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TOMOYO Linux Cross Reference
Linux/fs/file.c

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  1 /*
  2  *  linux/fs/file.c
  3  *
  4  *  Copyright (C) 1998-1999, Stephen Tweedie and Bill Hawes
  5  *
  6  *  Manage the dynamic fd arrays in the process files_struct.
  7  */
  8 
  9 #include <linux/syscalls.h>
 10 #include <linux/export.h>
 11 #include <linux/fs.h>
 12 #include <linux/mm.h>
 13 #include <linux/mmzone.h>
 14 #include <linux/time.h>
 15 #include <linux/sched/signal.h>
 16 #include <linux/slab.h>
 17 #include <linux/vmalloc.h>
 18 #include <linux/file.h>
 19 #include <linux/fdtable.h>
 20 #include <linux/bitops.h>
 21 #include <linux/interrupt.h>
 22 #include <linux/spinlock.h>
 23 #include <linux/rcupdate.h>
 24 #include <linux/workqueue.h>
 25 
 26 unsigned int sysctl_nr_open __read_mostly = 1024*1024;
 27 unsigned int sysctl_nr_open_min = BITS_PER_LONG;
 28 /* our min() is unusable in constant expressions ;-/ */
 29 #define __const_min(x, y) ((x) < (y) ? (x) : (y))
 30 unsigned int sysctl_nr_open_max =
 31         __const_min(INT_MAX, ~(size_t)0/sizeof(void *)) & -BITS_PER_LONG;
 32 
 33 static void __free_fdtable(struct fdtable *fdt)
 34 {
 35         kvfree(fdt->fd);
 36         kvfree(fdt->open_fds);
 37         kfree(fdt);
 38 }
 39 
 40 static void free_fdtable_rcu(struct rcu_head *rcu)
 41 {
 42         __free_fdtable(container_of(rcu, struct fdtable, rcu));
 43 }
 44 
 45 #define BITBIT_NR(nr)   BITS_TO_LONGS(BITS_TO_LONGS(nr))
 46 #define BITBIT_SIZE(nr) (BITBIT_NR(nr) * sizeof(long))
 47 
 48 /*
 49  * Copy 'count' fd bits from the old table to the new table and clear the extra
 50  * space if any.  This does not copy the file pointers.  Called with the files
 51  * spinlock held for write.
 52  */
 53 static void copy_fd_bitmaps(struct fdtable *nfdt, struct fdtable *ofdt,
 54                             unsigned int count)
 55 {
 56         unsigned int cpy, set;
 57 
 58         cpy = count / BITS_PER_BYTE;
 59         set = (nfdt->max_fds - count) / BITS_PER_BYTE;
 60         memcpy(nfdt->open_fds, ofdt->open_fds, cpy);
 61         memset((char *)nfdt->open_fds + cpy, 0, set);
 62         memcpy(nfdt->close_on_exec, ofdt->close_on_exec, cpy);
 63         memset((char *)nfdt->close_on_exec + cpy, 0, set);
 64 
 65         cpy = BITBIT_SIZE(count);
 66         set = BITBIT_SIZE(nfdt->max_fds) - cpy;
 67         memcpy(nfdt->full_fds_bits, ofdt->full_fds_bits, cpy);
 68         memset((char *)nfdt->full_fds_bits + cpy, 0, set);
 69 }
 70 
 71 /*
 72  * Copy all file descriptors from the old table to the new, expanded table and
 73  * clear the extra space.  Called with the files spinlock held for write.
 74  */
 75 static void copy_fdtable(struct fdtable *nfdt, struct fdtable *ofdt)
 76 {
 77         unsigned int cpy, set;
 78 
 79         BUG_ON(nfdt->max_fds < ofdt->max_fds);
 80 
 81         cpy = ofdt->max_fds * sizeof(struct file *);
 82         set = (nfdt->max_fds - ofdt->max_fds) * sizeof(struct file *);
 83         memcpy(nfdt->fd, ofdt->fd, cpy);
 84         memset((char *)nfdt->fd + cpy, 0, set);
 85 
 86         copy_fd_bitmaps(nfdt, ofdt, ofdt->max_fds);
 87 }
 88 
 89 static struct fdtable * alloc_fdtable(unsigned int nr)
 90 {
 91         struct fdtable *fdt;
 92         void *data;
 93 
 94         /*
 95          * Figure out how many fds we actually want to support in this fdtable.
 96          * Allocation steps are keyed to the size of the fdarray, since it
 97          * grows far faster than any of the other dynamic data. We try to fit
 98          * the fdarray into comfortable page-tuned chunks: starting at 1024B
 99          * and growing in powers of two from there on.
100          */
101         nr /= (1024 / sizeof(struct file *));
102         nr = roundup_pow_of_two(nr + 1);
103         nr *= (1024 / sizeof(struct file *));
104         /*
105          * Note that this can drive nr *below* what we had passed if sysctl_nr_open
106          * had been set lower between the check in expand_files() and here.  Deal
107          * with that in caller, it's cheaper that way.
108          *
109          * We make sure that nr remains a multiple of BITS_PER_LONG - otherwise
110          * bitmaps handling below becomes unpleasant, to put it mildly...
111          */
112         if (unlikely(nr > sysctl_nr_open))
113                 nr = ((sysctl_nr_open - 1) | (BITS_PER_LONG - 1)) + 1;
114 
115         fdt = kmalloc(sizeof(struct fdtable), GFP_KERNEL_ACCOUNT);
116         if (!fdt)
117                 goto out;
118         fdt->max_fds = nr;
119         data = kvmalloc_array(nr, sizeof(struct file *), GFP_KERNEL_ACCOUNT);
120         if (!data)
121                 goto out_fdt;
122         fdt->fd = data;
123 
124         data = kvmalloc(max_t(size_t,
125                                  2 * nr / BITS_PER_BYTE + BITBIT_SIZE(nr), L1_CACHE_BYTES),
126                                  GFP_KERNEL_ACCOUNT);
127         if (!data)
128                 goto out_arr;
129         fdt->open_fds = data;
130         data += nr / BITS_PER_BYTE;
131         fdt->close_on_exec = data;
132         data += nr / BITS_PER_BYTE;
133         fdt->full_fds_bits = data;
134 
135         return fdt;
136 
137 out_arr:
138         kvfree(fdt->fd);
139 out_fdt:
140         kfree(fdt);
141 out:
142         return NULL;
143 }
144 
145 /*
146  * Expand the file descriptor table.
147  * This function will allocate a new fdtable and both fd array and fdset, of
148  * the given size.
149  * Return <0 error code on error; 1 on successful completion.
150  * The files->file_lock should be held on entry, and will be held on exit.
151  */
152 static int expand_fdtable(struct files_struct *files, unsigned int nr)
153         __releases(files->file_lock)
154         __acquires(files->file_lock)
155 {
156         struct fdtable *new_fdt, *cur_fdt;
157 
158         spin_unlock(&files->file_lock);
159         new_fdt = alloc_fdtable(nr);
160 
161         /* make sure all __fd_install() have seen resize_in_progress
162          * or have finished their rcu_read_lock_sched() section.
163          */
164         if (atomic_read(&files->count) > 1)
165                 synchronize_sched();
166 
167         spin_lock(&files->file_lock);
168         if (!new_fdt)
169                 return -ENOMEM;
170         /*
171          * extremely unlikely race - sysctl_nr_open decreased between the check in
172          * caller and alloc_fdtable().  Cheaper to catch it here...
173          */
174         if (unlikely(new_fdt->max_fds <= nr)) {
175                 __free_fdtable(new_fdt);
176                 return -EMFILE;
177         }
178         cur_fdt = files_fdtable(files);
179         BUG_ON(nr < cur_fdt->max_fds);
180         copy_fdtable(new_fdt, cur_fdt);
181         rcu_assign_pointer(files->fdt, new_fdt);
182         if (cur_fdt != &files->fdtab)
183                 call_rcu(&cur_fdt->rcu, free_fdtable_rcu);
184         /* coupled with smp_rmb() in __fd_install() */
185         smp_wmb();
186         return 1;
187 }
188 
189 /*
190  * Expand files.
191  * This function will expand the file structures, if the requested size exceeds
192  * the current capacity and there is room for expansion.
193  * Return <0 error code on error; 0 when nothing done; 1 when files were
194  * expanded and execution may have blocked.
195  * The files->file_lock should be held on entry, and will be held on exit.
196  */
197 static int expand_files(struct files_struct *files, unsigned int nr)
198         __releases(files->file_lock)
199         __acquires(files->file_lock)
200 {
201         struct fdtable *fdt;
202         int expanded = 0;
203 
204 repeat:
205         fdt = files_fdtable(files);
206 
207         /* Do we need to expand? */
208         if (nr < fdt->max_fds)
209                 return expanded;
210 
211         /* Can we expand? */
212         if (nr >= sysctl_nr_open)
213                 return -EMFILE;
214 
215         if (unlikely(files->resize_in_progress)) {
216                 spin_unlock(&files->file_lock);
217                 expanded = 1;
218                 wait_event(files->resize_wait, !files->resize_in_progress);
219                 spin_lock(&files->file_lock);
220                 goto repeat;
221         }
222 
223         /* All good, so we try */
224         files->resize_in_progress = true;
225         expanded = expand_fdtable(files, nr);
226         files->resize_in_progress = false;
227 
228         wake_up_all(&files->resize_wait);
229         return expanded;
230 }
231 
232 static inline void __set_close_on_exec(unsigned int fd, struct fdtable *fdt)
233 {
234         __set_bit(fd, fdt->close_on_exec);
235 }
236 
237 static inline void __clear_close_on_exec(unsigned int fd, struct fdtable *fdt)
238 {
239         if (test_bit(fd, fdt->close_on_exec))
240                 __clear_bit(fd, fdt->close_on_exec);
241 }
242 
243 static inline void __set_open_fd(unsigned int fd, struct fdtable *fdt)
244 {
245         __set_bit(fd, fdt->open_fds);
246         fd /= BITS_PER_LONG;
247         if (!~fdt->open_fds[fd])
248                 __set_bit(fd, fdt->full_fds_bits);
249 }
250 
251 static inline void __clear_open_fd(unsigned int fd, struct fdtable *fdt)
252 {
253         __clear_bit(fd, fdt->open_fds);
254         __clear_bit(fd / BITS_PER_LONG, fdt->full_fds_bits);
255 }
256 
257 static unsigned int count_open_files(struct fdtable *fdt)
258 {
259         unsigned int size = fdt->max_fds;
260         unsigned int i;
261 
262         /* Find the last open fd */
263         for (i = size / BITS_PER_LONG; i > 0; ) {
264                 if (fdt->open_fds[--i])
265                         break;
266         }
267         i = (i + 1) * BITS_PER_LONG;
268         return i;
269 }
270 
271 /*
272  * Allocate a new files structure and copy contents from the
273  * passed in files structure.
274  * errorp will be valid only when the returned files_struct is NULL.
275  */
276 struct files_struct *dup_fd(struct files_struct *oldf, int *errorp)
277 {
278         struct files_struct *newf;
279         struct file **old_fds, **new_fds;
280         unsigned int open_files, i;
281         struct fdtable *old_fdt, *new_fdt;
282 
283         *errorp = -ENOMEM;
284         newf = kmem_cache_alloc(files_cachep, GFP_KERNEL);
285         if (!newf)
286                 goto out;
287 
288         atomic_set(&newf->count, 1);
289 
290         spin_lock_init(&newf->file_lock);
291         newf->resize_in_progress = false;
292         init_waitqueue_head(&newf->resize_wait);
293         newf->next_fd = 0;
294         new_fdt = &newf->fdtab;
295         new_fdt->max_fds = NR_OPEN_DEFAULT;
296         new_fdt->close_on_exec = newf->close_on_exec_init;
297         new_fdt->open_fds = newf->open_fds_init;
298         new_fdt->full_fds_bits = newf->full_fds_bits_init;
299         new_fdt->fd = &newf->fd_array[0];
300 
301         spin_lock(&oldf->file_lock);
302         old_fdt = files_fdtable(oldf);
303         open_files = count_open_files(old_fdt);
304 
305         /*
306          * Check whether we need to allocate a larger fd array and fd set.
307          */
308         while (unlikely(open_files > new_fdt->max_fds)) {
309                 spin_unlock(&oldf->file_lock);
310 
311                 if (new_fdt != &newf->fdtab)
312                         __free_fdtable(new_fdt);
313 
314                 new_fdt = alloc_fdtable(open_files - 1);
315                 if (!new_fdt) {
316                         *errorp = -ENOMEM;
317                         goto out_release;
318                 }
319 
320                 /* beyond sysctl_nr_open; nothing to do */
321                 if (unlikely(new_fdt->max_fds < open_files)) {
322                         __free_fdtable(new_fdt);
323                         *errorp = -EMFILE;
324                         goto out_release;
325                 }
326 
327                 /*
328                  * Reacquire the oldf lock and a pointer to its fd table
329                  * who knows it may have a new bigger fd table. We need
330                  * the latest pointer.
331                  */
332                 spin_lock(&oldf->file_lock);
333                 old_fdt = files_fdtable(oldf);
334                 open_files = count_open_files(old_fdt);
335         }
336 
337         copy_fd_bitmaps(new_fdt, old_fdt, open_files);
338 
339         old_fds = old_fdt->fd;
340         new_fds = new_fdt->fd;
341 
342         for (i = open_files; i != 0; i--) {
343                 struct file *f = *old_fds++;
344                 if (f) {
345                         get_file(f);
346                 } else {
347                         /*
348                          * The fd may be claimed in the fd bitmap but not yet
349                          * instantiated in the files array if a sibling thread
350                          * is partway through open().  So make sure that this
351                          * fd is available to the new process.
352                          */
353                         __clear_open_fd(open_files - i, new_fdt);
354                 }
355                 rcu_assign_pointer(*new_fds++, f);
356         }
357         spin_unlock(&oldf->file_lock);
358 
359         /* clear the remainder */
360         memset(new_fds, 0, (new_fdt->max_fds - open_files) * sizeof(struct file *));
361 
362         rcu_assign_pointer(newf->fdt, new_fdt);
363 
364         return newf;
365 
366 out_release:
367         kmem_cache_free(files_cachep, newf);
368 out:
369         return NULL;
370 }
371 
372 static struct fdtable *close_files(struct files_struct * files)
373 {
374         /*
375          * It is safe to dereference the fd table without RCU or
376          * ->file_lock because this is the last reference to the
377          * files structure.
378          */
379         struct fdtable *fdt = rcu_dereference_raw(files->fdt);
380         unsigned int i, j = 0;
381 
382         for (;;) {
383                 unsigned long set;
384                 i = j * BITS_PER_LONG;
385                 if (i >= fdt->max_fds)
386                         break;
387                 set = fdt->open_fds[j++];
388                 while (set) {
389                         if (set & 1) {
390                                 struct file * file = xchg(&fdt->fd[i], NULL);
391                                 if (file) {
392                                         filp_close(file, files);
393                                         cond_resched_rcu_qs();
394                                 }
395                         }
396                         i++;
397                         set >>= 1;
398                 }
399         }
400 
401         return fdt;
402 }
403 
404 struct files_struct *get_files_struct(struct task_struct *task)
405 {
406         struct files_struct *files;
407 
408         task_lock(task);
409         files = task->files;
410         if (files)
411                 atomic_inc(&files->count);
412         task_unlock(task);
413 
414         return files;
415 }
416 
417 void put_files_struct(struct files_struct *files)
418 {
419         if (atomic_dec_and_test(&files->count)) {
420                 struct fdtable *fdt = close_files(files);
421 
422                 /* free the arrays if they are not embedded */
423                 if (fdt != &files->fdtab)
424                         __free_fdtable(fdt);
425                 kmem_cache_free(files_cachep, files);
426         }
427 }
428 
429 void reset_files_struct(struct files_struct *files)
430 {
431         struct task_struct *tsk = current;
432         struct files_struct *old;
433 
434         old = tsk->files;
435         task_lock(tsk);
436         tsk->files = files;
437         task_unlock(tsk);
438         put_files_struct(old);
439 }
440 
441 void exit_files(struct task_struct *tsk)
442 {
443         struct files_struct * files = tsk->files;
444 
445         if (files) {
446                 task_lock(tsk);
447                 tsk->files = NULL;
448                 task_unlock(tsk);
449                 put_files_struct(files);
450         }
451 }
452 
453 struct files_struct init_files = {
454         .count          = ATOMIC_INIT(1),
455         .fdt            = &init_files.fdtab,
456         .fdtab          = {
457                 .max_fds        = NR_OPEN_DEFAULT,
458                 .fd             = &init_files.fd_array[0],
459                 .close_on_exec  = init_files.close_on_exec_init,
460                 .open_fds       = init_files.open_fds_init,
461                 .full_fds_bits  = init_files.full_fds_bits_init,
462         },
463         .file_lock      = __SPIN_LOCK_UNLOCKED(init_files.file_lock),
464 };
465 
466 static unsigned int find_next_fd(struct fdtable *fdt, unsigned int start)
467 {
468         unsigned int maxfd = fdt->max_fds;
469         unsigned int maxbit = maxfd / BITS_PER_LONG;
470         unsigned int bitbit = start / BITS_PER_LONG;
471 
472         bitbit = find_next_zero_bit(fdt->full_fds_bits, maxbit, bitbit) * BITS_PER_LONG;
473         if (bitbit > maxfd)
474                 return maxfd;
475         if (bitbit > start)
476                 start = bitbit;
477         return find_next_zero_bit(fdt->open_fds, maxfd, start);
478 }
479 
480 /*
481  * allocate a file descriptor, mark it busy.
482  */
483 int __alloc_fd(struct files_struct *files,
484                unsigned start, unsigned end, unsigned flags)
485 {
486         unsigned int fd;
487         int error;
488         struct fdtable *fdt;
489 
490         spin_lock(&files->file_lock);
491 repeat:
492         fdt = files_fdtable(files);
493         fd = start;
494         if (fd < files->next_fd)
495                 fd = files->next_fd;
496 
497         if (fd < fdt->max_fds)
498                 fd = find_next_fd(fdt, fd);
499 
500         /*
501          * N.B. For clone tasks sharing a files structure, this test
502          * will limit the total number of files that can be opened.
503          */
504         error = -EMFILE;
505         if (fd >= end)
506                 goto out;
507 
508         error = expand_files(files, fd);
509         if (error < 0)
510                 goto out;
511 
512         /*
513          * If we needed to expand the fs array we
514          * might have blocked - try again.
515          */
516         if (error)
517                 goto repeat;
518 
519         if (start <= files->next_fd)
520                 files->next_fd = fd + 1;
521 
522         __set_open_fd(fd, fdt);
523         if (flags & O_CLOEXEC)
524                 __set_close_on_exec(fd, fdt);
525         else
526                 __clear_close_on_exec(fd, fdt);
527         error = fd;
528 #if 1
529         /* Sanity check */
530         if (rcu_access_pointer(fdt->fd[fd]) != NULL) {
531                 printk(KERN_WARNING "alloc_fd: slot %d not NULL!\n", fd);
532                 rcu_assign_pointer(fdt->fd[fd], NULL);
533         }
534 #endif
535 
536 out:
537         spin_unlock(&files->file_lock);
538         return error;
539 }
540 
541 static int alloc_fd(unsigned start, unsigned flags)
542 {
543         return __alloc_fd(current->files, start, rlimit(RLIMIT_NOFILE), flags);
544 }
545 
546 int get_unused_fd_flags(unsigned flags)
547 {
548         return __alloc_fd(current->files, 0, rlimit(RLIMIT_NOFILE), flags);
549 }
550 EXPORT_SYMBOL(get_unused_fd_flags);
551 
552 static void __put_unused_fd(struct files_struct *files, unsigned int fd)
553 {
554         struct fdtable *fdt = files_fdtable(files);
555         __clear_open_fd(fd, fdt);
556         if (fd < files->next_fd)
557                 files->next_fd = fd;
558 }
559 
560 void put_unused_fd(unsigned int fd)
561 {
562         struct files_struct *files = current->files;
563         spin_lock(&files->file_lock);
564         __put_unused_fd(files, fd);
565         spin_unlock(&files->file_lock);
566 }
567 
568 EXPORT_SYMBOL(put_unused_fd);
569 
570 /*
571  * Install a file pointer in the fd array.
572  *
573  * The VFS is full of places where we drop the files lock between
574  * setting the open_fds bitmap and installing the file in the file
575  * array.  At any such point, we are vulnerable to a dup2() race
576  * installing a file in the array before us.  We need to detect this and
577  * fput() the struct file we are about to overwrite in this case.
578  *
579  * It should never happen - if we allow dup2() do it, _really_ bad things
580  * will follow.
581  *
582  * NOTE: __fd_install() variant is really, really low-level; don't
583  * use it unless you are forced to by truly lousy API shoved down
584  * your throat.  'files' *MUST* be either current->files or obtained
585  * by get_files_struct(current) done by whoever had given it to you,
586  * or really bad things will happen.  Normally you want to use
587  * fd_install() instead.
588  */
589 
590 void __fd_install(struct files_struct *files, unsigned int fd,
591                 struct file *file)
592 {
593         struct fdtable *fdt;
594 
595         might_sleep();
596         rcu_read_lock_sched();
597 
598         while (unlikely(files->resize_in_progress)) {
599                 rcu_read_unlock_sched();
600                 wait_event(files->resize_wait, !files->resize_in_progress);
601                 rcu_read_lock_sched();
602         }
603         /* coupled with smp_wmb() in expand_fdtable() */
604         smp_rmb();
605         fdt = rcu_dereference_sched(files->fdt);
606         BUG_ON(fdt->fd[fd] != NULL);
607         rcu_assign_pointer(fdt->fd[fd], file);
608         rcu_read_unlock_sched();
609 }
610 
611 void fd_install(unsigned int fd, struct file *file)
612 {
613         __fd_install(current->files, fd, file);
614 }
615 
616 EXPORT_SYMBOL(fd_install);
617 
618 /*
619  * The same warnings as for __alloc_fd()/__fd_install() apply here...
620  */
621 int __close_fd(struct files_struct *files, unsigned fd)
622 {
623         struct file *file;
624         struct fdtable *fdt;
625 
626         spin_lock(&files->file_lock);
627         fdt = files_fdtable(files);
628         if (fd >= fdt->max_fds)
629                 goto out_unlock;
630         file = fdt->fd[fd];
631         if (!file)
632                 goto out_unlock;
633         rcu_assign_pointer(fdt->fd[fd], NULL);
634         __clear_close_on_exec(fd, fdt);
635         __put_unused_fd(files, fd);
636         spin_unlock(&files->file_lock);
637         return filp_close(file, files);
638 
639 out_unlock:
640         spin_unlock(&files->file_lock);
641         return -EBADF;
642 }
643 
644 void do_close_on_exec(struct files_struct *files)
645 {
646         unsigned i;
647         struct fdtable *fdt;
648 
649         /* exec unshares first */
650         spin_lock(&files->file_lock);
651         for (i = 0; ; i++) {
652                 unsigned long set;
653                 unsigned fd = i * BITS_PER_LONG;
654                 fdt = files_fdtable(files);
655                 if (fd >= fdt->max_fds)
656                         break;
657                 set = fdt->close_on_exec[i];
658                 if (!set)
659                         continue;
660                 fdt->close_on_exec[i] = 0;
661                 for ( ; set ; fd++, set >>= 1) {
662                         struct file *file;
663                         if (!(set & 1))
664                                 continue;
665                         file = fdt->fd[fd];
666                         if (!file)
667                                 continue;
668                         rcu_assign_pointer(fdt->fd[fd], NULL);
669                         __put_unused_fd(files, fd);
670                         spin_unlock(&files->file_lock);
671                         filp_close(file, files);
672                         cond_resched();
673                         spin_lock(&files->file_lock);
674                 }
675 
676         }
677         spin_unlock(&files->file_lock);
678 }
679 
680 static struct file *__fget(unsigned int fd, fmode_t mask)
681 {
682         struct files_struct *files = current->files;
683         struct file *file;
684 
685         rcu_read_lock();
686 loop:
687         file = fcheck_files(files, fd);
688         if (file) {
689                 /* File object ref couldn't be taken.
690                  * dup2() atomicity guarantee is the reason
691                  * we loop to catch the new file (or NULL pointer)
692                  */
693                 if (file->f_mode & mask)
694                         file = NULL;
695                 else if (!get_file_rcu(file))
696                         goto loop;
697         }
698         rcu_read_unlock();
699 
700         return file;
701 }
702 
703 struct file *fget(unsigned int fd)
704 {
705         return __fget(fd, FMODE_PATH);
706 }
707 EXPORT_SYMBOL(fget);
708 
709 struct file *fget_raw(unsigned int fd)
710 {
711         return __fget(fd, 0);
712 }
713 EXPORT_SYMBOL(fget_raw);
714 
715 /*
716  * Lightweight file lookup - no refcnt increment if fd table isn't shared.
717  *
718  * You can use this instead of fget if you satisfy all of the following
719  * conditions:
720  * 1) You must call fput_light before exiting the syscall and returning control
721  *    to userspace (i.e. you cannot remember the returned struct file * after
722  *    returning to userspace).
723  * 2) You must not call filp_close on the returned struct file * in between
724  *    calls to fget_light and fput_light.
725  * 3) You must not clone the current task in between the calls to fget_light
726  *    and fput_light.
727  *
728  * The fput_needed flag returned by fget_light should be passed to the
729  * corresponding fput_light.
730  */
731 static unsigned long __fget_light(unsigned int fd, fmode_t mask)
732 {
733         struct files_struct *files = current->files;
734         struct file *file;
735 
736         if (atomic_read(&files->count) == 1) {
737                 file = __fcheck_files(files, fd);
738                 if (!file || unlikely(file->f_mode & mask))
739                         return 0;
740                 return (unsigned long)file;
741         } else {
742                 file = __fget(fd, mask);
743                 if (!file)
744                         return 0;
745                 return FDPUT_FPUT | (unsigned long)file;
746         }
747 }
748 unsigned long __fdget(unsigned int fd)
749 {
750         return __fget_light(fd, FMODE_PATH);
751 }
752 EXPORT_SYMBOL(__fdget);
753 
754 unsigned long __fdget_raw(unsigned int fd)
755 {
756         return __fget_light(fd, 0);
757 }
758 
759 unsigned long __fdget_pos(unsigned int fd)
760 {
761         unsigned long v = __fdget(fd);
762         struct file *file = (struct file *)(v & ~3);
763 
764         if (file && (file->f_mode & FMODE_ATOMIC_POS)) {
765                 if (file_count(file) > 1) {
766                         v |= FDPUT_POS_UNLOCK;
767                         mutex_lock(&file->f_pos_lock);
768                 }
769         }
770         return v;
771 }
772 
773 void __f_unlock_pos(struct file *f)
774 {
775         mutex_unlock(&f->f_pos_lock);
776 }
777 
778 /*
779  * We only lock f_pos if we have threads or if the file might be
780  * shared with another process. In both cases we'll have an elevated
781  * file count (done either by fdget() or by fork()).
782  */
783 
784 void set_close_on_exec(unsigned int fd, int flag)
785 {
786         struct files_struct *files = current->files;
787         struct fdtable *fdt;
788         spin_lock(&files->file_lock);
789         fdt = files_fdtable(files);
790         if (flag)
791                 __set_close_on_exec(fd, fdt);
792         else
793                 __clear_close_on_exec(fd, fdt);
794         spin_unlock(&files->file_lock);
795 }
796 
797 bool get_close_on_exec(unsigned int fd)
798 {
799         struct files_struct *files = current->files;
800         struct fdtable *fdt;
801         bool res;
802         rcu_read_lock();
803         fdt = files_fdtable(files);
804         res = close_on_exec(fd, fdt);
805         rcu_read_unlock();
806         return res;
807 }
808 
809 static int do_dup2(struct files_struct *files,
810         struct file *file, unsigned fd, unsigned flags)
811 __releases(&files->file_lock)
812 {
813         struct file *tofree;
814         struct fdtable *fdt;
815 
816         /*
817          * We need to detect attempts to do dup2() over allocated but still
818          * not finished descriptor.  NB: OpenBSD avoids that at the price of
819          * extra work in their equivalent of fget() - they insert struct
820          * file immediately after grabbing descriptor, mark it larval if
821          * more work (e.g. actual opening) is needed and make sure that
822          * fget() treats larval files as absent.  Potentially interesting,
823          * but while extra work in fget() is trivial, locking implications
824          * and amount of surgery on open()-related paths in VFS are not.
825          * FreeBSD fails with -EBADF in the same situation, NetBSD "solution"
826          * deadlocks in rather amusing ways, AFAICS.  All of that is out of
827          * scope of POSIX or SUS, since neither considers shared descriptor
828          * tables and this condition does not arise without those.
829          */
830         fdt = files_fdtable(files);
831         tofree = fdt->fd[fd];
832         if (!tofree && fd_is_open(fd, fdt))
833                 goto Ebusy;
834         get_file(file);
835         rcu_assign_pointer(fdt->fd[fd], file);
836         __set_open_fd(fd, fdt);
837         if (flags & O_CLOEXEC)
838                 __set_close_on_exec(fd, fdt);
839         else
840                 __clear_close_on_exec(fd, fdt);
841         spin_unlock(&files->file_lock);
842 
843         if (tofree)
844                 filp_close(tofree, files);
845 
846         return fd;
847 
848 Ebusy:
849         spin_unlock(&files->file_lock);
850         return -EBUSY;
851 }
852 
853 int replace_fd(unsigned fd, struct file *file, unsigned flags)
854 {
855         int err;
856         struct files_struct *files = current->files;
857 
858         if (!file)
859                 return __close_fd(files, fd);
860 
861         if (fd >= rlimit(RLIMIT_NOFILE))
862                 return -EBADF;
863 
864         spin_lock(&files->file_lock);
865         err = expand_files(files, fd);
866         if (unlikely(err < 0))
867                 goto out_unlock;
868         return do_dup2(files, file, fd, flags);
869 
870 out_unlock:
871         spin_unlock(&files->file_lock);
872         return err;
873 }
874 
875 SYSCALL_DEFINE3(dup3, unsigned int, oldfd, unsigned int, newfd, int, flags)
876 {
877         int err = -EBADF;
878         struct file *file;
879         struct files_struct *files = current->files;
880 
881         if ((flags & ~O_CLOEXEC) != 0)
882                 return -EINVAL;
883 
884         if (unlikely(oldfd == newfd))
885                 return -EINVAL;
886 
887         if (newfd >= rlimit(RLIMIT_NOFILE))
888                 return -EBADF;
889 
890         spin_lock(&files->file_lock);
891         err = expand_files(files, newfd);
892         file = fcheck(oldfd);
893         if (unlikely(!file))
894                 goto Ebadf;
895         if (unlikely(err < 0)) {
896                 if (err == -EMFILE)
897                         goto Ebadf;
898                 goto out_unlock;
899         }
900         return do_dup2(files, file, newfd, flags);
901 
902 Ebadf:
903         err = -EBADF;
904 out_unlock:
905         spin_unlock(&files->file_lock);
906         return err;
907 }
908 
909 SYSCALL_DEFINE2(dup2, unsigned int, oldfd, unsigned int, newfd)
910 {
911         if (unlikely(newfd == oldfd)) { /* corner case */
912                 struct files_struct *files = current->files;
913                 int retval = oldfd;
914 
915                 rcu_read_lock();
916                 if (!fcheck_files(files, oldfd))
917                         retval = -EBADF;
918                 rcu_read_unlock();
919                 return retval;
920         }
921         return sys_dup3(oldfd, newfd, 0);
922 }
923 
924 SYSCALL_DEFINE1(dup, unsigned int, fildes)
925 {
926         int ret = -EBADF;
927         struct file *file = fget_raw(fildes);
928 
929         if (file) {
930                 ret = get_unused_fd_flags(0);
931                 if (ret >= 0)
932                         fd_install(ret, file);
933                 else
934                         fput(file);
935         }
936         return ret;
937 }
938 
939 int f_dupfd(unsigned int from, struct file *file, unsigned flags)
940 {
941         int err;
942         if (from >= rlimit(RLIMIT_NOFILE))
943                 return -EINVAL;
944         err = alloc_fd(from, flags);
945         if (err >= 0) {
946                 get_file(file);
947                 fd_install(err, file);
948         }
949         return err;
950 }
951 
952 int iterate_fd(struct files_struct *files, unsigned n,
953                 int (*f)(const void *, struct file *, unsigned),
954                 const void *p)
955 {
956         struct fdtable *fdt;
957         int res = 0;
958         if (!files)
959                 return 0;
960         spin_lock(&files->file_lock);
961         for (fdt = files_fdtable(files); n < fdt->max_fds; n++) {
962                 struct file *file;
963                 file = rcu_dereference_check_fdtable(files, fdt->fd[n]);
964                 if (!file)
965                         continue;
966                 res = f(p, file, n);
967                 if (res)
968                         break;
969         }
970         spin_unlock(&files->file_lock);
971         return res;
972 }
973 EXPORT_SYMBOL(iterate_fd);
974 

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