lib/libstand/dosfs.c

   1 /*
   2  * Copyright (c) 1996, 1998 Robert Nordier
   3  * All rights reserved.
   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
  12  *    the documentation and/or other materials provided with the
  13  *    distribution.
  14  *
  15  * THIS SOFTWARE IS PROVIDED BY THE AUTHOR(S) ``AS IS'' AND ANY EXPRESS
  16  * OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
  17  * WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
  18  * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR(S) BE LIABLE FOR ANY
  19  * DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
  20  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE
  21  * GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
  22  * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER
  23  * IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR
  24  * OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN
  25  * IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  26  *
  27  * $FreeBSD: src/lib/libstand/dosfs.c,v 1.4.2.1 2000/05/04 13:47:49 ps Exp $
  28  * $DragonFly: src/lib/libstand/dosfs.c,v 1.2 2003/06/17 04:26:51 dillon Exp $
  29  */
  30
  31 /*
  32  * Readonly filesystem for Microsoft FAT12/FAT16/FAT32 filesystems,
  33  * also supports VFAT.
  34  */
  35
  36 #include <sys/types.h>
  37 #include <string.h>
  38 #include <stddef.h>
  39
  40 #include "stand.h"
  41
  42 #include "dosfs.h"
  43
  44
  45 static int      dos_open(const char *path, struct open_file *fd);
  46 static int      dos_close(struct open_file *fd);
  47 static int      dos_read(struct open_file *fd, void *buf, size_t size, size_t *resid);
  48 static off_t    dos_seek(struct open_file *fd, off_t offset, int whence);
  49 static int      dos_stat(struct open_file *fd, struct stat *sb);
  50
  51 struct fs_ops dosfs_fsops = {
  52         "dosfs",
  53         dos_open,
  54         dos_close,
  55         dos_read,
  56         null_write,
  57         dos_seek,
  58         dos_stat,
  59         null_readdir
  60 };
  61
  62 #define SECSIZ  512             /* sector size */
  63 #define SSHIFT    9             /* SECSIZ shift */
  64 #define DEPSEC   16             /* directory entries per sector */
  65 #define DSHIFT    4             /* DEPSEC shift */
  66 #define LOCLUS    2             /* lowest cluster number */
  67
  68 /* DOS "BIOS Parameter Block" */
  69 typedef struct {
  70     u_char secsiz[2];           /* sector size */
  71     u_char spc;                 /* sectors per cluster */
  72     u_char ressec[2];           /* reserved sectors */
  73     u_char fats;                /* FATs */
  74     u_char dirents[2];          /* root directory entries */
  75     u_char secs[2];             /* total sectors */
  76     u_char media;               /* media descriptor */
  77     u_char spf[2];              /* sectors per FAT */
  78     u_char spt[2];              /* sectors per track */
  79     u_char heads[2];            /* drive heads */
  80     u_char hidsec[4];           /* hidden sectors */
  81     u_char lsecs[4];            /* huge sectors */
  82     u_char lspf[4];             /* huge sectors per FAT */
  83     u_char xflg[2];             /* flags */
  84     u_char vers[2];             /* filesystem version */
  85     u_char rdcl[4];             /* root directory start cluster */
  86     u_char infs[2];             /* filesystem info sector */
  87     u_char bkbs[2];             /* backup boot sector */
  88 } DOS_BPB;
  89
  90 /* Initial portion of DOS boot sector */
  91 typedef struct {
  92     u_char jmp[3];              /* usually 80x86 'jmp' opcode */
  93     u_char oem[8];              /* OEM name and version */
  94     DOS_BPB bpb;                /* BPB */
  95 } DOS_BS;
  96
  97 /* Supply missing "." and ".." root directory entries */
  98 static const char *const dotstr[2] = {".", ".."};
  99 static DOS_DE dot[2] = {
 100     {".       ", "   ", FA_DIR, {0, 0, {0, 0}, {0, 0}, {0, 0}, {0, 0}},
 101      {0, 0}, {0x21, 0}, {0, 0}, {0, 0, 0, 0}},
 102     {"..      ", "   ", FA_DIR, {0, 0, {0, 0}, {0, 0}, {0, 0}, {0, 0}},
 103      {0, 0}, {0x21, 0}, {0, 0}, {0, 0, 0, 0}}
 104 };
 105
 106 /* The usual conversion macros to avoid multiplication and division */
 107 #define bytsec(n)      ((n) >> SSHIFT)
 108 #define secbyt(s)      ((s) << SSHIFT)
 109 #define entsec(e)      ((e) >> DSHIFT)
 110 #define bytblk(fs, n)  ((n) >> (fs)->bshift)
 111 #define blkbyt(fs, b)  ((b) << (fs)->bshift)
 112 #define secblk(fs, s)  ((s) >> ((fs)->bshift - SSHIFT))
 113 #define blksec(fs, b)  ((b) << ((fs)->bshift - SSHIFT))
 114
 115 /* Convert cluster number to offset within filesystem */
 116 #define blkoff(fs, b) (secbyt((fs)->lsndta) + blkbyt(fs, (b) - LOCLUS))
 117
 118 /* Convert cluster number to logical sector number */
 119 #define blklsn(fs, b)  ((fs)->lsndta + blksec(fs, (b) - LOCLUS))
 120
 121 /* Convert cluster number to offset within FAT */
 122 #define fatoff(sz, c)  ((sz) == 12 ? (c) + ((c) >> 1) :  \
 123                         (sz) == 16 ? (c) << 1 :          \
 124                         (c) << 2)
 125
 126 /* Does cluster number reference a valid data cluster? */
 127 #define okclus(fs, c)  ((c) >= LOCLUS && (c) <= (fs)->xclus)
 128
 129 /* Get start cluster from directory entry */
 130 #define stclus(sz, de)  ((sz) != 32 ? cv2((de)->clus) :          \
 131                          ((u_int)cv2((de)->dex.h_clus) << 16) |  \
 132                          cv2((de)->clus))
 133
 134 static int dosunmount(DOS_FS *);
 135 static int parsebs(DOS_FS *, DOS_BS *);
 136 static int namede(DOS_FS *, const char *, DOS_DE **);
 137 static int lookup(DOS_FS *, u_int, const char *, DOS_DE **);
 138 static void cp_xdnm(u_char *, DOS_XDE *);
 139 static void cp_sfn(u_char *, DOS_DE *);
 140 static off_t fsize(DOS_FS *, DOS_DE *);
 141 static int fatcnt(DOS_FS *, u_int);
 142 static int fatget(DOS_FS *, u_int *);
 143 static int fatend(u_int, u_int);
 144 static int ioread(DOS_FS *, u_int, void *, u_int);
 145 static int iobuf(DOS_FS *, u_int);
 146 static int ioget(struct open_file *, u_int, void *, u_int);
 147
 148 /*
 149  * Mount DOS filesystem
 150  */
 151 static int
 152 dos_mount(DOS_FS *fs, struct open_file *fd)
 153 {
 154     int err;
 155
 156     bzero(fs, sizeof(DOS_FS));
 157     fs->fd = fd;
 158     if ((err = !(fs->buf = malloc(SECSIZ)) ? errno : 0) ||
 159         (err = ioget(fs->fd, 0, fs->buf, 1)) ||
 160         (err = parsebs(fs, (DOS_BS *)fs->buf))) {
 161         (void)dosunmount(fs);
 162         return(err);
 163     }
 164     return 0;
 165 }
 166
 167 /*
 168  * Unmount mounted filesystem
 169  */
 170 static int
 171 dos_unmount(DOS_FS *fs)
 172 {
 173     int err;
 174
 175     if (fs->links)
 176         return(EBUSY);
 177     if ((err = dosunmount(fs)))
 178         return(err);
 179     return 0;
 180 }
 181
 182 /*
 183  * Common code shared by dos_mount() and dos_unmount()
 184  */
 185 static int
 186 dosunmount(DOS_FS *fs)
 187 {
 188     if (fs->buf)
 189         free(fs->buf);
 190     free(fs);
 191     return(0);
 192 }
 193
 194 /*
 195  * Open DOS file
 196  */
 197 static int
 198 dos_open(const char *path, struct open_file *fd)
 199 {
 200     DOS_DE *de;
 201     DOS_FILE *f;
 202     DOS_FS *fs;
 203     u_int size, clus;
 204     int err = 0;
 205
 206     /* Allocate mount structure, associate with open */
 207     fs = malloc(sizeof(DOS_FS));
 208
 209     if ((err = dos_mount(fs, fd)))
 210         goto out;
 211
 212     if ((err = namede(fs, path, &de)))
 213         goto out;
 214
 215     clus = stclus(fs->fatsz, de);
 216     size = cv4(de->size);
 217
 218     if ((!(de->attr & FA_DIR) && (!clus != !size)) ||
 219         ((de->attr & FA_DIR) && size) ||
 220         (clus && !okclus(fs, clus))) {
 221         err = EINVAL;
 222         goto out;
 223     }
 224     f = malloc(sizeof(DOS_FILE));
 225     bzero(f, sizeof(DOS_FILE));
 226     f->fs = fs;
 227     fs->links++;
 228     f->de = *de;
 229     fd->f_fsdata = (void *)f;
 230
 231  out:
 232     return(err);
 233 }
 234
 235 /*
 236  * Read from file
 237  */
 238 static int
 239 dos_read(struct open_file *fd, void *buf, size_t nbyte, size_t *resid)
 240 {
 241     off_t size;
 242     u_int nb, off, clus, c, cnt, n;
 243     DOS_FILE *f = (DOS_FILE *)fd->f_fsdata;
 244     int err = 0;
 245
 246     nb = (u_int)nbyte;
 247     if ((size = fsize(f->fs, &f->de)) == -1)
 248         return EINVAL;
 249     if (nb > (n = size - f->offset))
 250         nb = n;
 251     off = f->offset;
 252     if ((clus = stclus(f->fs->fatsz, &f->de)))
 253         off &= f->fs->bsize - 1;
 254     c = f->c;
 255     cnt = nb;
 256     while (cnt) {
 257         n = 0;
 258         if (!c) {
 259             if ((c = clus))
 260                 n = bytblk(f->fs, f->offset);
 261         } else if (!off)
 262             n++;
 263         while (n--) {
 264             if ((err = fatget(f->fs, &c)))
 265                 goto out;
 266             if (!okclus(f->fs, c)) {
 267                 err = EINVAL;
 268                 goto out;
 269             }
 270         }
 271         if (!clus || (n = f->fs->bsize - off) > cnt)
 272             n = cnt;
 273         if ((err = ioread(f->fs, (c ? blkoff(f->fs, c) :
 274                                       secbyt(f->fs->lsndir)) + off,
 275                           buf, n)))
 276             goto out;
 277         f->offset += n;
 278         f->c = c;
 279         off = 0;
 280         buf += n;
 281         cnt -= n;
 282     }
 283  out:
 284     if (resid)
 285         *resid = nbyte - nb + cnt;
 286     return(err);
 287 }
 288
 289 /*
 290  * Reposition within file
 291  */
 292 static off_t
 293 dos_seek(struct open_file *fd, off_t offset, int whence)
 294 {
 295     off_t off;
 296     u_int size;
 297     DOS_FILE *f = (DOS_FILE *)fd->f_fsdata;
 298
 299     size = cv4(f->de.size);
 300     switch (whence) {
 301     case SEEK_SET:
 302         off = 0;
 303         break;
 304     case SEEK_CUR:
 305         off = f->offset;
 306         break;
 307     case SEEK_END:
 308         off = size;
 309         break;
 310     default:
 311         return(-1);
 312     }
 313     off += offset;
 314     if (off < 0 || off > size)
 315         return(-1);
 316     f->offset = (u_int)off;
 317     f->c = 0;
 318     return(off);
 319 }
 320
 321 /*
 322  * Close open file
 323  */
 324 static int
 325 dos_close(struct open_file *fd)
 326 {
 327     DOS_FILE *f = (DOS_FILE *)fd->f_fsdata;
 328     DOS_FS *fs;
 329
 330     fd->f_fsdata = NULL;
 331     if (f) {
 332         fs = f->fs;
 333         f->fs = NULL;
 334         fs->links--;
 335         free(f);
 336         dos_unmount(fs);
 337     }
 338     return 0;
 339 }
 340
 341 /*
 342  * Return some stat information on a file.
 343  */
 344 static int
 345 dos_stat(struct open_file *fd, struct stat *sb)
 346 {
 347     DOS_FILE *f = (DOS_FILE *)fd->f_fsdata;
 348
 349     /* only important stuff */
 350     sb->st_mode = f->de.attr & FA_DIR ? S_IFDIR | 0555 : S_IFREG | 0444;
 351     sb->st_nlink = 1;
 352     sb->st_uid = 0;
 353     sb->st_gid = 0;
 354     if ((sb->st_size = fsize(f->fs, &f->de)) == -1)
 355         return EINVAL;
 356     return (0);
 357 }
 358
 359 /*
 360  * Parse DOS boot sector
 361  */
 362 static int
 363 parsebs(DOS_FS *fs, DOS_BS *bs)
 364 {
 365     u_int sc;
 366
 367     if ((bs->jmp[0] != 0x69 &&
 368          bs->jmp[0] != 0xe9 &&
 369          (bs->jmp[0] != 0xeb || bs->jmp[2] != 0x90)) ||
 370         bs->bpb.media < 0xf0)
 371         return EINVAL;
 372     if (cv2(bs->bpb.secsiz) != SECSIZ)
 373         return EINVAL;
 374     if (!(fs->spc = bs->bpb.spc) || fs->spc & (fs->spc - 1))
 375         return EINVAL;
 376     fs->bsize = secbyt(fs->spc);
 377     fs->bshift = ffs(fs->bsize) - 1;
 378     if ((fs->spf = cv2(bs->bpb.spf))) {
 379         if (bs->bpb.fats != 2)
 380             return EINVAL;
 381         if (!(fs->dirents = cv2(bs->bpb.dirents)))
 382             return EINVAL;
 383     } else {
 384         if (!(fs->spf = cv4(bs->bpb.lspf)))
 385             return EINVAL;
 386         if (!bs->bpb.fats || bs->bpb.fats > 16)
 387             return EINVAL;
 388         if ((fs->rdcl = cv4(bs->bpb.rdcl)) < LOCLUS)
 389             return EINVAL;
 390     }
 391     if (!(fs->lsnfat = cv2(bs->bpb.ressec)))
 392         return EINVAL;
 393     fs->lsndir = fs->lsnfat + fs->spf * bs->bpb.fats;
 394     fs->lsndta = fs->lsndir + entsec(fs->dirents);
 395     if (!(sc = cv2(bs->bpb.secs)) && !(sc = cv4(bs->bpb.lsecs)))
 396         return EINVAL;
 397     if (fs->lsndta > sc)
 398         return EINVAL;
 399     if ((fs->xclus = secblk(fs, sc - fs->lsndta) + 1) < LOCLUS)
 400         return EINVAL;
 401     fs->fatsz = fs->dirents ? fs->xclus < 0xff6 ? 12 : 16 : 32;
 402     sc = (secbyt(fs->spf) << 1) / (fs->fatsz >> 2) - 1;
 403     if (fs->xclus > sc)
 404         fs->xclus = sc;
 405     return 0;
 406 }
 407
 408 /*
 409  * Return directory entry from path
 410  */
 411 static int
 412 namede(DOS_FS *fs, const char *path, DOS_DE **dep)
 413 {
 414     char name[256];
 415     DOS_DE *de;
 416     char *s;
 417     size_t n;
 418     int err;
 419
 420     err = 0;
 421     de = dot;
 422     if (*path == '/')
 423         path++;
 424     while (*path) {
 425         if (!(s = strchr(path, '/')))
 426             s = strchr(path, 0);
 427         if ((n = s - path) > 255)
 428             return ENAMETOOLONG;
 429         memcpy(name, path, n);
 430         name[n] = 0;
 431         path = s;
 432         if (!(de->attr & FA_DIR))
 433             return ENOTDIR;
 434         if ((err = lookup(fs, stclus(fs->fatsz, de), name, &de)))
 435             return err;
 436         if (*path == '/')
 437             path++;
 438     }
 439     *dep = de;
 440     return 0;
 441 }
 442
 443 /*
 444  * Lookup path segment
 445  */
 446 static int
 447 lookup(DOS_FS *fs, u_int clus, const char *name, DOS_DE **dep)
 448 {
 449     static DOS_DIR dir[DEPSEC];
 450     u_char lfn[261];
 451     u_char sfn[13];
 452     u_int nsec, lsec, xdn, chk, sec, ent, x;
 453     int err, ok, i;
 454
 455     if (!clus)
 456         for (ent = 0; ent < 2; ent++)
 457             if (!strcasecmp(name, dotstr[ent])) {
 458                 *dep = dot + ent;
 459                 return 0;
 460             }
 461     if (!clus && fs->fatsz == 32)
 462         clus = fs->rdcl;
 463     nsec = !clus ? entsec(fs->dirents) : fs->spc;
 464     lsec = 0;
 465     xdn = chk = 0;
 466     for (;;) {
 467         if (!clus && !lsec)
 468             lsec = fs->lsndir;
 469         else if (okclus(fs, clus))
 470             lsec = blklsn(fs, clus);
 471         else
 472             return EINVAL;
 473         for (sec = 0; sec < nsec; sec++) {
 474             if ((err = ioget(fs->fd, lsec + sec, dir, 1)))
 475                 return err;
 476             for (ent = 0; ent < DEPSEC; ent++) {
 477                 if (!*dir[ent].de.name)
 478                     return ENOENT;
 479                 if (*dir[ent].de.name != 0xe5) {
 480                     if ((dir[ent].de.attr & FA_MASK) == FA_XDE) {
 481                         x = dir[ent].xde.seq;
 482                         if (x & 0x40 || (x + 1 == xdn &&
 483                                          dir[ent].xde.chk == chk)) {
 484                             if (x & 0x40) {
 485                                 chk = dir[ent].xde.chk;
 486                                 x &= ~0x40;
 487                             }
 488                             if (x >= 1 && x <= 20) {
 489                                 cp_xdnm(lfn, &dir[ent].xde);
 490                                 xdn = x;
 491                                 continue;
 492                             }
 493                         }
 494                     } else if (!(dir[ent].de.attr & FA_LABEL)) {
 495                         if ((ok = xdn == 1)) {
 496                             for (x = 0, i = 0; i < 11; i++)
 497                                 x = ((((x & 1) << 7) | (x >> 1)) +
 498                                      dir[ent].de.name[i]) & 0xff;
 499                             ok = chk == x &&
 500                                 !strcasecmp(name, (const char *)lfn);
 501                         }
 502                         if (!ok) {
 503                             cp_sfn(sfn, &dir[ent].de);
 504                             ok = !strcasecmp(name, (const char *)sfn);
 505                         }
 506                         if (ok) {
 507                             *dep = &dir[ent].de;
 508                             return 0;
 509                         }
 510                     }
 511                 }
 512                 xdn = 0;
 513             }
 514         }
 515         if (!clus)
 516             break;
 517         if ((err = fatget(fs, &clus)))
 518             return err;
 519         if (fatend(fs->fatsz, clus))
 520             break;
 521     }
 522     return ENOENT;
 523 }
 524
 525 /*
 526  * Copy name from extended directory entry
 527  */
 528 static void
 529 cp_xdnm(u_char *lfn, DOS_XDE *xde)
 530 {
 531     static struct {
 532         u_int off;
 533         u_int dim;
 534     } ix[3] = {
 535         {offsetof(DOS_XDE, name1), sizeof(xde->name1) / 2},
 536         {offsetof(DOS_XDE, name2), sizeof(xde->name2) / 2},
 537         {offsetof(DOS_XDE, name3), sizeof(xde->name3) / 2}
 538     };
 539     u_char *p;
 540     u_int n, x, c;
 541
 542     lfn += 13 * ((xde->seq & ~0x40) - 1);
 543     for (n = 0; n < 3; n++)
 544         for (p = (u_char *)xde + ix[n].off, x = ix[n].dim; x;
 545              p += 2, x--) {
 546             if ((c = cv2(p)) && (c < 32 || c > 127))
 547                 c = '?';
 548             if (!(*lfn++ = c))
 549                 return;
 550         }
 551     if (xde->seq & 0x40)
 552         *lfn = 0;
 553 }
 554
 555 /*
 556  * Copy short filename
 557  */
 558 static void
 559 cp_sfn(u_char *sfn, DOS_DE *de)
 560 {
 561     u_char *p;
 562     int j, i;
 563
 564     p = sfn;
 565     if (*de->name != ' ') {
 566         for (j = 7; de->name[j] == ' '; j--);
 567         for (i = 0; i <= j; i++)
 568             *p++ = de->name[i];
 569         if (*de->ext != ' ') {
 570             *p++ = '.';
 571             for (j = 2; de->ext[j] == ' '; j--);
 572             for (i = 0; i <= j; i++)
 573                 *p++ = de->ext[i];
 574         }
 575     }
 576     *p = 0;
 577     if (*sfn == 5)
 578         *sfn = 0xe5;
 579 }
 580
 581 /*
 582  * Return size of file in bytes
 583  */
 584 static off_t
 585 fsize(DOS_FS *fs, DOS_DE *de)
 586 {
 587    u_long size;
 588    u_int c;
 589    int n;
 590
 591    if (!(size = cv4(de->size)) && de->attr & FA_DIR) {
 592       if (!(c = cv2(de->clus)))
 593          size = fs->dirents * sizeof(DOS_DE);
 594       else {
 595          if ((n = fatcnt(fs, c)) == -1)
 596             return n;
 597          size = blkbyt(fs, n);
 598       }
 599    }
 600    return size;
 601 }
 602
 603 /*
 604  * Count number of clusters in chain
 605  */
 606 static int
 607 fatcnt(DOS_FS *fs, u_int c)
 608 {
 609    int n;
 610
 611    for (n = 0; okclus(fs, c); n++)
 612       if (fatget(fs, &c))
 613           return -1;
 614    return fatend(fs->fatsz, c) ? n : -1;
 615 }
 616
 617 /*
 618  * Get next cluster in cluster chain
 619  */
 620 static int
 621 fatget(DOS_FS *fs, u_int *c)
 622 {
 623     u_char buf[4];
 624     u_int x;
 625     int err;
 626
 627     err = ioread(fs, secbyt(fs->lsnfat) + fatoff(fs->fatsz, *c), buf,
 628                  fs->fatsz != 32 ? 2 : 4);
 629     if (err)
 630         return err;
 631     x = fs->fatsz != 32 ? cv2(buf) : cv4(buf);
 632     *c = fs->fatsz == 12 ? *c & 1 ? x >> 4 : x & 0xfff : x;
 633     return 0;
 634 }
 635
 636 /*
 637  * Is cluster an end-of-chain marker?
 638  */
 639 static int
 640 fatend(u_int sz, u_int c)
 641 {
 642     return c > (sz == 12 ? 0xff7U : sz == 16 ? 0xfff7U : 0xffffff7);
 643 }
 644
 645 /*
 646  * Offset-based I/O primitive
 647  */
 648 static int
 649 ioread(DOS_FS *fs, u_int offset, void *buf, u_int nbyte)
 650 {
 651     char *s;
 652     u_int off, n;
 653     int err;
 654
 655     s = buf;
 656     if ((off = offset & (SECSIZ - 1))) {
 657         offset -= off;
 658         if ((err = iobuf(fs, bytsec(offset))))
 659             return err;
 660         offset += SECSIZ;
 661         if ((n = SECSIZ - off) > nbyte)
 662             n = nbyte;
 663         memcpy(s, fs->buf + off, n);
 664         s += n;
 665         nbyte -= n;
 666     }
 667     n = nbyte & (SECSIZ - 1);
 668     if (nbyte -= n) {
 669         if ((err = ioget(fs->fd, bytsec(offset), s, bytsec(nbyte))))
 670             return err;
 671         offset += nbyte;
 672         s += nbyte;
 673     }
 674     if (n) {
 675         if ((err = iobuf(fs, bytsec(offset))))
 676             return err;
 677         memcpy(s, fs->buf, n);
 678     }
 679     return 0;
 680 }
 681
 682 /*
 683  * Buffered sector-based I/O primitive
 684  */
 685 static int
 686 iobuf(DOS_FS *fs, u_int lsec)
 687 {
 688     int err;
 689
 690     if (fs->bufsec != lsec) {
 691         if ((err = ioget(fs->fd, lsec, fs->buf, 1)))
 692             return err;
 693         fs->bufsec = lsec;
 694     }
 695     return 0;
 696 }
 697
 698 /*
 699  * Sector-based I/O primitive
 700  */
 701 static int
 702 ioget(struct open_file *fd, u_int lsec, void *buf, u_int nsec)
 703 {
 704     int err;
 705
 706     if ((err = (fd->f_dev->dv_strategy)(fd->f_devdata, F_READ, lsec,
 707                                         secbyt(nsec), buf, NULL)))
 708         return(err);
 709     return(0);
 710 }