u-tools/apps/fsck_msdos/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: src/sbin/fsck_msdosfs/fat.c,v 1.9 2008/01/31 13:22:13 yar Exp $";
  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(1, "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         printf("Attempting to allocate %u KB for FAT\n",
 174                 (boot->FATsecs * boot->BytesPerSec) / 1024);
 175
 176         *buffer = malloc(boot->FATsecs * boot->BytesPerSec);
 177         if (*buffer == NULL) {
 178                 perror("No space for FAT");
 179                 return 0;
 180         }
 181
 182         off = boot->ResSectors + no * boot->FATsecs;
 183         off *= boot->BytesPerSec;
 184
 185         if (lseek(fs, off, SEEK_SET) != off) {
 186                 perror("Unable to read FAT");
 187                 goto err;
 188         }
 189
 190         if (read(fs, *buffer, boot->FATsecs * boot->BytesPerSec)
 191             != boot->FATsecs * boot->BytesPerSec) {
 192                 perror("Unable to read FAT");
 193                 goto err;
 194         }
 195
 196         return 1;
 197
 198     err:
 199         free(*buffer);
 200         return 0;
 201 }
 202
 203 /*
 204  * Read a FAT and decode it into internal format
 205  */
 206 int
 207 readfat(int fs, struct bootblock *boot, int no, struct fatEntry **fp)
 208 {
 209         struct fatEntry *fat;
 210         u_char *buffer, *p;
 211         cl_t cl;
 212         int ret = FSOK;
 213
 214         boot->NumFree = boot->NumBad = 0;
 215
 216         if (!_readfat(fs, boot, no, &buffer))
 217                 return FSFATAL;
 218
 219         fat = calloc(boot->NumClusters, sizeof(struct fatEntry));
 220         if (fat == NULL) {
 221                 perror("No space for FAT");
 222                 free(buffer);
 223                 return FSFATAL;
 224         }
 225
 226         if (buffer[0] != boot->Media
 227             || buffer[1] != 0xff || buffer[2] != 0xff
 228             || (boot->ClustMask == CLUST16_MASK && buffer[3] != 0xff)
 229             || (boot->ClustMask == CLUST32_MASK
 230                 && ((buffer[3]&0x0f) != 0x0f
 231                     || buffer[4] != 0xff || buffer[5] != 0xff
 232                     || buffer[6] != 0xff || (buffer[7]&0x0f) != 0x0f))) {
 233
 234                 /* Windows 95 OSR2 (and possibly any later) changes
 235                  * the FAT signature to 0xXXffff7f for FAT16 and to
 236                  * 0xXXffff0fffffff07 for FAT32 upon boot, to know that the
 237                  * file system is dirty if it doesn't reboot cleanly.
 238                  * Check this special condition before errorring out.
 239                  */
 240                 if (buffer[0] == boot->Media && buffer[1] == 0xff
 241                     && buffer[2] == 0xff
 242                     && ((boot->ClustMask == CLUST16_MASK && buffer[3] == 0x7f)
 243                         || (boot->ClustMask == CLUST32_MASK
 244                             && buffer[3] == 0x0f && buffer[4] == 0xff
 245                             && buffer[5] == 0xff && buffer[6] == 0xff
 246                             && buffer[7] == 0x07)))
 247                         ret |= FSDIRTY;
 248                 else {
 249                         /* just some odd byte sequence in FAT */
 250
 251                         switch (boot->ClustMask) {
 252                         case CLUST32_MASK:
 253                                 pwarn("%s (%02x%02x%02x%02x%02x%02x%02x%02x)\n",
 254                                       "FAT starts with odd byte sequence",
 255                                       buffer[0], buffer[1], buffer[2], buffer[3],
 256                                       buffer[4], buffer[5], buffer[6], buffer[7]);
 257                                 break;
 258                         case CLUST16_MASK:
 259                                 pwarn("%s (%02x%02x%02x%02x)\n",
 260                                     "FAT starts with odd byte sequence",
 261                                     buffer[0], buffer[1], buffer[2], buffer[3]);
 262                                 break;
 263                         default:
 264                                 pwarn("%s (%02x%02x%02x)\n",
 265                                     "FAT starts with odd byte sequence",
 266                                     buffer[0], buffer[1], buffer[2]);
 267                                 break;
 268                         }
 269
 270
 271                         if (ask(1, "Correct"))
 272                                 ret |= FSFIXFAT;
 273                 }
 274         }
 275         switch (boot->ClustMask) {
 276         case CLUST32_MASK:
 277                 p = buffer + 8;
 278                 break;
 279         case CLUST16_MASK:
 280                 p = buffer + 4;
 281                 break;
 282         default:
 283                 p = buffer + 3;
 284                 break;
 285         }
 286         for (cl = CLUST_FIRST; cl < boot->NumClusters;) {
 287                 switch (boot->ClustMask) {
 288                 case CLUST32_MASK:
 289                         fat[cl].next = p[0] + (p[1] << 8)
 290                                        + (p[2] << 16) + (p[3] << 24);
 291                         fat[cl].next &= boot->ClustMask;
 292                         ret |= checkclnum(boot, no, cl, &fat[cl].next);
 293                         cl++;
 294                         p += 4;
 295                         break;
 296                 case CLUST16_MASK:
 297                         fat[cl].next = p[0] + (p[1] << 8);
 298                         ret |= checkclnum(boot, no, cl, &fat[cl].next);
 299                         cl++;
 300                         p += 2;
 301                         break;
 302                 default:
 303                         fat[cl].next = (p[0] + (p[1] << 8)) & 0x0fff;
 304                         ret |= checkclnum(boot, no, cl, &fat[cl].next);
 305                         cl++;
 306                         if (cl >= boot->NumClusters)
 307                                 break;
 308                         fat[cl].next = ((p[1] >> 4) + (p[2] << 4)) & 0x0fff;
 309                         ret |= checkclnum(boot, no, cl, &fat[cl].next);
 310                         cl++;
 311                         p += 3;
 312                         break;
 313                 }
 314         }
 315
 316         free(buffer);
 317         *fp = fat;
 318         return ret;
 319 }
 320
 321 /*
 322  * Get type of reserved cluster
 323  */
 324 char *
 325 rsrvdcltype(cl_t cl)
 326 {
 327         if (cl == CLUST_FREE)
 328                 return "free";
 329         if (cl < CLUST_BAD)
 330                 return "reserved";
 331         if (cl > CLUST_BAD)
 332                 return "as EOF";
 333         return "bad";
 334 }
 335
 336 static int
 337 clustdiffer(cl_t cl, cl_t *cp1, cl_t *cp2, int fatnum)
 338 {
 339         if (*cp1 == CLUST_FREE || *cp1 >= CLUST_RSRVD) {
 340                 if (*cp2 == CLUST_FREE || *cp2 >= CLUST_RSRVD) {
 341                         if ((*cp1 != CLUST_FREE && *cp1 < CLUST_BAD
 342                              && *cp2 != CLUST_FREE && *cp2 < CLUST_BAD)
 343                             || (*cp1 > CLUST_BAD && *cp2 > CLUST_BAD)) {
 344                                 pwarn("Cluster %u is marked %s with different indicators\n",
 345                                       cl, rsrvdcltype(*cp1));
 346                                 if (ask(1, "Fix")) {
 347                                         *cp2 = *cp1;
 348                                         return FSFATMOD;
 349                                 }
 350                                 return FSFATAL;
 351                         }
 352                         pwarn("Cluster %u is marked %s in FAT 0, %s in FAT %d\n",
 353                               cl, rsrvdcltype(*cp1), rsrvdcltype(*cp2), fatnum);
 354                         if (ask(1, "Use FAT 0's entry")) {
 355                                 *cp2 = *cp1;
 356                                 return FSFATMOD;
 357                         }
 358                         if (ask(1, "Use FAT %d's entry", fatnum)) {
 359                                 *cp1 = *cp2;
 360                                 return FSFATMOD;
 361                         }
 362                         return FSFATAL;
 363                 }
 364                 pwarn("Cluster %u is marked %s in FAT 0, but continues with cluster %u in FAT %d\n",
 365                       cl, rsrvdcltype(*cp1), *cp2, fatnum);
 366                 if (ask(1, "Use continuation from FAT %d", fatnum)) {
 367                         *cp1 = *cp2;
 368                         return FSFATMOD;
 369                 }
 370                 if (ask(1, "Use mark from FAT 0")) {
 371                         *cp2 = *cp1;
 372                         return FSFATMOD;
 373                 }
 374                 return FSFATAL;
 375         }
 376         if (*cp2 == CLUST_FREE || *cp2 >= CLUST_RSRVD) {
 377                 pwarn("Cluster %u continues with cluster %u in FAT 0, but is marked %s in FAT %d\n",
 378                       cl, *cp1, rsrvdcltype(*cp2), fatnum);
 379                 if (ask(1, "Use continuation from FAT 0")) {
 380                         *cp2 = *cp1;
 381                         return FSFATMOD;
 382                 }
 383                 if (ask(1, "Use mark from FAT %d", fatnum)) {
 384                         *cp1 = *cp2;
 385                         return FSFATMOD;
 386                 }
 387                 return FSERROR;
 388         }
 389         pwarn("Cluster %u continues with cluster %u in FAT 0, but with cluster %u in FAT %d\n",
 390               cl, *cp1, *cp2, fatnum);
 391         if (ask(1, "Use continuation from FAT 0")) {
 392                 *cp2 = *cp1;
 393                 return FSFATMOD;
 394         }
 395         if (ask(1, "Use continuation from FAT %d", fatnum)) {
 396                 *cp1 = *cp2;
 397                 return FSFATMOD;
 398         }
 399         return FSERROR;
 400 }
 401
 402 /*
 403  * Compare two FAT copies in memory. Resolve any conflicts and merge them
 404  * into the first one.
 405  */
 406 int
 407 comparefat(struct bootblock *boot, struct fatEntry *first,
 408     struct fatEntry *second, int fatnum)
 409 {
 410         cl_t cl;
 411         int ret = FSOK;
 412
 413         for (cl = CLUST_FIRST; cl < boot->NumClusters; cl++)
 414                 if (first[cl].next != second[cl].next)
 415                         ret |= clustdiffer(cl, &first[cl].next, &second[cl].next, fatnum);
 416         return ret;
 417 }
 418
 419 void
 420 clearchain(struct bootblock *boot, struct fatEntry *fat, cl_t head)
 421 {
 422         cl_t p, q;
 423
 424         for (p = head; p >= CLUST_FIRST && p < boot->NumClusters; p = q) {
 425                 if (fat[p].head != head)
 426                         break;
 427                 q = fat[p].next;
 428                 fat[p].next = fat[p].head = CLUST_FREE;
 429                 fat[p].length = 0;
 430         }
 431 }
 432
 433 int
 434 tryclear(struct bootblock *boot, struct fatEntry *fat, cl_t head, cl_t *trunc)
 435 {
 436         if (ask(1, "Clear chain starting at %u", head)) {
 437                 clearchain(boot, fat, head);
 438                 return FSFATMOD;
 439         } else if (ask(1, "Truncate")) {
 440                 *trunc = CLUST_EOF;
 441                 return FSFATMOD;
 442         } else
 443                 return FSERROR;
 444 }
 445
 446 /*
 447  * Check a complete FAT in-memory for crosslinks
 448  */
 449 int
 450 checkfat(struct bootblock *boot, struct fatEntry *fat)
 451 {
 452         cl_t head, p, h, n, wdk;
 453         u_int len;
 454         int ret = 0;
 455         int conf;
 456
 457         /*
 458          * pass 1: figure out the cluster chains.
 459          */
 460         for (head = CLUST_FIRST; head < boot->NumClusters; head++) {
 461                 /* find next untravelled chain */
 462                 if (fat[head].head != 0         /* cluster already belongs to some chain */
 463                     || fat[head].next == CLUST_FREE
 464                     || fat[head].next == CLUST_BAD)
 465                         continue;               /* skip it. */
 466
 467                 /* follow the chain and mark all clusters on the way */
 468                 for (len = 0, p = head;
 469                          p >= CLUST_FIRST && p < boot->NumClusters;
 470                          p = fat[p].next) {
 471                                 /* we have to check the len, to avoid infinite loop */
 472                                 if (len > boot->NumClusters) {
 473                                         printf("detect cluster chain loop: head %u for p %u\n", head, p);
 474                                         break;
 475                         }
 476
 477                         fat[p].head = head;
 478                         len++;
 479                 }
 480
 481                 /* the head record gets the length */
 482                 fat[head].length = fat[head].next == CLUST_FREE ? 0 : len;
 483         }
 484
 485         /*
 486          * pass 2: check for crosslinked chains (we couldn't do this in pass 1 because
 487          * we didn't know the real start of the chain then - would have treated partial
 488          * chains as interlinked with their main chain)
 489          */
 490         for (head = CLUST_FIRST; head < boot->NumClusters; head++) {
 491                 /* find next untravelled chain */
 492                 if (fat[head].head != head)
 493                         continue;
 494
 495                 /* follow the chain to its end (hopefully) */
 496                 /* also possible infinite loop, that's why I insert wdk counter */
 497                 for (p = head,wdk=boot->NumClusters;
 498                      (n = fat[p].next) >= CLUST_FIRST && n < boot->NumClusters && wdk;
 499                                  p = n,wdk--) {
 500                         if (fat[n].head != head)
 501                                 break;
 502                 }
 503
 504                 if (n >= CLUST_EOFS)
 505                         continue;
 506
 507                 if (n == CLUST_FREE || n >= CLUST_RSRVD) {
 508                         pwarn("Cluster chain starting at %u ends with cluster marked %s\n",
 509                               head, rsrvdcltype(n));
 510                         ret |= tryclear(boot, fat, head, &fat[p].next);
 511                         continue;
 512                 }
 513                 if (n < CLUST_FIRST || n >= boot->NumClusters) {
 514                         pwarn("Cluster chain starting at %u ends with cluster out of range (%u)\n",
 515                               head, n);
 516                         ret |= tryclear(boot, fat, head, &fat[p].next);
 517                         continue;
 518                 }
 519                 pwarn("Cluster chains starting at %u and %u are linked at cluster %u\n",
 520                       head, fat[n].head, n);
 521                 conf = tryclear(boot, fat, head, &fat[p].next);
 522                 if (ask(1, "Clear chain starting at %u", h = fat[n].head)) {
 523                         if (conf == FSERROR) {
 524                                 /*
 525                                  * Transfer the common chain to the one not cleared above.
 526                                  */
 527                                 for (p = n;
 528                                      p >= CLUST_FIRST && p < boot->NumClusters;
 529                                      p = fat[p].next) {
 530                                         if (h != fat[p].head) {
 531                                                 /*
 532                                                  * Have to reexamine this chain.
 533                                                  */
 534                                                 head--;
 535                                                 break;
 536                                         }
 537                                         fat[p].head = head;
 538                                 }
 539                         }
 540                         clearchain(boot, fat, h);
 541                         conf |= FSFATMOD;
 542                 }
 543                 ret |= conf;
 544         }
 545
 546         return ret;
 547 }
 548
 549 /*
 550  * Write out FATs encoding them from the internal format
 551  */
 552 int
 553 writefat(int fs, struct bootblock *boot, struct fatEntry *fat, int correct_fat)
 554 {
 555         u_char *buffer, *p;
 556         cl_t cl;
 557         int i;
 558         u_int32_t fatsz;
 559         off_t off;
 560         int ret = FSOK;
 561
 562         buffer = malloc(fatsz = boot->FATsecs * boot->BytesPerSec);
 563         if (buffer == NULL) {
 564                 perror("No space for FAT");
 565                 return FSFATAL;
 566         }
 567         memset(buffer, 0, fatsz);
 568         boot->NumFree = 0;
 569         p = buffer;
 570         if (correct_fat) {
 571                 *p++ = (u_char)boot->Media;
 572                 *p++ = 0xff;
 573                 *p++ = 0xff;
 574                 switch (boot->ClustMask) {
 575                 case CLUST16_MASK:
 576                         *p++ = 0xff;
 577                         break;
 578                 case CLUST32_MASK:
 579                         *p++ = 0x0f;
 580                         *p++ = 0xff;
 581                         *p++ = 0xff;
 582                         *p++ = 0xff;
 583                         *p++ = 0x0f;
 584                         break;
 585                 }
 586         } else {
 587                 /* use same FAT signature as the old FAT has */
 588                 int count;
 589                 u_char *old_fat;
 590
 591                 switch (boot->ClustMask) {
 592                 case CLUST32_MASK:
 593                         count = 8;
 594                         break;
 595                 case CLUST16_MASK:
 596                         count = 4;
 597                         break;
 598                 default:
 599                         count = 3;
 600                         break;
 601                 }
 602
 603                 if (!_readfat(fs, boot, boot->ValidFat >= 0 ? boot->ValidFat :0,
 604                                          &old_fat)) {
 605                         free(buffer);
 606                         return FSFATAL;
 607                 }
 608
 609                 memcpy(p, old_fat, count);
 610                 free(old_fat);
 611                 p += count;
 612         }
 613
 614         for (cl = CLUST_FIRST; cl < boot->NumClusters; cl++) {
 615                 switch (boot->ClustMask) {
 616                 case CLUST32_MASK:
 617                         if (fat[cl].next == CLUST_FREE)
 618                                 boot->NumFree++;
 619                         *p++ = (u_char)fat[cl].next;
 620                         *p++ = (u_char)(fat[cl].next >> 8);
 621                         *p++ = (u_char)(fat[cl].next >> 16);
 622                         *p &= 0xf0;
 623                         *p++ |= (fat[cl].next >> 24)&0x0f;
 624                         break;
 625                 case CLUST16_MASK:
 626                         if (fat[cl].next == CLUST_FREE)
 627                                 boot->NumFree++;
 628                         *p++ = (u_char)fat[cl].next;
 629                         *p++ = (u_char)(fat[cl].next >> 8);
 630                         break;
 631                 default:
 632                         if (fat[cl].next == CLUST_FREE)
 633                                 boot->NumFree++;
 634                         if (cl + 1 < boot->NumClusters
 635                             && fat[cl + 1].next == CLUST_FREE)
 636                                 boot->NumFree++;
 637                         *p++ = (u_char)fat[cl].next;
 638                         *p++ = (u_char)((fat[cl].next >> 8) & 0xf)
 639                                |(u_char)(fat[cl+1].next << 4);
 640                         *p++ = (u_char)(fat[++cl].next >> 4);
 641                         break;
 642                 }
 643         }
 644         for (i = 0; i < boot->FATs; i++) {
 645                 off = boot->ResSectors + i * boot->FATsecs;
 646                 off *= boot->BytesPerSec;
 647                 if (lseek(fs, off, SEEK_SET) != off
 648                     || write(fs, buffer, fatsz) != fatsz) {
 649                         perror("Unable to write FAT");
 650                         ret = FSFATAL; /* Return immediately?           XXX */
 651                 }
 652         }
 653         free(buffer);
 654         return ret;
 655 }
 656
 657 /*
 658  * Check a complete in-memory FAT for lost cluster chains
 659  */
 660 int
 661 checklost(int dosfs, struct bootblock *boot, struct fatEntry *fat)
 662 {
 663         cl_t head;
 664         int mod = FSOK;
 665         int ret;
 666
 667         for (head = CLUST_FIRST; head < boot->NumClusters; head++) {
 668                 /* find next untravelled chain */
 669                 if (fat[head].head != head
 670                     || fat[head].next == CLUST_FREE
 671                     || (fat[head].next >= CLUST_RSRVD
 672                         && fat[head].next < CLUST_EOFS)
 673                     || (fat[head].flags & FAT_USED))
 674                         continue;
 675
 676                 pwarn("Lost cluster chain at cluster %u\n%d Cluster(s) lost\n",
 677                       head, fat[head].length);
 678                 mod |= ret = reconnect(dosfs, boot, fat, head);
 679                 if (mod & FSFATAL) {
 680                         /* If the reconnect failed, then just clear the chain */
 681                         pwarn("Error reconnecting chain - clearing\n");
 682                         mod &= ~FSFATAL;
 683                         clearchain(boot, fat, head);
 684                         mod |= FSFATMOD;
 685                         continue;
 686                 }
 687                 if (ret == FSERROR && ask(1, "Clear")) {
 688                         clearchain(boot, fat, head);
 689                         mod |= FSFATMOD;
 690                 }
 691         }
 692         finishlf();
 693
 694         if (boot->FSInfo) {
 695                 ret = 0;
 696                 if (boot->FSFree != boot->NumFree) {
 697                         pwarn("Free space in FSInfo block (%d) not correct (%d)\n",
 698                               boot->FSFree, boot->NumFree);
 699                         if (ask(1, "Fix")) {
 700                                 boot->FSFree = boot->NumFree;
 701                                 ret = 1;
 702                         }
 703                 }
 704
 705                 if (boot->FSNext > boot->NumClusters  ) {
 706                         pwarn("FSNext block (%d) not correct NumClusters (%d)\n",
 707                                         boot->FSNext, boot->NumClusters);
 708                         boot->FSNext=CLUST_FIRST; // boot->FSNext can have -1 value.
 709                 }
 710
 711                 if (boot->NumFree && fat[boot->FSNext].next != CLUST_FREE) {
 712                         pwarn("Next free cluster in FSInfo block (%u) not free\n",
 713                                         boot->FSNext);
 714                         if (ask(1, "Fix"))
 715                                 for (head = CLUST_FIRST; head < boot->NumClusters; head++)
 716                                         if (fat[head].next == CLUST_FREE) {
 717                                                 boot->FSNext = head;
 718                                                 ret = 1;
 719                                                 break;
 720                                         }
 721                 }
 722                 if (ret)
 723                         mod |= writefsinfo(dosfs, boot);
 724         }
 725
 726         return mod;
 727 }