sbin/fsck_msdosfs/fat.c

   1 /*
   2  * Copyright (C) 1995, 1996, 1997 Wolfgang Solfrank
   3  * Copyright (c) 1995 Martin Husemann
   4  *
   5  * Redistribution and use in source and binary forms, with or without
   6  * modification, are permitted provided that the following conditions
   7  * are met:
   8  * 1. Redistributions of source code must retain the above copyright
   9  *    notice, this list of conditions and the following disclaimer.
  10  * 2. Redistributions in binary form must reproduce the above copyright
  11  *    notice, this list of conditions and the following disclaimer in the
  12  *    documentation and/or other materials provided with the distribution.
  13  * 3. All advertising materials mentioning features or use of this software
  14  *    must display the following acknowledgement:
  15  *      This product includes software developed by Martin Husemann
  16  *      and Wolfgang Solfrank.
  17  * 4. Neither the name of the University nor the names of its contributors
  18  *    may be used to endorse or promote products derived from this software
  19  *    without specific prior written permission.
  20  *
  21  * THIS SOFTWARE IS PROVIDED BY THE AUTHORS ``AS IS'' AND ANY EXPRESS OR
  22  * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
  23  * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
  24  * IN NO EVENT SHALL THE AUTHORS BE LIABLE FOR ANY DIRECT, INDIRECT,
  25  * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
  26  * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
  27  * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
  28  * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
  29  * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
  30  * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  31  */
  32
  33
  34 #include <sys/cdefs.h>
  35 #ifndef lint
  36 __RCSID("$NetBSD: fat.c,v 1.12 2000/10/10 20:24:52 is Exp $");
  37 static const char rcsid[] =
  38   "$FreeBSD$";
  39 #endif /* not lint */
  40
  41 #include <stdlib.h>
  42 #include <string.h>
  43 #include <ctype.h>
  44 #include <stdio.h>
  45 #include <unistd.h>
  46
  47 #include "ext.h"
  48 #include "fsutil.h"
  49
  50 static int checkclnum(struct bootblock *, int, cl_t, cl_t *);
  51 static int clustdiffer(cl_t, cl_t *, cl_t *, int);
  52 static int tryclear(struct bootblock *, struct fatEntry *, cl_t, cl_t *);
  53 static int _readfat(int, struct bootblock *, int, u_char **);
  54
  55 /*-
  56  * The first 2 FAT entries contain pseudo-cluster numbers with the following
  57  * layout:
  58  *
  59  * 31...... ........ ........ .......0
  60  * rrrr1111 11111111 11111111 mmmmmmmm         FAT32 entry 0
  61  * rrrrsh11 11111111 11111111 11111xxx         FAT32 entry 1
  62  *
  63  *                   11111111 mmmmmmmm         FAT16 entry 0
  64  *                   sh111111 11111xxx         FAT16 entry 1
  65  *
  66  * r = reserved
  67  * m = BPB media ID byte
  68  * s = clean flag (1 = dismounted; 0 = still mounted)
  69  * h = hard error flag (1 = ok; 0 = I/O error)
  70  * x = any value ok
  71  */
  72
  73 int
  74 checkdirty(int fs, struct bootblock *boot)
  75 {
  76         off_t off;
  77         u_char *buffer;
  78         int ret = 0;
  79
  80         if (boot->ClustMask != CLUST16_MASK && boot->ClustMask != CLUST32_MASK)
  81                 return 0;
  82
  83         off = boot->ResSectors;
  84         off *= boot->BytesPerSec;
  85
  86         buffer = malloc(boot->BytesPerSec);
  87         if (buffer == NULL) {
  88                 perror("No space for FAT");
  89                 return 1;
  90         }
  91
  92         if (lseek(fs, off, SEEK_SET) != off) {
  93                 perror("Unable to read FAT");
  94                 goto err;
  95         }
  96
  97         if (read(fs, buffer, boot->BytesPerSec) != boot->BytesPerSec) {
  98                 perror("Unable to read FAT");
  99                 goto err;
 100         }
 101
 102         /*
 103          * If we don't understand the FAT, then the file system must be
 104          * assumed to be unclean.
 105          */
 106         if (buffer[0] != boot->Media || buffer[1] != 0xff)
 107                 goto err;
 108         if (boot->ClustMask == CLUST16_MASK) {
 109                 if ((buffer[2] & 0xf8) != 0xf8 || (buffer[3] & 0x3f) != 0x3f)
 110                         goto err;
 111         } else {
 112                 if (buffer[2] != 0xff || (buffer[3] & 0x0f) != 0x0f
 113                     || (buffer[4] & 0xf8) != 0xf8 || buffer[5] != 0xff
 114                     || buffer[6] != 0xff || (buffer[7] & 0x03) != 0x03)
 115                         goto err;
 116         }
 117
 118         /*
 119          * Now check the actual clean flag (and the no-error flag).
 120          */
 121         if (boot->ClustMask == CLUST16_MASK) {
 122                 if ((buffer[3] & 0xc0) == 0xc0)
 123                         ret = 1;
 124         } else {
 125                 if ((buffer[7] & 0x0c) == 0x0c)
 126                         ret = 1;
 127         }
 128
 129 err:
 130         free(buffer);
 131         return ret;
 132 }
 133
 134 /*
 135  * Check a cluster number for valid value
 136  */
 137 static int
 138 checkclnum(struct bootblock *boot, int fat, cl_t cl, cl_t *next)
 139 {
 140         if (*next >= (CLUST_RSRVD&boot->ClustMask))
 141                 *next |= ~boot->ClustMask;
 142         if (*next == CLUST_FREE) {
 143                 boot->NumFree++;
 144                 return FSOK;
 145         }
 146         if (*next == CLUST_BAD) {
 147                 boot->NumBad++;
 148                 return FSOK;
 149         }
 150         if (*next < CLUST_FIRST
 151             || (*next >= boot->NumClusters && *next < CLUST_EOFS)) {
 152                 pwarn("Cluster %u in FAT %d continues with %s cluster number %u\n",
 153                       cl, fat,
 154                       *next < CLUST_RSRVD ? "out of range" : "reserved",
 155                       *next&boot->ClustMask);
 156                 if (ask(0, "Truncate")) {
 157                         *next = CLUST_EOF;
 158                         return FSFATMOD;
 159                 }
 160                 return FSERROR;
 161         }
 162         return FSOK;
 163 }
 164
 165 /*
 166  * Read a FAT from disk. Returns 1 if successful, 0 otherwise.
 167  */
 168 static int
 169 _readfat(int fs, struct bootblock *boot, int no, u_char **buffer)
 170 {
 171         off_t off;
 172
 173         *buffer = malloc(boot->FATsecs * boot->BytesPerSec);
 174         if (*buffer == NULL) {
 175                 perror("No space for FAT");
 176                 return 0;
 177         }
 178
 179         off = boot->ResSectors + no * boot->FATsecs;
 180         off *= boot->BytesPerSec;
 181
 182         if (lseek(fs, off, SEEK_SET) != off) {
 183                 perror("Unable to read FAT");
 184                 goto err;
 185         }
 186
 187         if (read(fs, *buffer, boot->FATsecs * boot->BytesPerSec)
 188             != boot->FATsecs * boot->BytesPerSec) {
 189                 perror("Unable to read FAT");
 190                 goto err;
 191         }
 192
 193         return 1;
 194
 195     err:
 196         free(*buffer);
 197         return 0;
 198 }
 199
 200 /*
 201  * Read a FAT and decode it into internal format
 202  */
 203 int
 204 readfat(int fs, struct bootblock *boot, int no, struct fatEntry **fp)
 205 {
 206         struct fatEntry *fat;
 207         u_char *buffer, *p;
 208         cl_t cl;
 209         int ret = FSOK;
 210
 211         boot->NumFree = boot->NumBad = 0;
 212
 213         if (!_readfat(fs, boot, no, &buffer))
 214                 return FSFATAL;
 215
 216         fat = calloc(boot->NumClusters, sizeof(struct fatEntry));
 217         if (fat == NULL) {
 218                 perror("No space for FAT");
 219                 free(buffer);
 220                 return FSFATAL;
 221         }
 222
 223         if (buffer[0] != boot->Media
 224             || buffer[1] != 0xff || buffer[2] != 0xff
 225             || (boot->ClustMask == CLUST16_MASK && buffer[3] != 0xff)
 226             || (boot->ClustMask == CLUST32_MASK
 227                 && ((buffer[3]&0x0f) != 0x0f
 228                     || buffer[4] != 0xff || buffer[5] != 0xff
 229                     || buffer[6] != 0xff || (buffer[7]&0x0f) != 0x0f))) {
 230
 231                 /* Windows 95 OSR2 (and possibly any later) changes
 232                  * the FAT signature to 0xXXffff7f for FAT16 and to
 233                  * 0xXXffff0fffffff07 for FAT32 upon boot, to know that the
 234                  * file system is dirty if it doesn't reboot cleanly.
 235                  * Check this special condition before errorring out.
 236                  */
 237                 if (buffer[0] == boot->Media && buffer[1] == 0xff
 238                     && buffer[2] == 0xff
 239                     && ((boot->ClustMask == CLUST16_MASK && buffer[3] == 0x7f)
 240                         || (boot->ClustMask == CLUST32_MASK
 241                             && buffer[3] == 0x0f && buffer[4] == 0xff
 242                             && buffer[5] == 0xff && buffer[6] == 0xff
 243                             && buffer[7] == 0x07)))
 244                         ret |= FSDIRTY;
 245                 else {
 246                         /* just some odd byte sequence in FAT */
 247
 248                         switch (boot->ClustMask) {
 249                         case CLUST32_MASK:
 250                                 pwarn("%s (%02x%02x%02x%02x%02x%02x%02x%02x)\n",
 251                                       "FAT starts with odd byte sequence",
 252                                       buffer[0], buffer[1], buffer[2], buffer[3],
 253                                       buffer[4], buffer[5], buffer[6], buffer[7]);
 254                                 break;
 255                         case CLUST16_MASK:
 256                                 pwarn("%s (%02x%02x%02x%02x)\n",
 257                                     "FAT starts with odd byte sequence",
 258                                     buffer[0], buffer[1], buffer[2], buffer[3]);
 259                                 break;
 260                         default:
 261                                 pwarn("%s (%02x%02x%02x)\n",
 262                                     "FAT starts with odd byte sequence",
 263                                     buffer[0], buffer[1], buffer[2]);
 264                                 break;
 265                         }
 266
 267
 268                         if (ask(1, "Correct"))
 269                                 ret |= FSFIXFAT;
 270                 }
 271         }
 272         switch (boot->ClustMask) {
 273         case CLUST32_MASK:
 274                 p = buffer + 8;
 275                 break;
 276         case CLUST16_MASK:
 277                 p = buffer + 4;
 278                 break;
 279         default:
 280                 p = buffer + 3;
 281                 break;
 282         }
 283         for (cl = CLUST_FIRST; cl < boot->NumClusters;) {
 284                 switch (boot->ClustMask) {
 285                 case CLUST32_MASK:
 286                         fat[cl].next = p[0] + (p[1] << 8)
 287                                        + (p[2] << 16) + (p[3] << 24);
 288                         fat[cl].next &= boot->ClustMask;
 289                         ret |= checkclnum(boot, no, cl, &fat[cl].next);
 290                         cl++;
 291                         p += 4;
 292                         break;
 293                 case CLUST16_MASK:
 294                         fat[cl].next = p[0] + (p[1] << 8);
 295                         ret |= checkclnum(boot, no, cl, &fat[cl].next);
 296                         cl++;
 297                         p += 2;
 298                         break;
 299                 default:
 300                         fat[cl].next = (p[0] + (p[1] << 8)) & 0x0fff;
 301                         ret |= checkclnum(boot, no, cl, &fat[cl].next);
 302                         cl++;
 303                         if (cl >= boot->NumClusters)
 304                                 break;
 305                         fat[cl].next = ((p[1] >> 4) + (p[2] << 4)) & 0x0fff;
 306                         ret |= checkclnum(boot, no, cl, &fat[cl].next);
 307                         cl++;
 308                         p += 3;
 309                         break;
 310                 }
 311         }
 312
 313         free(buffer);
 314         *fp = fat;
 315         return ret;
 316 }
 317
 318 /*
 319  * Get type of reserved cluster
 320  */
 321 char *
 322 rsrvdcltype(cl_t cl)
 323 {
 324         if (cl == CLUST_FREE)
 325                 return "free";
 326         if (cl < CLUST_BAD)
 327                 return "reserved";
 328         if (cl > CLUST_BAD)
 329                 return "as EOF";
 330         return "bad";
 331 }
 332
 333 static int
 334 clustdiffer(cl_t cl, cl_t *cp1, cl_t *cp2, int fatnum)
 335 {
 336         if (*cp1 == CLUST_FREE || *cp1 >= CLUST_RSRVD) {
 337                 if (*cp2 == CLUST_FREE || *cp2 >= CLUST_RSRVD) {
 338                         if ((*cp1 != CLUST_FREE && *cp1 < CLUST_BAD
 339                              && *cp2 != CLUST_FREE && *cp2 < CLUST_BAD)
 340                             || (*cp1 > CLUST_BAD && *cp2 > CLUST_BAD)) {
 341                                 pwarn("Cluster %u is marked %s with different indicators\n",
 342                                       cl, rsrvdcltype(*cp1));
 343                                 if (ask(1, "Fix")) {
 344                                         *cp2 = *cp1;
 345                                         return FSFATMOD;
 346                                 }
 347                                 return FSFATAL;
 348                         }
 349                         pwarn("Cluster %u is marked %s in FAT 0, %s in FAT %d\n",
 350                               cl, rsrvdcltype(*cp1), rsrvdcltype(*cp2), fatnum);
 351                         if (ask(0, "Use FAT 0's entry")) {
 352                                 *cp2 = *cp1;
 353                                 return FSFATMOD;
 354                         }
 355                         if (ask(0, "Use FAT %d's entry", fatnum)) {
 356                                 *cp1 = *cp2;
 357                                 return FSFATMOD;
 358                         }
 359                         return FSFATAL;
 360                 }
 361                 pwarn("Cluster %u is marked %s in FAT 0, but continues with cluster %u in FAT %d\n",
 362                       cl, rsrvdcltype(*cp1), *cp2, fatnum);
 363                 if (ask(0, "Use continuation from FAT %d", fatnum)) {
 364                         *cp1 = *cp2;
 365                         return FSFATMOD;
 366                 }
 367                 if (ask(0, "Use mark from FAT 0")) {
 368                         *cp2 = *cp1;
 369                         return FSFATMOD;
 370                 }
 371                 return FSFATAL;
 372         }
 373         if (*cp2 == CLUST_FREE || *cp2 >= CLUST_RSRVD) {
 374                 pwarn("Cluster %u continues with cluster %u in FAT 0, but is marked %s in FAT %d\n",
 375                       cl, *cp1, rsrvdcltype(*cp2), fatnum);
 376                 if (ask(0, "Use continuation from FAT 0")) {
 377                         *cp2 = *cp1;
 378                         return FSFATMOD;
 379                 }
 380                 if (ask(0, "Use mark from FAT %d", fatnum)) {
 381                         *cp1 = *cp2;
 382                         return FSFATMOD;
 383                 }
 384                 return FSERROR;
 385         }
 386         pwarn("Cluster %u continues with cluster %u in FAT 0, but with cluster %u in FAT %d\n",
 387               cl, *cp1, *cp2, fatnum);
 388         if (ask(0, "Use continuation from FAT 0")) {
 389                 *cp2 = *cp1;
 390                 return FSFATMOD;
 391         }
 392         if (ask(0, "Use continuation from FAT %d", fatnum)) {
 393                 *cp1 = *cp2;
 394                 return FSFATMOD;
 395         }
 396         return FSERROR;
 397 }
 398
 399 /*
 400  * Compare two FAT copies in memory. Resolve any conflicts and merge them
 401  * into the first one.
 402  */
 403 int
 404 comparefat(struct bootblock *boot, struct fatEntry *first,
 405     struct fatEntry *second, int fatnum)
 406 {
 407         cl_t cl;
 408         int ret = FSOK;
 409
 410         for (cl = CLUST_FIRST; cl < boot->NumClusters; cl++)
 411                 if (first[cl].next != second[cl].next)
 412                         ret |= clustdiffer(cl, &first[cl].next, &second[cl].next, fatnum);
 413         return ret;
 414 }
 415
 416 void
 417 clearchain(struct bootblock *boot, struct fatEntry *fat, cl_t head)
 418 {
 419         cl_t p, q;
 420
 421         for (p = head; p >= CLUST_FIRST && p < boot->NumClusters; p = q) {
 422                 if (fat[p].head != head)
 423                         break;
 424                 q = fat[p].next;
 425                 fat[p].next = fat[p].head = CLUST_FREE;
 426                 fat[p].length = 0;
 427         }
 428 }
 429
 430 int
 431 tryclear(struct bootblock *boot, struct fatEntry *fat, cl_t head, cl_t *trunc)
 432 {
 433         if (ask(0, "Clear chain starting at %u", head)) {
 434                 clearchain(boot, fat, head);
 435                 return FSFATMOD;
 436         } else if (ask(0, "Truncate")) {
 437                 *trunc = CLUST_EOF;
 438                 return FSFATMOD;
 439         } else
 440                 return FSERROR;
 441 }
 442
 443 /*
 444  * Check a complete FAT in-memory for crosslinks
 445  */
 446 int
 447 checkfat(struct bootblock *boot, struct fatEntry *fat)
 448 {
 449         cl_t head, p, h, n;
 450         u_int len;
 451         int ret = 0;
 452         int conf;
 453
 454         /*
 455          * pass 1: figure out the cluster chains.
 456          */
 457         for (head = CLUST_FIRST; head < boot->NumClusters; head++) {
 458                 /* find next untravelled chain */
 459                 if (fat[head].head != 0         /* cluster already belongs to some chain */
 460                     || fat[head].next == CLUST_FREE
 461                     || fat[head].next == CLUST_BAD)
 462                         continue;               /* skip it. */
 463
 464                 /* follow the chain and mark all clusters on the way */
 465                 for (len = 0, p = head;
 466                      p >= CLUST_FIRST && p < boot->NumClusters;
 467                      p = fat[p].next) {
 468                         fat[p].head = head;
 469                         len++;
 470                 }
 471
 472                 /* the head record gets the length */
 473                 fat[head].length = fat[head].next == CLUST_FREE ? 0 : len;
 474         }
 475
 476         /*
 477          * pass 2: check for crosslinked chains (we couldn't do this in pass 1 because
 478          * we didn't know the real start of the chain then - would have treated partial
 479          * chains as interlinked with their main chain)
 480          */
 481         for (head = CLUST_FIRST; head < boot->NumClusters; head++) {
 482                 /* find next untravelled chain */
 483                 if (fat[head].head != head)
 484                         continue;
 485
 486                 /* follow the chain to its end (hopefully) */
 487                 for (p = head;
 488                      (n = fat[p].next) >= CLUST_FIRST && n < boot->NumClusters;
 489                      p = n)
 490                         if (fat[n].head != head)
 491                                 break;
 492                 if (n >= CLUST_EOFS)
 493                         continue;
 494
 495                 if (n == CLUST_FREE || n >= CLUST_RSRVD) {
 496                         pwarn("Cluster chain starting at %u ends with cluster marked %s\n",
 497                               head, rsrvdcltype(n));
 498                         ret |= tryclear(boot, fat, head, &fat[p].next);
 499                         continue;
 500                 }
 501                 if (n < CLUST_FIRST || n >= boot->NumClusters) {
 502                         pwarn("Cluster chain starting at %u ends with cluster out of range (%u)\n",
 503                               head, n);
 504                         ret |= tryclear(boot, fat, head, &fat[p].next);
 505                         continue;
 506                 }
 507                 pwarn("Cluster chains starting at %u and %u are linked at cluster %u\n",
 508                       head, fat[n].head, n);
 509                 conf = tryclear(boot, fat, head, &fat[p].next);
 510                 if (ask(0, "Clear chain starting at %u", h = fat[n].head)) {
 511                         if (conf == FSERROR) {
 512                                 /*
 513                                  * Transfer the common chain to the one not cleared above.
 514                                  */
 515                                 for (p = n;
 516                                      p >= CLUST_FIRST && p < boot->NumClusters;
 517                                      p = fat[p].next) {
 518                                         if (h != fat[p].head) {
 519                                                 /*
 520                                                  * Have to reexamine this chain.
 521                                                  */
 522                                                 head--;
 523                                                 break;
 524                                         }
 525                                         fat[p].head = head;
 526                                 }
 527                         }
 528                         clearchain(boot, fat, h);
 529                         conf |= FSFATMOD;
 530                 }
 531                 ret |= conf;
 532         }
 533
 534         return ret;
 535 }
 536
 537 /*
 538  * Write out FATs encoding them from the internal format
 539  */
 540 int
 541 writefat(int fs, struct bootblock *boot, struct fatEntry *fat, int correct_fat)
 542 {
 543         u_char *buffer, *p;
 544         cl_t cl;
 545         int i;
 546         u_int32_t fatsz;
 547         off_t off;
 548         int ret = FSOK;
 549
 550         buffer = malloc(fatsz = boot->FATsecs * boot->BytesPerSec);
 551         if (buffer == NULL) {
 552                 perror("No space for FAT");
 553                 return FSFATAL;
 554         }
 555         memset(buffer, 0, fatsz);
 556         boot->NumFree = 0;
 557         p = buffer;
 558         if (correct_fat) {
 559                 *p++ = (u_char)boot->Media;
 560                 *p++ = 0xff;
 561                 *p++ = 0xff;
 562                 switch (boot->ClustMask) {
 563                 case CLUST16_MASK:
 564                         *p++ = 0xff;
 565                         break;
 566                 case CLUST32_MASK:
 567                         *p++ = 0x0f;
 568                         *p++ = 0xff;
 569                         *p++ = 0xff;
 570                         *p++ = 0xff;
 571                         *p++ = 0x0f;
 572                         break;
 573                 }
 574         } else {
 575                 /* use same FAT signature as the old FAT has */
 576                 int count;
 577                 u_char *old_fat;
 578
 579                 switch (boot->ClustMask) {
 580                 case CLUST32_MASK:
 581                         count = 8;
 582                         break;
 583                 case CLUST16_MASK:
 584                         count = 4;
 585                         break;
 586                 default:
 587                         count = 3;
 588                         break;
 589                 }
 590
 591                 if (!_readfat(fs, boot, boot->ValidFat >= 0 ? boot->ValidFat :0,
 592                                          &old_fat)) {
 593                         free(buffer);
 594                         return FSFATAL;
 595                 }
 596
 597                 memcpy(p, old_fat, count);
 598                 free(old_fat);
 599                 p += count;
 600         }
 601
 602         for (cl = CLUST_FIRST; cl < boot->NumClusters; cl++) {
 603                 switch (boot->ClustMask) {
 604                 case CLUST32_MASK:
 605                         if (fat[cl].next == CLUST_FREE)
 606                                 boot->NumFree++;
 607                         *p++ = (u_char)fat[cl].next;
 608                         *p++ = (u_char)(fat[cl].next >> 8);
 609                         *p++ = (u_char)(fat[cl].next >> 16);
 610                         *p &= 0xf0;
 611                         *p++ |= (fat[cl].next >> 24)&0x0f;
 612                         break;
 613                 case CLUST16_MASK:
 614                         if (fat[cl].next == CLUST_FREE)
 615                                 boot->NumFree++;
 616                         *p++ = (u_char)fat[cl].next;
 617                         *p++ = (u_char)(fat[cl].next >> 8);
 618                         break;
 619                 default:
 620                         if (fat[cl].next == CLUST_FREE)
 621                                 boot->NumFree++;
 622                         if (cl + 1 < boot->NumClusters
 623                             && fat[cl + 1].next == CLUST_FREE)
 624                                 boot->NumFree++;
 625                         *p++ = (u_char)fat[cl].next;
 626                         *p++ = (u_char)((fat[cl].next >> 8) & 0xf)
 627                                |(u_char)(fat[cl+1].next << 4);
 628                         *p++ = (u_char)(fat[++cl].next >> 4);
 629                         break;
 630                 }
 631         }
 632         for (i = 0; i < boot->FATs; i++) {
 633                 off = boot->ResSectors + i * boot->FATsecs;
 634                 off *= boot->BytesPerSec;
 635                 if (lseek(fs, off, SEEK_SET) != off
 636                     || write(fs, buffer, fatsz) != fatsz) {
 637                         perror("Unable to write FAT");
 638                         ret = FSFATAL; /* Return immediately?           XXX */
 639                 }
 640         }
 641         free(buffer);
 642         return ret;
 643 }
 644
 645 /*
 646  * Check a complete in-memory FAT for lost cluster chains
 647  */
 648 int
 649 checklost(int dosfs, struct bootblock *boot, struct fatEntry *fat)
 650 {
 651         cl_t head;
 652         int mod = FSOK;
 653         int ret;
 654
 655         for (head = CLUST_FIRST; head < boot->NumClusters; head++) {
 656                 /* find next untravelled chain */
 657                 if (fat[head].head != head
 658                     || fat[head].next == CLUST_FREE
 659                     || (fat[head].next >= CLUST_RSRVD
 660                         && fat[head].next < CLUST_EOFS)
 661                     || (fat[head].flags & FAT_USED))
 662                         continue;
 663
 664                 pwarn("Lost cluster chain at cluster %u\n%d Cluster(s) lost\n",
 665                       head, fat[head].length);
 666                 mod |= ret = reconnect(dosfs, boot, fat, head);
 667                 if (mod & FSFATAL)
 668                         break;
 669                 if (ret == FSERROR && ask(0, "Clear")) {
 670                         clearchain(boot, fat, head);
 671                         mod |= FSFATMOD;
 672                 }
 673         }
 674         finishlf();
 675
 676         if (boot->FSInfo) {
 677                 ret = 0;
 678                 if (boot->FSFree != boot->NumFree) {
 679                         pwarn("Free space in FSInfo block (%d) not correct (%d)\n",
 680                               boot->FSFree, boot->NumFree);
 681                         if (ask(1, "Fix")) {
 682                                 boot->FSFree = boot->NumFree;
 683                                 ret = 1;
 684                         }
 685                 }
 686                 if (boot->NumFree && fat[boot->FSNext].next != CLUST_FREE) {
 687                         pwarn("Next free cluster in FSInfo block (%u) not free\n",
 688                               boot->FSNext);
 689                         if (ask(1, "Fix"))
 690                                 for (head = CLUST_FIRST; head < boot->NumClusters; head++)
 691                                         if (fat[head].next == CLUST_FREE) {
 692                                                 boot->FSNext = head;
 693                                                 ret = 1;
 694                                                 break;
 695                                         }
 696                 }
 697                 if (ret)
 698                         mod |= writefsinfo(dosfs, boot);
 699         }
 700
 701         return mod;
 702 }