exit_files(struct task_struct * tsk)456 void exit_files(struct task_struct *tsk)
457 {
458 	struct files_struct * files = tsk->files;
459 
460 	if (files) {
461 		task_lock(tsk);
462 		tsk->files = NULL;
463 		task_unlock(tsk);
464 		put_files_struct(files);
465 	}
466 }
467 
468 struct files_struct init_files = {
469 	.count		= ATOMIC_INIT(1),
470 	.fdt		= &init_files.fdtab,
471 	.fdtab		= {
472 		.max_fds	= NR_OPEN_DEFAULT,
473 		.fd		= &init_files.fd_array[0],
474 		.close_on_exec	= init_files.close_on_exec_init,
475 		.open_fds	= init_files.open_fds_init,
476 		.full_fds_bits	= init_files.full_fds_bits_init,
477 	},
478 	.file_lock	= __SPIN_LOCK_UNLOCKED(init_files.file_lock),
479 	.resize_wait	= __WAIT_QUEUE_HEAD_INITIALIZER(init_files.resize_wait),
480 };
481 
find_next_fd(struct fdtable * fdt,unsigned int start)482 static unsigned int find_next_fd(struct fdtable *fdt, unsigned int start)
483 {
484 	unsigned int maxfd = fdt->max_fds;
485 	unsigned int maxbit = maxfd / BITS_PER_LONG;
486 	unsigned int bitbit = start / BITS_PER_LONG;
487 
488 	bitbit = find_next_zero_bit(fdt->full_fds_bits, maxbit, bitbit) * BITS_PER_LONG;
489 	if (bitbit > maxfd)
490 		return maxfd;
491 	if (bitbit > start)
492 		start = bitbit;
493 	return find_next_zero_bit(fdt->open_fds, maxfd, start);
494 }
495 
496 /*
497  * allocate a file descriptor, mark it busy.
498  */
alloc_fd(unsigned start,unsigned end,unsigned flags)499 static int alloc_fd(unsigned start, unsigned end, unsigned flags)
500 {
501 	struct files_struct *files = current->files;
502 	unsigned int fd;
503 	int error;
504 	struct fdtable *fdt;
505 
506 	spin_lock(&files->file_lock);
507 repeat:
508 	fdt = files_fdtable(files);
509 	fd = start;
510 	if (fd < files->next_fd)
511 		fd = files->next_fd;
512 
513 	if (fd < fdt->max_fds)
514 		fd = find_next_fd(fdt, fd);
515 
516 	/*
517 	 * N.B. For clone tasks sharing a files structure, this test
518 	 * will limit the total number of files that can be opened.
519 	 */
520 	error = -EMFILE;
521 	if (fd >= end)
522 		goto out;
523 
524 	error = expand_files(files, fd);
525 	if (error < 0)
526 		goto out;
527 
528 	/*
529 	 * If we needed to expand the fs array we
530 	 * might have blocked - try again.
531 	 */
532 	if (error)
533 		goto repeat;
534 
535 	if (start <= files->next_fd)
536 		files->next_fd = fd + 1;
537 
538 	__set_open_fd(fd, fdt);
539 	if (flags & O_CLOEXEC)
540 		__set_close_on_exec(fd, fdt);
541 	else
542 		__clear_close_on_exec(fd, fdt);
543 	error = fd;
544 #if 1
545 	/* Sanity check */
546 	if (rcu_access_pointer(fdt->fd[fd]) != NULL) {
547 		printk(KERN_WARNING "alloc_fd: slot %d not NULL!\n", fd);
548 		rcu_assign_pointer(fdt->fd[fd], NULL);
549 	}
550 #endif
551 
552 out:
553 	spin_unlock(&files->file_lock);
554 	return error;
555 }
556 
__get_unused_fd_flags(unsigned flags,unsigned long nofile)557 int __get_unused_fd_flags(unsigned flags, unsigned long nofile)
558 {
559 	return alloc_fd(0, nofile, flags);
560 }
561 
get_unused_fd_flags(unsigned flags)562 int get_unused_fd_flags(unsigned flags)
563 {
564 	return __get_unused_fd_flags(flags, rlimit(RLIMIT_NOFILE));
565 }
566 EXPORT_SYMBOL(get_unused_fd_flags);
567 
__put_unused_fd(struct files_struct * files,unsigned int fd)568 static void __put_unused_fd(struct files_struct *files, unsigned int fd)
569 {
570 	struct fdtable *fdt = files_fdtable(files);
571 	__clear_open_fd(fd, fdt);
572 	if (fd < files->next_fd)
573 		files->next_fd = fd;
574 }
575 
put_unused_fd(unsigned int fd)576 void put_unused_fd(unsigned int fd)
577 {
578 	struct files_struct *files = current->files;
579 	spin_lock(&files->file_lock);
580 	__put_unused_fd(files, fd);
581 	spin_unlock(&files->file_lock);
582 }
583 
584 EXPORT_SYMBOL(put_unused_fd);
585 
586 /*
587  * Install a file pointer in the fd array.
588  *
589  * The VFS is full of places where we drop the files lock between
590  * setting the open_fds bitmap and installing the file in the file
591  * array.  At any such point, we are vulnerable to a dup2() race
592  * installing a file in the array before us.  We need to detect this and
593  * fput() the struct file we are about to overwrite in this case.
594  *
595  * It should never happen - if we allow dup2() do it, _really_ bad things
596  * will follow.
597  *
598  * This consumes the "file" refcount, so callers should treat it
599  * as if they had called fput(file).
600  */
601 
fd_install(unsigned int fd,struct file * file)602 void fd_install(unsigned int fd, struct file *file)
603 {
604 	struct files_struct *files = current->files;
605 	struct fdtable *fdt;
606 
607 	rcu_read_lock_sched();
608 
609 	if (unlikely(files->resize_in_progress)) {
610 		rcu_read_unlock_sched();
611 		spin_lock(&files->file_lock);
612 		fdt = files_fdtable(files);
613 		BUG_ON(fdt->fd[fd] != NULL);
614 		rcu_assign_pointer(fdt->fd[fd], file);
615 		spin_unlock(&files->file_lock);
616 		return;
617 	}
618 	/* coupled with smp_wmb() in expand_fdtable() */
619 	smp_rmb();
620 	fdt = rcu_dereference_sched(files->fdt);
621 	BUG_ON(fdt->fd[fd] != NULL);
622 	rcu_assign_pointer(fdt->fd[fd], file);
623 	rcu_read_unlock_sched();
624 }
625 
626 EXPORT_SYMBOL(fd_install);
627 
628 /**
629  * pick_file - return file associatd with fd
630  * @files: file struct to retrieve file from
631  * @fd: file descriptor to retrieve file for
632  *
633  * Context: files_lock must be held.
634  *
635  * Returns: The file associated with @fd (NULL if @fd is not open)
636  */
pick_file(struct files_struct * files,unsigned fd)637 static struct file *pick_file(struct files_struct *files, unsigned fd)
638 {
639 	struct fdtable *fdt = files_fdtable(files);
640 	struct file *file;
641 
642 	if (fd >= fdt->max_fds)
643 		return NULL;
644 
645 	fd = array_index_nospec(fd, fdt->max_fds);
646 	file = fdt->fd[fd];
647 	if (file) {
648 		rcu_assign_pointer(fdt->fd[fd], NULL);
649 		__put_unused_fd(files, fd);
650 	}
651 	return file;
652 }
653 
close_fd(unsigned fd)654 int close_fd(unsigned fd)
655 {
656 	struct files_struct *files = current->files;
657 	struct file *file;
658 
659 	spin_lock(&files->file_lock);
660 	file = pick_file(files, fd);
661 	spin_unlock(&files->file_lock);
662 	if (!file)
663 		return -EBADF;
664 
665 	return filp_close(file, files);
666 }
667 EXPORT_SYMBOL(close_fd); /* for ksys_close() */
668 
669 /**
670  * last_fd - return last valid index into fd table
671  * @fdt: File descriptor table.
672  *
673  * Context: Either rcu read lock or files_lock must be held.
674  *
675  * Returns: Last valid index into fdtable.
676  */
last_fd(struct fdtable * fdt)677 static inline unsigned last_fd(struct fdtable *fdt)
678 {
679 	return fdt->max_fds - 1;
680 }
681 
__range_cloexec(struct files_struct * cur_fds,unsigned int fd,unsigned int max_fd)682 static inline void __range_cloexec(struct files_struct *cur_fds,
683 				   unsigned int fd, unsigned int max_fd)
684 {
685 	struct fdtable *fdt;
686 
687 	/* make sure we're using the correct maximum value */
688 	spin_lock(&cur_fds->file_lock);
689 	fdt = files_fdtable(cur_fds);
690 	max_fd = min(last_fd(fdt), max_fd);
691 	if (fd <= max_fd)
692 		bitmap_set(fdt->close_on_exec, fd, max_fd - fd + 1);
693 	spin_unlock(&cur_fds->file_lock);
694 }
695 
__range_close(struct files_struct * files,unsigned int fd,unsigned int max_fd)696 static inline void __range_close(struct files_struct *files, unsigned int fd,
697 				 unsigned int max_fd)
698 {
699 	struct file *file;
700 	unsigned n;
701 
702 	spin_lock(&files->file_lock);
703 	n = last_fd(files_fdtable(files));
704 	max_fd = min(max_fd, n);
705 
706 	for (; fd <= max_fd; fd++) {
707 		file = pick_file(files, fd);
708 		if (file) {
709 			spin_unlock(&files->file_lock);
710 			filp_close(file, files);
711 			cond_resched();
712 			spin_lock(&files->file_lock);
713 		} else if (need_resched()) {
714 			spin_unlock(&files->file_lock);
715 			cond_resched();
716 			spin_lock(&files->file_lock);
717 		}
718 	}
719 	spin_unlock(&files->file_lock);
720 }
721 
722 /**
723  * __close_range() - Close all file descriptors in a given range.
724  *
725  * @fd:     starting file descriptor to close
726  * @max_fd: last file descriptor to close
727  * @flags:  CLOSE_RANGE flags.
728  *
729  * This closes a range of file descriptors. All file descriptors
730  * from @fd up to and including @max_fd are closed.
731  */
__close_range(unsigned fd,unsigned max_fd,unsigned int flags)732 int __close_range(unsigned fd, unsigned max_fd, unsigned int flags)
733 {
734 	struct task_struct *me = current;
735 	struct files_struct *cur_fds = me->files, *fds = NULL;
736 
737 	if (flags & ~(CLOSE_RANGE_UNSHARE | CLOSE_RANGE_CLOEXEC))
738 		return -EINVAL;
739 
740 	if (fd > max_fd)
741 		return -EINVAL;
742 
743 	if (flags & CLOSE_RANGE_UNSHARE) {
744 		int ret;
745 		unsigned int max_unshare_fds = NR_OPEN_MAX;
746 
747 		/*
748 		 * If the caller requested all fds to be made cloexec we always
749 		 * copy all of the file descriptors since they still want to
750 		 * use them.
751 		 */
752 		if (!(flags & CLOSE_RANGE_CLOEXEC)) {
753 			/*
754 			 * If the requested range is greater than the current
755 			 * maximum, we're closing everything so only copy all
756 			 * file descriptors beneath the lowest file descriptor.
757 			 */
758 			rcu_read_lock();
759 			if (max_fd >= last_fd(files_fdtable(cur_fds)))
760 				max_unshare_fds = fd;
761 			rcu_read_unlock();
762 		}
763 
764 		ret = unshare_fd(CLONE_FILES, max_unshare_fds, &fds);
765 		if (ret)
766 			return ret;
767 
768 		/*
769 		 * We used to share our file descriptor table, and have now
770 		 * created a private one, make sure we're using it below.
771 		 */
772 		if (fds)
773 			swap(cur_fds, fds);
774 	}
775 
776 	if (flags & CLOSE_RANGE_CLOEXEC)
777 		__range_cloexec(cur_fds, fd, max_fd);
778 	else
779 		__range_close(cur_fds, fd, max_fd);
780 
781 	if (fds) {
782 		/*
783 		 * We're done closing the files we were supposed to. Time to install
784 		 * the new file descriptor table and drop the old one.
785 		 */
786 		task_lock(me);
787 		me->files = cur_fds;
788 		task_unlock(me);
789 		put_files_struct(fds);
790 	}
791 
792 	return 0;
793 }
794 
795 /*
796  * See close_fd_get_file() below, this variant assumes current->files->file_lock
797  * is held.
798  */
__close_fd_get_file(unsigned int fd)799 struct file *__close_fd_get_file(unsigned int fd)
800 {
801 	return pick_file(current->files, fd);
802 }
803 
804 /*
805  * variant of close_fd that gets a ref on the file for later fput.
806  * The caller must ensure that filp_close() called on the file.
807  */
close_fd_get_file(unsigned int fd)808 struct file *close_fd_get_file(unsigned int fd)
809 {
810 	struct files_struct *files = current->files;
811 	struct file *file;
812 
813 	spin_lock(&files->file_lock);
814 	file = pick_file(files, fd);
815 	spin_unlock(&files->file_lock);
816 
817 	return file;
818 }
819 
do_close_on_exec(struct files_struct * files)820 void do_close_on_exec(struct files_struct *files)
821 {
822 	unsigned i;
823 	struct fdtable *fdt;
824 
825 	/* exec unshares first */
826 	spin_lock(&files->file_lock);
827 	for (i = 0; ; i++) {
828 		unsigned long set;
829 		unsigned fd = i * BITS_PER_LONG;
830 		fdt = files_fdtable(files);
831 		if (fd >= fdt->max_fds)
832 			break;
833 		set = fdt->close_on_exec[i];
834 		if (!set)
835 			continue;
836 		fdt->close_on_exec[i] = 0;
837 		for ( ; set ; fd++, set >>= 1) {
838 			struct file *file;
839 			if (!(set & 1))
840 				continue;
841 			file = fdt->fd[fd];
842 			if (!file)
843 				continue;
844 			rcu_assign_pointer(fdt->fd[fd], NULL);
845 			__put_unused_fd(files, fd);
846 			spin_unlock(&files->file_lock);
847 			filp_close(file, files);
848 			cond_resched();
849 			spin_lock(&files->file_lock);
850 		}
851 
852 	}
853 	spin_unlock(&files->file_lock);
854 }
855 
__fget_files_rcu(struct files_struct * files,unsigned int fd,fmode_t mask)856 static inline struct file *__fget_files_rcu(struct files_struct *files,
857 	unsigned int fd, fmode_t mask)
858 {
859 	for (;;) {
860 		struct file *file;
861 		struct fdtable *fdt = rcu_dereference_raw(files->fdt);
862 		struct file __rcu **fdentry;
863 
864 		if (unlikely(fd >= fdt->max_fds))
865 			return NULL;
866 
867 		fdentry = fdt->fd + array_index_nospec(fd, fdt->max_fds);
868 		file = rcu_dereference_raw(*fdentry);
869 		if (unlikely(!file))
870 			return NULL;
871 
872 		if (unlikely(file->f_mode & mask))
873 			return NULL;
874 
875 		/*
876 		 * Ok, we have a file pointer. However, because we do
877 		 * this all locklessly under RCU, we may be racing with
878 		 * that file being closed.
879 		 *
880 		 * Such a race can take two forms:
881 		 *
882 		 *  (a) the file ref already went down to zero,
883 		 *      and get_file_rcu() fails. Just try again:
884 		 */
885 		if (unlikely(!get_file_rcu(file)))
886 			continue;
887 
888 		/*
889 		 *  (b) the file table entry has changed under us.
890 		 *       Note that we don't need to re-check the 'fdt->fd'
891 		 *       pointer having changed, because it always goes
892 		 *       hand-in-hand with 'fdt'.
893 		 *
894 		 * If so, we need to put our ref and try again.
895 		 */
896 		if (unlikely(rcu_dereference_raw(files->fdt) != fdt) ||
897 		    unlikely(rcu_dereference_raw(*fdentry) != file)) {
898 			fput(file);
899 			continue;
900 		}
901 
902 		/*
903 		 * Ok, we have a ref to the file, and checked that it
904 		 * still exists.
905 		 */
906 		return file;
907 	}
908 }
909 
__fget_files(struct files_struct * files,unsigned int fd,fmode_t mask)910 static struct file *__fget_files(struct files_struct *files, unsigned int fd,
911 				 fmode_t mask)
912 {
913 	struct file *file;
914 
915 	rcu_read_lock();
916 	file = __fget_files_rcu(files, fd, mask);
917 	rcu_read_unlock();
918 
919 	return file;
920 }
921 
__fget(unsigned int fd,fmode_t mask)922 static inline struct file *__fget(unsigned int fd, fmode_t mask)
923 {
924 	return __fget_files(current->files, fd, mask);
925 }
926 
fget(unsigned int fd)927 struct file *fget(unsigned int fd)
928 {
929 	return __fget(fd, FMODE_PATH);
930 }
931 EXPORT_SYMBOL(fget);
932 
fget_raw(unsigned int fd)933 struct file *fget_raw(unsigned int fd)
934 {
935 	return __fget(fd, 0);
936 }
937 EXPORT_SYMBOL(fget_raw);
938 
fget_task(struct task_struct * task,unsigned int fd)939 struct file *fget_task(struct task_struct *task, unsigned int fd)
940 {
941 	struct file *file = NULL;
942 
943 	task_lock(task);
944 	if (task->files)
945 		file = __fget_files(task->files, fd, 0);
946 	task_unlock(task);
947 
948 	return file;
949 }
950 
task_lookup_fd_rcu(struct task_struct * task,unsigned int fd)951 struct file *task_lookup_fd_rcu(struct task_struct *task, unsigned int fd)
952 {
953 	/* Must be called with rcu_read_lock held */
954 	struct files_struct *files;
955 	struct file *file = NULL;
956 
957 	task_lock(task);
958 	files = task->files;
959 	if (files)
960 		file = files_lookup_fd_rcu(files, fd);
961 	task_unlock(task);
962 
963 	return file;
964 }
965 
task_lookup_next_fd_rcu(struct task_struct * task,unsigned int * ret_fd)966 struct file *task_lookup_next_fd_rcu(struct task_struct *task, unsigned int *ret_fd)
967 {
968 	/* Must be called with rcu_read_lock held */
969 	struct files_struct *files;
970 	unsigned int fd = *ret_fd;
971 	struct file *file = NULL;
972 
973 	task_lock(task);
974 	files = task->files;
975 	if (files) {
976 		for (; fd < files_fdtable(files)->max_fds; fd++) {
977 			file = files_lookup_fd_rcu(files, fd);
978 			if (file)
979 				break;
980 		}
981 	}
982 	task_unlock(task);
983 	*ret_fd = fd;
984 	return file;
985 }
986 EXPORT_SYMBOL(task_lookup_next_fd_rcu);
987 
988 /*
989  * Lightweight file lookup - no refcnt increment if fd table isn't shared.
990  *
991  * You can use this instead of fget if you satisfy all of the following
992  * conditions:
993  * 1) You must call fput_light before exiting the syscall and returning control
994  *    to userspace (i.e. you cannot remember the returned struct file * after
995  *    returning to userspace).
996  * 2) You must not call filp_close on the returned struct file * in between
997  *    calls to fget_light and fput_light.
998  * 3) You must not clone the current task in between the calls to fget_light
999  *    and fput_light.
1000  *
1001  * The fput_needed flag returned by fget_light should be passed to the
1002  * corresponding fput_light.
1003  */
__fget_light(unsigned int fd,fmode_t mask)1004 static unsigned long __fget_light(unsigned int fd, fmode_t mask)
1005 {
1006 	struct files_struct *files = current->files;
1007 	struct file *file;
1008 
1009 	/*
1010 	 * If another thread is concurrently calling close_fd() followed
1011 	 * by put_files_struct(), we must not observe the old table
1012 	 * entry combined with the new refcount - otherwise we could
1013 	 * return a file that is concurrently being freed.
1014 	 *
1015 	 * atomic_read_acquire() pairs with atomic_dec_and_test() in
1016 	 * put_files_struct().
1017 	 */
1018 	if (atomic_read_acquire(&files->count) == 1) {
1019 		file = files_lookup_fd_raw(files, fd);
1020 		if (!file || unlikely(file->f_mode & mask))
1021 			return 0;
1022 		return (unsigned long)file;
1023 	} else {
1024 		file = __fget(fd, mask);
1025 		if (!file)
1026 			return 0;
1027 		return FDPUT_FPUT | (unsigned long)file;
1028 	}
1029 }
__fdget(unsigned int fd)1030 unsigned long __fdget(unsigned int fd)
1031 {
1032 	return __fget_light(fd, FMODE_PATH);
1033 }
1034 EXPORT_SYMBOL(__fdget);
1035 
__fdget_raw(unsigned int fd)1036 unsigned long __fdget_raw(unsigned int fd)
1037 {
1038 	return __fget_light(fd, 0);
1039 }
1040 
1041 /*
1042  * Try to avoid f_pos locking. We only need it if the
1043  * file is marked for FMODE_ATOMIC_POS, and it can be
1044  * accessed multiple ways.
1045  *
1046  * Always do it for directories, because pidfd_getfd()
1047  * can make a file accessible even if it otherwise would
1048  * not be, and for directories this is a correctness
1049  * issue, not a "POSIX requirement".
1050  */
file_needs_f_pos_lock(struct file * file)1051 static inline bool file_needs_f_pos_lock(struct file *file)
1052 {
1053 	return (file->f_mode & FMODE_ATOMIC_POS) &&
1054 		(file_count(file) > 1 || file->f_op->iterate_shared);
1055 }
1056 
__fdget_pos(unsigned int fd)1057 unsigned long __fdget_pos(unsigned int fd)
1058 {
1059 	unsigned long v = __fdget(fd);
1060 	struct file *file = (struct file *)(v & ~3);
1061 
1062 	if (file && file_needs_f_pos_lock(file)) {
1063 		v |= FDPUT_POS_UNLOCK;
1064 		mutex_lock(&file->f_pos_lock);
1065 	}
1066 	return v;
1067 }
1068 
__f_unlock_pos(struct file * f)1069 void __f_unlock_pos(struct file *f)
1070 {
1071 	mutex_unlock(&f->f_pos_lock);
1072 }
1073 
1074 /*
1075  * We only lock f_pos if we have threads or if the file might be
1076  * shared with another process. In both cases we'll have an elevated
1077  * file count (done either by fdget() or by fork()).
1078  */
1079 
set_close_on_exec(unsigned int fd,int flag)1080 void set_close_on_exec(unsigned int fd, int flag)
1081 {
1082 	struct files_struct *files = current->files;
1083 	struct fdtable *fdt;
1084 	spin_lock(&files->file_lock);
1085 	fdt = files_fdtable(files);
1086 	if (flag)
1087 		__set_close_on_exec(fd, fdt);
1088 	else
1089 		__clear_close_on_exec(fd, fdt);
1090 	spin_unlock(&files->file_lock);
1091 }
1092 
get_close_on_exec(unsigned int fd)1093 bool get_close_on_exec(unsigned int fd)
1094 {
1095 	struct files_struct *files = current->files;
1096 	struct fdtable *fdt;
1097 	bool res;
1098 	rcu_read_lock();
1099 	fdt = files_fdtable(files);
1100 	res = close_on_exec(fd, fdt);
1101 	rcu_read_unlock();
1102 	return res;
1103 }
1104 
do_dup2(struct files_struct * files,struct file * file,unsigned fd,unsigned flags)1105 static int do_dup2(struct files_struct *files,
1106 	struct file *file, unsigned fd, unsigned flags)
1107 __releases(&files->file_lock)
1108 {
1109 	struct file *tofree;
1110 	struct fdtable *fdt;
1111 
1112 	/*
1113 	 * We need to detect attempts to do dup2() over allocated but still
1114 	 * not finished descriptor.  NB: OpenBSD avoids that at the price of
1115 	 * extra work in their equivalent of fget() - they insert struct
1116 	 * file immediately after grabbing descriptor, mark it larval if
1117 	 * more work (e.g. actual opening) is needed and make sure that
1118 	 * fget() treats larval files as absent.  Potentially interesting,
1119 	 * but while extra work in fget() is trivial, locking implications
1120 	 * and amount of surgery on open()-related paths in VFS are not.
1121 	 * FreeBSD fails with -EBADF in the same situation, NetBSD "solution"
1122 	 * deadlocks in rather amusing ways, AFAICS.  All of that is out of
1123 	 * scope of POSIX or SUS, since neither considers shared descriptor
1124 	 * tables and this condition does not arise without those.
1125 	 */
1126 	fdt = files_fdtable(files);
1127 	tofree = fdt->fd[fd];
1128 	if (!tofree && fd_is_open(fd, fdt))
1129 		goto Ebusy;
1130 	get_file(file);
1131 	rcu_assign_pointer(fdt->fd[fd], file);
1132 	__set_open_fd(fd, fdt);
1133 	if (flags & O_CLOEXEC)
1134 		__set_close_on_exec(fd, fdt);
1135 	else
1136 		__clear_close_on_exec(fd, fdt);
1137 	spin_unlock(&files->file_lock);
1138 
1139 	if (tofree)
1140 		filp_close(tofree, files);
1141 
1142 	return fd;
1143 
1144 Ebusy:
1145 	spin_unlock(&files->file_lock);
1146 	return -EBUSY;
1147 }
1148 
replace_fd(unsigned fd,struct file * file,unsigned flags)1149 int replace_fd(unsigned fd, struct file *file, unsigned flags)
1150 {
1151 	int err;
1152 	struct files_struct *files = current->files;
1153 
1154 	if (!file)
1155 		return close_fd(fd);
1156 
1157 	if (fd >= rlimit(RLIMIT_NOFILE))
1158 		return -EBADF;
1159 
1160 	spin_lock(&files->file_lock);
1161 	err = expand_files(files, fd);
1162 	if (unlikely(err < 0))
1163 		goto out_unlock;
1164 	return do_dup2(files, file, fd, flags);
1165 
1166 out_unlock:
1167 	spin_unlock(&files->file_lock);
1168 	return err;
1169 }
1170 
1171 /**
1172  * __receive_fd() - Install received file into file descriptor table
1173  * @file: struct file that was received from another process
1174  * @ufd: __user pointer to write new fd number to
1175  * @o_flags: the O_* flags to apply to the new fd entry
1176  *
1177  * Installs a received file into the file descriptor table, with appropriate
1178  * checks and count updates. Optionally writes the fd number to userspace, if
1179  * @ufd is non-NULL.
1180  *
1181  * This helper handles its own reference counting of the incoming
1182  * struct file.
1183  *
1184  * Returns newly install fd or -ve on error.
1185  */
__receive_fd(struct file * file,int __user * ufd,unsigned int o_flags)1186 int __receive_fd(struct file *file, int __user *ufd, unsigned int o_flags)
1187 {
1188 	int new_fd;
1189 	int error;
1190 
1191 	error = security_file_receive(file);
1192 	if (error)
1193 		return error;
1194 
1195 	new_fd = get_unused_fd_flags(o_flags);
1196 	if (new_fd < 0)
1197 		return new_fd;
1198 
1199 	if (ufd) {
1200 		error = put_user(new_fd, ufd);
1201 		if (error) {
1202 			put_unused_fd(new_fd);
1203 			return error;
1204 		}
1205 	}
1206 
1207 	fd_install(new_fd, get_file(file));
1208 	__receive_sock(file);
1209 	return new_fd;
1210 }
1211 
receive_fd_replace(int new_fd,struct file * file,unsigned int o_flags)1212 int receive_fd_replace(int new_fd, struct file *file, unsigned int o_flags)
1213 {
1214 	int error;
1215 
1216 	error = security_file_receive(file);
1217 	if (error)
1218 		return error;
1219 	error = replace_fd(new_fd, file, o_flags);
1220 	if (error)
1221 		return error;
1222 	__receive_sock(file);
1223 	return new_fd;
1224 }
1225 
receive_fd(struct file * file,unsigned int o_flags)1226 int receive_fd(struct file *file, unsigned int o_flags)
1227 {
1228 	return __receive_fd(file, NULL, o_flags);
1229 }
1230 EXPORT_SYMBOL_GPL(receive_fd);
1231 
ksys_dup3(unsigned int oldfd,unsigned int newfd,int flags)1232 static int ksys_dup3(unsigned int oldfd, unsigned int newfd, int flags)
1233 {
1234 	int err = -EBADF;
1235 	struct file *file;
1236 	struct files_struct *files = current->files;
1237 
1238 	if ((flags & ~O_CLOEXEC) != 0)
1239 		return -EINVAL;
1240 
1241 	if (unlikely(oldfd == newfd))
1242 		return -EINVAL;
1243 
1244 	if (newfd >= rlimit(RLIMIT_NOFILE))
1245 		return -EBADF;
1246 
1247 	spin_lock(&files->file_lock);
1248 	err = expand_files(files, newfd);
1249 	file = files_lookup_fd_locked(files, oldfd);
1250 	if (unlikely(!file))
1251 		goto Ebadf;
1252 	if (unlikely(err < 0)) {
1253 		if (err == -EMFILE)
1254 			goto Ebadf;
1255 		goto out_unlock;
1256 	}
1257 	return do_dup2(files, file, newfd, flags);
1258 
1259 Ebadf:
1260 	err = -EBADF;
1261 out_unlock:
1262 	spin_unlock(&files->file_lock);
1263 	return err;
1264 }
1265 
SYSCALL_DEFINE3(dup3,unsigned int,oldfd,unsigned int,newfd,int,flags)1266 SYSCALL_DEFINE3(dup3, unsigned int, oldfd, unsigned int, newfd, int, flags)
1267 {
1268 	return ksys_dup3(oldfd, newfd, flags);
1269 }
1270 
SYSCALL_DEFINE2(dup2,unsigned int,oldfd,unsigned int,newfd)1271 SYSCALL_DEFINE2(dup2, unsigned int, oldfd, unsigned int, newfd)
1272 {
1273 	if (unlikely(newfd == oldfd)) { /* corner case */
1274 		struct files_struct *files = current->files;
1275 		int retval = oldfd;
1276 
1277 		rcu_read_lock();
1278 		if (!files_lookup_fd_rcu(files, oldfd))
1279 			retval = -EBADF;
1280 		rcu_read_unlock();
1281 		return retval;
1282 	}
1283 	return ksys_dup3(oldfd, newfd, 0);
1284 }
1285 
SYSCALL_DEFINE1(dup,unsigned int,fildes)1286 SYSCALL_DEFINE1(dup, unsigned int, fildes)
1287 {
1288 	int ret = -EBADF;
1289 	struct file *file = fget_raw(fildes);
1290 
1291 	if (file) {
1292 		ret = get_unused_fd_flags(0);
1293 		if (ret >= 0)
1294 			fd_install(ret, file);
1295 		else
1296 			fput(file);
1297 	}
1298 	return ret;
1299 }
1300 
f_dupfd(unsigned int from,struct file * file,unsigned flags)1301 int f_dupfd(unsigned int from, struct file *file, unsigned flags)
1302 {
1303 	unsigned long nofile = rlimit(RLIMIT_NOFILE);
1304 	int err;
1305 	if (from >= nofile)
1306 		return -EINVAL;
1307 	err = alloc_fd(from, nofile, flags);
1308 	if (err >= 0) {
1309 		get_file(file);
1310 		fd_install(err, file);
1311 	}
1312 	return err;
1313 }
1314 
iterate_fd(struct files_struct * files,unsigned n,int (* f)(const void *,struct file *,unsigned),const void * p)1315 int iterate_fd(struct files_struct *files, unsigned n,
1316 		int (*f)(const void *, struct file *, unsigned),
1317 		const void *p)
1318 {
1319 	struct fdtable *fdt;
1320 	int res = 0;
1321 	if (!files)
1322 		return 0;
1323 	spin_lock(&files->file_lock);
1324 	for (fdt = files_fdtable(files); n < fdt->max_fds; n++) {
1325 		struct file *file;
1326 		file = rcu_dereference_check_fdtable(files, fdt->fd[n]);
1327 		if (!file)
1328 			continue;
1329 		res = f(p, file, n);
1330 		if (res)
1331 			break;
1332 	}
1333 	spin_unlock(&files->file_lock);
1334 	return res;
1335 }
1336 EXPORT_SYMBOL(iterate_fd);
1337