block/partition-generic.c

   1 // SPDX-License-Identifier: GPL-2.0
   2 /*
   3  *  Code extracted from drivers/block/genhd.c
   4  *  Copyright (C) 1991-1998  Linus Torvalds
   5  *  Re-organised Feb 1998 Russell King
   6  *
   7  *  We now have independent partition support from the
   8  *  block drivers, which allows all the partition code to
   9  *  be grouped in one location, and it to be mostly self
  10  *  contained.
  11  */
  12
  13 #include <linux/init.h>
  14 #include <linux/module.h>
  15 #include <linux/fs.h>
  16 #include <linux/slab.h>
  17 #include <linux/kmod.h>
  18 #include <linux/ctype.h>
  19 #include <linux/genhd.h>
  20 #include <linux/blktrace_api.h>
  21
  22 #include "partitions/check.h"
  23
  24 #ifdef CONFIG_BLK_DEV_MD
  25 extern void md_autodetect_dev(dev_t dev);
  26 #endif
  27
  28 /*
  29  * disk_name() is used by partition check code and the genhd driver.
  30  * It formats the devicename of the indicated disk into
  31  * the supplied buffer (of size at least 32), and returns
  32  * a pointer to that same buffer (for convenience).
  33  */
  34
  35 char *disk_name(struct gendisk *hd, int partno, char *buf)
  36 {
  37         if (!partno)
  38                 snprintf(buf, BDEVNAME_SIZE, "%s", hd->disk_name);
  39         else if (isdigit(hd->disk_name[strlen(hd->disk_name)-1]))
  40                 snprintf(buf, BDEVNAME_SIZE, "%sp%d", hd->disk_name, partno);
  41         else
  42                 snprintf(buf, BDEVNAME_SIZE, "%s%d", hd->disk_name, partno);
  43
  44         return buf;
  45 }
  46
  47 const char *bdevname(struct block_device *bdev, char *buf)
  48 {
  49         return disk_name(bdev->bd_disk, bdev->bd_part->partno, buf);
  50 }
  51
  52 EXPORT_SYMBOL(bdevname);
  53
  54 const char *bio_devname(struct bio *bio, char *buf)
  55 {
  56         return disk_name(bio->bi_disk, bio->bi_partno, buf);
  57 }
  58 EXPORT_SYMBOL(bio_devname);
  59
  60 /*
  61  * There's very little reason to use this, you should really
  62  * have a struct block_device just about everywhere and use
  63  * bdevname() instead.
  64  */
  65 const char *__bdevname(dev_t dev, char *buffer)
  66 {
  67         scnprintf(buffer, BDEVNAME_SIZE, "unknown-block(%u,%u)",
  68                                 MAJOR(dev), MINOR(dev));
  69         return buffer;
  70 }
  71
  72 EXPORT_SYMBOL(__bdevname);
  73
  74 static ssize_t part_partition_show(struct device *dev,
  75                                    struct device_attribute *attr, char *buf)
  76 {
  77         struct hd_struct *p = dev_to_part(dev);
  78
  79         return sprintf(buf, "%d\n", p->partno);
  80 }
  81
  82 static ssize_t part_start_show(struct device *dev,
  83                                struct device_attribute *attr, char *buf)
  84 {
  85         struct hd_struct *p = dev_to_part(dev);
  86
  87         return sprintf(buf, "%llu\n",(unsigned long long)p->start_sect);
  88 }
  89
  90 ssize_t part_size_show(struct device *dev,
  91                        struct device_attribute *attr, char *buf)
  92 {
  93         struct hd_struct *p = dev_to_part(dev);
  94         return sprintf(buf, "%llu\n",(unsigned long long)part_nr_sects_read(p));
  95 }
  96
  97 static ssize_t part_ro_show(struct device *dev,
  98                             struct device_attribute *attr, char *buf)
  99 {
 100         struct hd_struct *p = dev_to_part(dev);
 101         return sprintf(buf, "%d\n", p->policy ? 1 : 0);
 102 }
 103
 104 static ssize_t part_alignment_offset_show(struct device *dev,
 105                                           struct device_attribute *attr, char *buf)
 106 {
 107         struct hd_struct *p = dev_to_part(dev);
 108         return sprintf(buf, "%llu\n", (unsigned long long)p->alignment_offset);
 109 }
 110
 111 static ssize_t part_discard_alignment_show(struct device *dev,
 112                                            struct device_attribute *attr, char *buf)
 113 {
 114         struct hd_struct *p = dev_to_part(dev);
 115         return sprintf(buf, "%u\n", p->discard_alignment);
 116 }
 117
 118 ssize_t part_stat_show(struct device *dev,
 119                        struct device_attribute *attr, char *buf)
 120 {
 121         struct hd_struct *p = dev_to_part(dev);
 122         struct request_queue *q = part_to_disk(p)->queue;
 123         unsigned int inflight[2];
 124         int cpu;
 125
 126         cpu = part_stat_lock();
 127         part_round_stats(q, cpu, p);
 128         part_stat_unlock();
 129         part_in_flight(q, p, inflight);
 130         return sprintf(buf,
 131                 "%8lu %8lu %8llu %8u "
 132                 "%8lu %8lu %8llu %8u "
 133                 "%8u %8u %8u"
 134                 "\n",
 135                 part_stat_read(p, ios[READ]),
 136                 part_stat_read(p, merges[READ]),
 137                 (unsigned long long)part_stat_read(p, sectors[READ]),
 138                 jiffies_to_msecs(part_stat_read(p, ticks[READ])),
 139                 part_stat_read(p, ios[WRITE]),
 140                 part_stat_read(p, merges[WRITE]),
 141                 (unsigned long long)part_stat_read(p, sectors[WRITE]),
 142                 jiffies_to_msecs(part_stat_read(p, ticks[WRITE])),
 143                 inflight[0],
 144                 jiffies_to_msecs(part_stat_read(p, io_ticks)),
 145                 jiffies_to_msecs(part_stat_read(p, time_in_queue)));
 146 }
 147
 148 ssize_t part_inflight_show(struct device *dev, struct device_attribute *attr,
 149                            char *buf)
 150 {
 151         struct hd_struct *p = dev_to_part(dev);
 152         struct request_queue *q = part_to_disk(p)->queue;
 153         unsigned int inflight[2];
 154
 155         part_in_flight_rw(q, p, inflight);
 156         return sprintf(buf, "%8u %8u\n", inflight[0], inflight[1]);
 157 }
 158
 159 #ifdef CONFIG_FAIL_MAKE_REQUEST
 160 ssize_t part_fail_show(struct device *dev,
 161                        struct device_attribute *attr, char *buf)
 162 {
 163         struct hd_struct *p = dev_to_part(dev);
 164
 165         return sprintf(buf, "%d\n", p->make_it_fail);
 166 }
 167
 168 ssize_t part_fail_store(struct device *dev,
 169                         struct device_attribute *attr,
 170                         const char *buf, size_t count)
 171 {
 172         struct hd_struct *p = dev_to_part(dev);
 173         int i;
 174
 175         if (count > 0 && sscanf(buf, "%d", &i) > 0)
 176                 p->make_it_fail = (i == 0) ? 0 : 1;
 177
 178         return count;
 179 }
 180 #endif
 181
 182 static DEVICE_ATTR(partition, S_IRUGO, part_partition_show, NULL);
 183 static DEVICE_ATTR(start, S_IRUGO, part_start_show, NULL);
 184 static DEVICE_ATTR(size, S_IRUGO, part_size_show, NULL);
 185 static DEVICE_ATTR(ro, S_IRUGO, part_ro_show, NULL);
 186 static DEVICE_ATTR(alignment_offset, S_IRUGO, part_alignment_offset_show, NULL);
 187 static DEVICE_ATTR(discard_alignment, S_IRUGO, part_discard_alignment_show,
 188                    NULL);
 189 static DEVICE_ATTR(stat, S_IRUGO, part_stat_show, NULL);
 190 static DEVICE_ATTR(inflight, S_IRUGO, part_inflight_show, NULL);
 191 #ifdef CONFIG_FAIL_MAKE_REQUEST
 192 static struct device_attribute dev_attr_fail =
 193         __ATTR(make-it-fail, S_IRUGO|S_IWUSR, part_fail_show, part_fail_store);
 194 #endif
 195
 196 static struct attribute *part_attrs[] = {
 197         &dev_attr_partition.attr,
 198         &dev_attr_start.attr,
 199         &dev_attr_size.attr,
 200         &dev_attr_ro.attr,
 201         &dev_attr_alignment_offset.attr,
 202         &dev_attr_discard_alignment.attr,
 203         &dev_attr_stat.attr,
 204         &dev_attr_inflight.attr,
 205 #ifdef CONFIG_FAIL_MAKE_REQUEST
 206         &dev_attr_fail.attr,
 207 #endif
 208         NULL
 209 };
 210
 211 static struct attribute_group part_attr_group = {
 212         .attrs = part_attrs,
 213 };
 214
 215 static const struct attribute_group *part_attr_groups[] = {
 216         &part_attr_group,
 217 #ifdef CONFIG_BLK_DEV_IO_TRACE
 218         &blk_trace_attr_group,
 219 #endif
 220         NULL
 221 };
 222
 223 static void part_release(struct device *dev)
 224 {
 225         struct hd_struct *p = dev_to_part(dev);
 226         blk_free_devt(dev->devt);
 227         hd_free_part(p);
 228         kfree(p);
 229 }
 230
 231 static int part_uevent(struct device *dev, struct kobj_uevent_env *env)
 232 {
 233         struct hd_struct *part = dev_to_part(dev);
 234
 235         add_uevent_var(env, "PARTN=%u", part->partno);
 236         if (part->info && part->info->volname[0])
 237                 add_uevent_var(env, "PARTNAME=%s", part->info->volname);
 238         return 0;
 239 }
 240
 241 struct device_type part_type = {
 242         .name           = "partition",
 243         .groups         = part_attr_groups,
 244         .release        = part_release,
 245         .uevent         = part_uevent,
 246 };
 247
 248 static void delete_partition_rcu_cb(struct rcu_head *head)
 249 {
 250         struct hd_struct *part = container_of(head, struct hd_struct, rcu_head);
 251
 252         part->start_sect = 0;
 253         part->nr_sects = 0;
 254         part_stat_set_all(part, 0);
 255         put_device(part_to_dev(part));
 256 }
 257
 258 void __delete_partition(struct percpu_ref *ref)
 259 {
 260         struct hd_struct *part = container_of(ref, struct hd_struct, ref);
 261         call_rcu(&part->rcu_head, delete_partition_rcu_cb);
 262 }
 263
 264 /*
 265  * Must be called either with bd_mutex held, before a disk can be opened or
 266  * after all disk users are gone.
 267  */
 268 void delete_partition(struct gendisk *disk, int partno)
 269 {
 270         struct disk_part_tbl *ptbl =
 271                 rcu_dereference_protected(disk->part_tbl, 1);
 272         struct hd_struct *part;
 273
 274         if (partno >= ptbl->len)
 275                 return;
 276
 277         part = rcu_dereference_protected(ptbl->part[partno], 1);
 278         if (!part)
 279                 return;
 280
 281         rcu_assign_pointer(ptbl->part[partno], NULL);
 282         rcu_assign_pointer(ptbl->last_lookup, NULL);
 283         kobject_put(part->holder_dir);
 284         device_del(part_to_dev(part));
 285
 286         hd_struct_kill(part);
 287 }
 288
 289 static ssize_t whole_disk_show(struct device *dev,
 290                                struct device_attribute *attr, char *buf)
 291 {
 292         return 0;
 293 }
 294 static DEVICE_ATTR(whole_disk, S_IRUSR | S_IRGRP | S_IROTH,
 295                    whole_disk_show, NULL);
 296
 297 /*
 298  * Must be called either with bd_mutex held, before a disk can be opened or
 299  * after all disk users are gone.
 300  */
 301 struct hd_struct *add_partition(struct gendisk *disk, int partno,
 302                                 sector_t start, sector_t len, int flags,
 303                                 struct partition_meta_info *info)
 304 {
 305         struct hd_struct *p;
 306         dev_t devt = MKDEV(0, 0);
 307         struct device *ddev = disk_to_dev(disk);
 308         struct device *pdev;
 309         struct disk_part_tbl *ptbl;
 310         const char *dname;
 311         int err;
 312
 313         err = disk_expand_part_tbl(disk, partno);
 314         if (err)
 315                 return ERR_PTR(err);
 316         ptbl = rcu_dereference_protected(disk->part_tbl, 1);
 317
 318         if (ptbl->part[partno])
 319                 return ERR_PTR(-EBUSY);
 320
 321         p = kzalloc(sizeof(*p), GFP_KERNEL);
 322         if (!p)
 323                 return ERR_PTR(-EBUSY);
 324
 325         if (!init_part_stats(p)) {
 326                 err = -ENOMEM;
 327                 goto out_free;
 328         }
 329
 330         seqcount_init(&p->nr_sects_seq);
 331         pdev = part_to_dev(p);
 332
 333         p->start_sect = start;
 334         p->alignment_offset =
 335                 queue_limit_alignment_offset(&disk->queue->limits, start);
 336         p->discard_alignment =
 337                 queue_limit_discard_alignment(&disk->queue->limits, start);
 338         p->nr_sects = len;
 339         p->partno = partno;
 340         p->policy = get_disk_ro(disk);
 341
 342         if (info) {
 343                 struct partition_meta_info *pinfo = alloc_part_info(disk);
 344                 if (!pinfo) {
 345                         err = -ENOMEM;
 346                         goto out_free_stats;
 347                 }
 348                 memcpy(pinfo, info, sizeof(*info));
 349                 p->info = pinfo;
 350         }
 351
 352         dname = dev_name(ddev);
 353         if (isdigit(dname[strlen(dname) - 1]))
 354                 dev_set_name(pdev, "%sp%d", dname, partno);
 355         else
 356                 dev_set_name(pdev, "%s%d", dname, partno);
 357
 358         device_initialize(pdev);
 359         pdev->class = &block_class;
 360         pdev->type = &part_type;
 361         pdev->parent = ddev;
 362
 363         err = blk_alloc_devt(p, &devt);
 364         if (err)
 365                 goto out_free_info;
 366         pdev->devt = devt;
 367
 368         /* delay uevent until 'holders' subdir is created */
 369         dev_set_uevent_suppress(pdev, 1);
 370         err = device_add(pdev);
 371         if (err)
 372                 goto out_put;
 373
 374         err = -ENOMEM;
 375         p->holder_dir = kobject_create_and_add("holders", &pdev->kobj);
 376         if (!p->holder_dir)
 377                 goto out_del;
 378
 379         dev_set_uevent_suppress(pdev, 0);
 380         if (flags & ADDPART_FLAG_WHOLEDISK) {
 381                 err = device_create_file(pdev, &dev_attr_whole_disk);
 382                 if (err)
 383                         goto out_del;
 384         }
 385
 386         err = hd_ref_init(p);
 387         if (err) {
 388                 if (flags & ADDPART_FLAG_WHOLEDISK)
 389                         goto out_remove_file;
 390                 goto out_del;
 391         }
 392
 393         /* everything is up and running, commence */
 394         rcu_assign_pointer(ptbl->part[partno], p);
 395
 396         /* suppress uevent if the disk suppresses it */
 397         if (!dev_get_uevent_suppress(ddev))
 398                 kobject_uevent(&pdev->kobj, KOBJ_ADD);
 399         return p;
 400
 401 out_free_info:
 402         free_part_info(p);
 403 out_free_stats:
 404         free_part_stats(p);
 405 out_free:
 406         kfree(p);
 407         return ERR_PTR(err);
 408 out_remove_file:
 409         device_remove_file(pdev, &dev_attr_whole_disk);
 410 out_del:
 411         kobject_put(p->holder_dir);
 412         device_del(pdev);
 413 out_put:
 414         put_device(pdev);
 415         return ERR_PTR(err);
 416 }
 417
 418 static bool disk_unlock_native_capacity(struct gendisk *disk)
 419 {
 420         const struct block_device_operations *bdops = disk->fops;
 421
 422         if (bdops->unlock_native_capacity &&
 423             !(disk->flags & GENHD_FL_NATIVE_CAPACITY)) {
 424                 printk(KERN_CONT "enabling native capacity\n");
 425                 bdops->unlock_native_capacity(disk);
 426                 disk->flags |= GENHD_FL_NATIVE_CAPACITY;
 427                 return true;
 428         } else {
 429                 printk(KERN_CONT "truncated\n");
 430                 return false;
 431         }
 432 }
 433
 434 static int drop_partitions(struct gendisk *disk, struct block_device *bdev)
 435 {
 436         struct disk_part_iter piter;
 437         struct hd_struct *part;
 438         int res;
 439
 440         if (bdev->bd_part_count || bdev->bd_super)
 441                 return -EBUSY;
 442         res = invalidate_partition(disk, 0);
 443         if (res)
 444                 return res;
 445
 446         disk_part_iter_init(&piter, disk, DISK_PITER_INCL_EMPTY);
 447         while ((part = disk_part_iter_next(&piter)))
 448                 delete_partition(disk, part->partno);
 449         disk_part_iter_exit(&piter);
 450
 451         return 0;
 452 }
 453
 454 static bool part_zone_aligned(struct gendisk *disk,
 455                               struct block_device *bdev,
 456                               sector_t from, sector_t size)
 457 {
 458         unsigned int zone_sectors = bdev_zone_sectors(bdev);
 459
 460         /*
 461          * If this function is called, then the disk is a zoned block device
 462          * (host-aware or host-managed). This can be detected even if the
 463          * zoned block device support is disabled (CONFIG_BLK_DEV_ZONED not
 464          * set). In this case, however, only host-aware devices will be seen
 465          * as a block device is not created for host-managed devices. Without
 466          * zoned block device support, host-aware drives can still be used as
 467          * regular block devices (no zone operation) and their zone size will
 468          * be reported as 0. Allow this case.
 469          */
 470         if (!zone_sectors)
 471                 return true;
 472
 473         /*
 474          * Check partition start and size alignement. If the drive has a
 475          * smaller last runt zone, ignore it and allow the partition to
 476          * use it. Check the zone size too: it should be a power of 2 number
 477          * of sectors.
 478          */
 479         if (WARN_ON_ONCE(!is_power_of_2(zone_sectors))) {
 480                 u32 rem;
 481
 482                 div_u64_rem(from, zone_sectors, &rem);
 483                 if (rem)
 484                         return false;
 485                 if ((from + size) < get_capacity(disk)) {
 486                         div_u64_rem(size, zone_sectors, &rem);
 487                         if (rem)
 488                                 return false;
 489                 }
 490
 491         } else {
 492
 493                 if (from & (zone_sectors - 1))
 494                         return false;
 495                 if ((from + size) < get_capacity(disk) &&
 496                     (size & (zone_sectors - 1)))
 497                         return false;
 498
 499         }
 500
 501         return true;
 502 }
 503
 504 int rescan_partitions(struct gendisk *disk, struct block_device *bdev)
 505 {
 506         struct parsed_partitions *state = NULL;
 507         struct hd_struct *part;
 508         int p, highest, res;
 509 rescan:
 510         if (state && !IS_ERR(state)) {
 511                 free_partitions(state);
 512                 state = NULL;
 513         }
 514
 515         res = drop_partitions(disk, bdev);
 516         if (res)
 517                 return res;
 518
 519         if (disk->fops->revalidate_disk)
 520                 disk->fops->revalidate_disk(disk);
 521         check_disk_size_change(disk, bdev);
 522         bdev->bd_invalidated = 0;
 523         if (!get_capacity(disk) || !(state = check_partition(disk, bdev)))
 524                 return 0;
 525         if (IS_ERR(state)) {
 526                 /*
 527                  * I/O error reading the partition table.  If any
 528                  * partition code tried to read beyond EOD, retry
 529                  * after unlocking native capacity.
 530                  */
 531                 if (PTR_ERR(state) == -ENOSPC) {
 532                         printk(KERN_WARNING "%s: partition table beyond EOD, ",
 533                                disk->disk_name);
 534                         if (disk_unlock_native_capacity(disk))
 535                                 goto rescan;
 536                 }
 537                 return -EIO;
 538         }
 539         /*
 540          * If any partition code tried to read beyond EOD, try
 541          * unlocking native capacity even if partition table is
 542          * successfully read as we could be missing some partitions.
 543          */
 544         if (state->access_beyond_eod) {
 545                 printk(KERN_WARNING
 546                        "%s: partition table partially beyond EOD, ",
 547                        disk->disk_name);
 548                 if (disk_unlock_native_capacity(disk))
 549                         goto rescan;
 550         }
 551
 552         /* tell userspace that the media / partition table may have changed */
 553         kobject_uevent(&disk_to_dev(disk)->kobj, KOBJ_CHANGE);
 554
 555         /* Detect the highest partition number and preallocate
 556          * disk->part_tbl.  This is an optimization and not strictly
 557          * necessary.
 558          */
 559         for (p = 1, highest = 0; p < state->limit; p++)
 560                 if (state->parts[p].size)
 561                         highest = p;
 562
 563         disk_expand_part_tbl(disk, highest);
 564
 565         /* add partitions */
 566         for (p = 1; p < state->limit; p++) {
 567                 sector_t size, from;
 568
 569                 size = state->parts[p].size;
 570                 if (!size)
 571                         continue;
 572
 573                 from = state->parts[p].from;
 574                 if (from >= get_capacity(disk)) {
 575                         printk(KERN_WARNING
 576                                "%s: p%d start %llu is beyond EOD, ",
 577                                disk->disk_name, p, (unsigned long long) from);
 578                         if (disk_unlock_native_capacity(disk))
 579                                 goto rescan;
 580                         continue;
 581                 }
 582
 583                 if (from + size > get_capacity(disk)) {
 584                         printk(KERN_WARNING
 585                                "%s: p%d size %llu extends beyond EOD, ",
 586                                disk->disk_name, p, (unsigned long long) size);
 587
 588                         if (disk_unlock_native_capacity(disk)) {
 589                                 /* free state and restart */
 590                                 goto rescan;
 591                         } else {
 592                                 /*
 593                                  * we can not ignore partitions of broken tables
 594                                  * created by for example camera firmware, but
 595                                  * we limit them to the end of the disk to avoid
 596                                  * creating invalid block devices
 597                                  */
 598                                 size = get_capacity(disk) - from;
 599                         }
 600                 }
 601
 602                 /*
 603                  * On a zoned block device, partitions should be aligned on the
 604                  * device zone size (i.e. zone boundary crossing not allowed).
 605                  * Otherwise, resetting the write pointer of the last zone of
 606                  * one partition may impact the following partition.
 607                  */
 608                 if (bdev_is_zoned(bdev) &&
 609                     !part_zone_aligned(disk, bdev, from, size)) {
 610                         printk(KERN_WARNING
 611                                "%s: p%d start %llu+%llu is not zone aligned\n",
 612                                disk->disk_name, p, (unsigned long long) from,
 613                                (unsigned long long) size);
 614                         continue;
 615                 }
 616
 617                 part = add_partition(disk, p, from, size,
 618                                      state->parts[p].flags,
 619                                      &state->parts[p].info);
 620                 if (IS_ERR(part)) {
 621                         printk(KERN_ERR " %s: p%d could not be added: %ld\n",
 622                                disk->disk_name, p, -PTR_ERR(part));
 623                         continue;
 624                 }
 625 #ifdef CONFIG_BLK_DEV_MD
 626                 if (state->parts[p].flags & ADDPART_FLAG_RAID)
 627                         md_autodetect_dev(part_to_dev(part)->devt);
 628 #endif
 629         }
 630         free_partitions(state);
 631         return 0;
 632 }
 633
 634 int invalidate_partitions(struct gendisk *disk, struct block_device *bdev)
 635 {
 636         int res;
 637
 638         if (!bdev->bd_invalidated)
 639                 return 0;
 640
 641         res = drop_partitions(disk, bdev);
 642         if (res)
 643                 return res;
 644
 645         set_capacity(disk, 0);
 646         check_disk_size_change(disk, bdev);
 647         bdev->bd_invalidated = 0;
 648         /* tell userspace that the media / partition table may have changed */
 649         kobject_uevent(&disk_to_dev(disk)->kobj, KOBJ_CHANGE);
 650
 651         return 0;
 652 }
 653
 654 unsigned char *read_dev_sector(struct block_device *bdev, sector_t n, Sector *p)
 655 {
 656         struct address_space *mapping = bdev->bd_inode->i_mapping;
 657         struct page *page;
 658
 659         page = read_mapping_page(mapping, (pgoff_t)(n >> (PAGE_SHIFT-9)), NULL);
 660         if (!IS_ERR(page)) {
 661                 if (PageError(page))
 662                         goto fail;
 663                 p->v = page;
 664                 return (unsigned char *)page_address(page) +  ((n & ((1 << (PAGE_SHIFT - 9)) - 1)) << 9);
 665 fail:
 666                 put_page(page);
 667         }
 668         p->v = NULL;
 669         return NULL;
 670 }
 671
 672 EXPORT_SYMBOL(read_dev_sector);