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 = f->fs;
 329
 330     f->fs->links--;
 331     free(f);
 332     dos_unmount(fs);
 333     return 0;
 334 }
 335
 336 /*
 337  * Return some stat information on a file.
 338  */
 339 static int
 340 dos_stat(struct open_file *fd, struct stat *sb)
 341 {
 342     DOS_FILE *f = (DOS_FILE *)fd->f_fsdata;
 343
 344     /* only important stuff */
 345     sb->st_mode = f->de.attr & FA_DIR ? S_IFDIR | 0555 : S_IFREG | 0444;
 346     sb->st_nlink = 1;
 347     sb->st_uid = 0;
 348     sb->st_gid = 0;
 349     if ((sb->st_size = fsize(f->fs, &f->de)) == -1)
 350         return EINVAL;
 351     return (0);
 352 }
 353
 354 /*
 355  * Parse DOS boot sector
 356  */
 357 static int
 358 parsebs(DOS_FS *fs, DOS_BS *bs)
 359 {
 360     u_int sc;
 361
 362     if ((bs->jmp[0] != 0x69 &&
 363          bs->jmp[0] != 0xe9 &&
 364          (bs->jmp[0] != 0xeb || bs->jmp[2] != 0x90)) ||
 365         bs->bpb.media < 0xf0)
 366         return EINVAL;
 367     if (cv2(bs->bpb.secsiz) != SECSIZ)
 368         return EINVAL;
 369     if (!(fs->spc = bs->bpb.spc) || fs->spc & (fs->spc - 1))
 370         return EINVAL;
 371     fs->bsize = secbyt(fs->spc);
 372     fs->bshift = ffs(fs->bsize) - 1;
 373     if ((fs->spf = cv2(bs->bpb.spf))) {
 374         if (bs->bpb.fats != 2)
 375             return EINVAL;
 376         if (!(fs->dirents = cv2(bs->bpb.dirents)))
 377             return EINVAL;
 378     } else {
 379         if (!(fs->spf = cv4(bs->bpb.lspf)))
 380             return EINVAL;
 381         if (!bs->bpb.fats || bs->bpb.fats > 16)
 382             return EINVAL;
 383         if ((fs->rdcl = cv4(bs->bpb.rdcl)) < LOCLUS)
 384             return EINVAL;
 385     }
 386     if (!(fs->lsnfat = cv2(bs->bpb.ressec)))
 387         return EINVAL;
 388     fs->lsndir = fs->lsnfat + fs->spf * bs->bpb.fats;
 389     fs->lsndta = fs->lsndir + entsec(fs->dirents);
 390     if (!(sc = cv2(bs->bpb.secs)) && !(sc = cv4(bs->bpb.lsecs)))
 391         return EINVAL;
 392     if (fs->lsndta > sc)
 393         return EINVAL;
 394     if ((fs->xclus = secblk(fs, sc - fs->lsndta) + 1) < LOCLUS)
 395         return EINVAL;
 396     fs->fatsz = fs->dirents ? fs->xclus < 0xff6 ? 12 : 16 : 32;
 397     sc = (secbyt(fs->spf) << 1) / (fs->fatsz >> 2) - 1;
 398     if (fs->xclus > sc)
 399         fs->xclus = sc;
 400     return 0;
 401 }
 402
 403 /*
 404  * Return directory entry from path
 405  */
 406 static int
 407 namede(DOS_FS *fs, const char *path, DOS_DE **dep)
 408 {
 409     char name[256];
 410     DOS_DE *de;
 411     char *s;
 412     size_t n;
 413     int err;
 414
 415     err = 0;
 416     de = dot;
 417     if (*path == '/')
 418         path++;
 419     while (*path) {
 420         if (!(s = strchr(path, '/')))
 421             s = strchr(path, 0);
 422         if ((n = s - path) > 255)
 423             return ENAMETOOLONG;
 424         memcpy(name, path, n);
 425         name[n] = 0;
 426         path = s;
 427         if (!(de->attr & FA_DIR))
 428             return ENOTDIR;
 429         if ((err = lookup(fs, stclus(fs->fatsz, de), name, &de)))
 430             return err;
 431         if (*path == '/')
 432             path++;
 433     }
 434     *dep = de;
 435     return 0;
 436 }
 437
 438 /*
 439  * Lookup path segment
 440  */
 441 static int
 442 lookup(DOS_FS *fs, u_int clus, const char *name, DOS_DE **dep)
 443 {
 444     static DOS_DIR dir[DEPSEC];
 445     u_char lfn[261];
 446     u_char sfn[13];
 447     u_int nsec, lsec, xdn, chk, sec, ent, x;
 448     int err, ok, i;
 449
 450     if (!clus)
 451         for (ent = 0; ent < 2; ent++)
 452             if (!strcasecmp(name, dotstr[ent])) {
 453                 *dep = dot + ent;
 454                 return 0;
 455             }
 456     if (!clus && fs->fatsz == 32)
 457         clus = fs->rdcl;
 458     nsec = !clus ? entsec(fs->dirents) : fs->spc;
 459     lsec = 0;
 460     xdn = chk = 0;
 461     for (;;) {
 462         if (!clus && !lsec)
 463             lsec = fs->lsndir;
 464         else if (okclus(fs, clus))
 465             lsec = blklsn(fs, clus);
 466         else
 467             return EINVAL;
 468         for (sec = 0; sec < nsec; sec++) {
 469             if ((err = ioget(fs->fd, lsec + sec, dir, 1)))
 470                 return err;
 471             for (ent = 0; ent < DEPSEC; ent++) {
 472                 if (!*dir[ent].de.name)
 473                     return ENOENT;
 474                 if (*dir[ent].de.name != 0xe5) {
 475                     if ((dir[ent].de.attr & FA_MASK) == FA_XDE) {
 476                         x = dir[ent].xde.seq;
 477                         if (x & 0x40 || (x + 1 == xdn &&
 478                                          dir[ent].xde.chk == chk)) {
 479                             if (x & 0x40) {
 480                                 chk = dir[ent].xde.chk;
 481                                 x &= ~0x40;
 482                             }
 483                             if (x >= 1 && x <= 20) {
 484                                 cp_xdnm(lfn, &dir[ent].xde);
 485                                 xdn = x;
 486                                 continue;
 487                             }
 488                         }
 489                     } else if (!(dir[ent].de.attr & FA_LABEL)) {
 490                         if ((ok = xdn == 1)) {
 491                             for (x = 0, i = 0; i < 11; i++)
 492                                 x = ((((x & 1) << 7) | (x >> 1)) +
 493                                      dir[ent].de.name[i]) & 0xff;
 494                             ok = chk == x &&
 495                                 !strcasecmp(name, (const char *)lfn);
 496                         }
 497                         if (!ok) {
 498                             cp_sfn(sfn, &dir[ent].de);
 499                             ok = !strcasecmp(name, (const char *)sfn);
 500                         }
 501                         if (ok) {
 502                             *dep = &dir[ent].de;
 503                             return 0;
 504                         }
 505                     }
 506                 }
 507                 xdn = 0;
 508             }
 509         }
 510         if (!clus)
 511             break;
 512         if ((err = fatget(fs, &clus)))
 513             return err;
 514         if (fatend(fs->fatsz, clus))
 515             break;
 516     }
 517     return ENOENT;
 518 }
 519
 520 /*
 521  * Copy name from extended directory entry
 522  */
 523 static void
 524 cp_xdnm(u_char *lfn, DOS_XDE *xde)
 525 {
 526     static struct {
 527         u_int off;
 528         u_int dim;
 529     } ix[3] = {
 530         {offsetof(DOS_XDE, name1), sizeof(xde->name1) / 2},
 531         {offsetof(DOS_XDE, name2), sizeof(xde->name2) / 2},
 532         {offsetof(DOS_XDE, name3), sizeof(xde->name3) / 2}
 533     };
 534     u_char *p;
 535     u_int n, x, c;
 536
 537     lfn += 13 * ((xde->seq & ~0x40) - 1);
 538     for (n = 0; n < 3; n++)
 539         for (p = (u_char *)xde + ix[n].off, x = ix[n].dim; x;
 540              p += 2, x--) {
 541             if ((c = cv2(p)) && (c < 32 || c > 127))
 542                 c = '?';
 543             if (!(*lfn++ = c))
 544                 return;
 545         }
 546     if (xde->seq & 0x40)
 547         *lfn = 0;
 548 }
 549
 550 /*
 551  * Copy short filename
 552  */
 553 static void
 554 cp_sfn(u_char *sfn, DOS_DE *de)
 555 {
 556     u_char *p;
 557     int j, i;
 558
 559     p = sfn;
 560     if (*de->name != ' ') {
 561         for (j = 7; de->name[j] == ' '; j--);
 562         for (i = 0; i <= j; i++)
 563             *p++ = de->name[i];
 564         if (*de->ext != ' ') {
 565             *p++ = '.';
 566             for (j = 2; de->ext[j] == ' '; j--);
 567             for (i = 0; i <= j; i++)
 568                 *p++ = de->ext[i];
 569         }
 570     }
 571     *p = 0;
 572     if (*sfn == 5)
 573         *sfn = 0xe5;
 574 }
 575
 576 /*
 577  * Return size of file in bytes
 578  */
 579 static off_t
 580 fsize(DOS_FS *fs, DOS_DE *de)
 581 {
 582    u_long size;
 583    u_int c;
 584    int n;
 585
 586    if (!(size = cv4(de->size)) && de->attr & FA_DIR) {
 587       if (!(c = cv2(de->clus)))
 588          size = fs->dirents * sizeof(DOS_DE);
 589       else {
 590          if ((n = fatcnt(fs, c)) == -1)
 591             return n;
 592          size = blkbyt(fs, n);
 593       }
 594    }
 595    return size;
 596 }
 597
 598 /*
 599  * Count number of clusters in chain
 600  */
 601 static int
 602 fatcnt(DOS_FS *fs, u_int c)
 603 {
 604    int n;
 605
 606    for (n = 0; okclus(fs, c); n++)
 607       if (fatget(fs, &c))
 608           return -1;
 609    return fatend(fs->fatsz, c) ? n : -1;
 610 }
 611
 612 /*
 613  * Get next cluster in cluster chain
 614  */
 615 static int
 616 fatget(DOS_FS *fs, u_int *c)
 617 {
 618     u_char buf[4];
 619     u_int x;
 620     int err;
 621
 622     err = ioread(fs, secbyt(fs->lsnfat) + fatoff(fs->fatsz, *c), buf,
 623                  fs->fatsz != 32 ? 2 : 4);
 624     if (err)
 625         return err;
 626     x = fs->fatsz != 32 ? cv2(buf) : cv4(buf);
 627     *c = fs->fatsz == 12 ? *c & 1 ? x >> 4 : x & 0xfff : x;
 628     return 0;
 629 }
 630
 631 /*
 632  * Is cluster an end-of-chain marker?
 633  */
 634 static int
 635 fatend(u_int sz, u_int c)
 636 {
 637     return c > (sz == 12 ? 0xff7U : sz == 16 ? 0xfff7U : 0xffffff7);
 638 }
 639
 640 /*
 641  * Offset-based I/O primitive
 642  */
 643 static int
 644 ioread(DOS_FS *fs, u_int offset, void *buf, u_int nbyte)
 645 {
 646     char *s;
 647     u_int off, n;
 648     int err;
 649
 650     s = buf;
 651     if ((off = offset & (SECSIZ - 1))) {
 652         offset -= off;
 653         if ((err = iobuf(fs, bytsec(offset))))
 654             return err;
 655         offset += SECSIZ;
 656         if ((n = SECSIZ - off) > nbyte)
 657             n = nbyte;
 658         memcpy(s, fs->buf + off, n);
 659         s += n;
 660         nbyte -= n;
 661     }
 662     n = nbyte & (SECSIZ - 1);
 663     if (nbyte -= n) {
 664         if ((err = ioget(fs->fd, bytsec(offset), s, bytsec(nbyte))))
 665             return err;
 666         offset += nbyte;
 667         s += nbyte;
 668     }
 669     if (n) {
 670         if ((err = iobuf(fs, bytsec(offset))))
 671             return err;
 672         memcpy(s, fs->buf, n);
 673     }
 674     return 0;
 675 }
 676
 677 /*
 678  * Buffered sector-based I/O primitive
 679  */
 680 static int
 681 iobuf(DOS_FS *fs, u_int lsec)
 682 {
 683     int err;
 684
 685     if (fs->bufsec != lsec) {
 686         if ((err = ioget(fs->fd, lsec, fs->buf, 1)))
 687             return err;
 688         fs->bufsec = lsec;
 689     }
 690     return 0;
 691 }
 692
 693 /*
 694  * Sector-based I/O primitive
 695  */
 696 static int
 697 ioget(struct open_file *fd, u_int lsec, void *buf, u_int nsec)
 698 {
 699     int err;
 700
 701     if ((err = (fd->f_dev->dv_strategy)(fd->f_devdata, F_READ, lsec,
 702                                         secbyt(nsec), buf, NULL)))
 703         return(err);
 704     return(0);
 705 }