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
28#include <sys/cdefs.h>
29__FBSDID("$FreeBSD$");
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
45static int	dos_open(const char *path, struct open_file *fd);
46static int	dos_close(struct open_file *fd);
47static int	dos_read(struct open_file *fd, void *buf, size_t size, size_t *resid);
48static off_t	dos_seek(struct open_file *fd, off_t offset, int whence);
49static int	dos_stat(struct open_file *fd, struct stat *sb);
50static int	dos_readdir(struct open_file *fd, struct dirent *d);
51
52struct fs_ops dosfs_fsops = {
53	"dosfs",
54	dos_open,
55	dos_close,
56	dos_read,
57	null_write,
58	dos_seek,
59	dos_stat,
60	dos_readdir
61};
62
63#define SECSIZ  512             /* sector size */
64#define SSHIFT    9             /* SECSIZ shift */
65#define DEPSEC   16             /* directory entries per sector */
66#define DSHIFT    4             /* DEPSEC shift */
67#define LOCLUS    2             /* lowest cluster number */
68
69/* DOS "BIOS Parameter Block" */
70typedef struct {
71    u_char secsiz[2];           /* sector size */
72    u_char spc;                 /* sectors per cluster */
73    u_char ressec[2];           /* reserved sectors */
74    u_char fats;                /* FATs */
75    u_char dirents[2];          /* root directory entries */
76    u_char secs[2];             /* total sectors */
77    u_char media;               /* media descriptor */
78    u_char spf[2];              /* sectors per FAT */
79    u_char spt[2];              /* sectors per track */
80    u_char heads[2];            /* drive heads */
81    u_char hidsec[4];           /* hidden sectors */
82    u_char lsecs[4];            /* huge sectors */
83    u_char lspf[4];             /* huge sectors per FAT */
84    u_char xflg[2];             /* flags */
85    u_char vers[2];             /* filesystem version */
86    u_char rdcl[4];             /* root directory start cluster */
87    u_char infs[2];             /* filesystem info sector */
88    u_char bkbs[2];             /* backup boot sector */
89} DOS_BPB;
90
91/* Initial portion of DOS boot sector */
92typedef struct {
93    u_char jmp[3];              /* usually 80x86 'jmp' opcode */
94    u_char oem[8];              /* OEM name and version */
95    DOS_BPB bpb;                /* BPB */
96} DOS_BS;
97
98/* Supply missing "." and ".." root directory entries */
99static const char *const dotstr[2] = {".", ".."};
100static DOS_DE dot[2] = {
101    {".       ", "   ", FA_DIR, {0, 0, {0, 0}, {0, 0}, {0, 0}, {0, 0}},
102     {0, 0}, {0x21, 0}, {0, 0}, {0, 0, 0, 0}},
103    {"..      ", "   ", FA_DIR, {0, 0, {0, 0}, {0, 0}, {0, 0}, {0, 0}},
104     {0, 0}, {0x21, 0}, {0, 0}, {0, 0, 0, 0}}
105};
106
107/* The usual conversion macros to avoid multiplication and division */
108#define bytsec(n)      ((n) >> SSHIFT)
109#define secbyt(s)      ((s) << SSHIFT)
110#define entsec(e)      ((e) >> DSHIFT)
111#define bytblk(fs, n)  ((n) >> (fs)->bshift)
112#define blkbyt(fs, b)  ((b) << (fs)->bshift)
113#define secblk(fs, s)  ((s) >> ((fs)->bshift - SSHIFT))
114#define blksec(fs, b)  ((b) << ((fs)->bshift - SSHIFT))
115
116/* Convert cluster number to offset within filesystem */
117#define blkoff(fs, b) (secbyt((fs)->lsndta) + blkbyt(fs, (b) - LOCLUS))
118
119/* Convert cluster number to logical sector number */
120#define blklsn(fs, b)  ((fs)->lsndta + blksec(fs, (b) - LOCLUS))
121
122/* Convert cluster number to offset within FAT */
123#define fatoff(sz, c)  ((sz) == 12 ? (c) + ((c) >> 1) :  \
124                        (sz) == 16 ? (c) << 1 :          \
125			(c) << 2)
126
127/* Does cluster number reference a valid data cluster? */
128#define okclus(fs, c)  ((c) >= LOCLUS && (c) <= (fs)->xclus)
129
130/* Get start cluster from directory entry */
131#define stclus(sz, de)  ((sz) != 32 ? cv2((de)->clus) :          \
132                         ((u_int)cv2((de)->dex.h_clus) << 16) |  \
133			 cv2((de)->clus))
134
135static int dosunmount(DOS_FS *);
136static int parsebs(DOS_FS *, DOS_BS *);
137static int namede(DOS_FS *, const char *, DOS_DE **);
138static int lookup(DOS_FS *, u_int, const char *, DOS_DE **);
139static void cp_xdnm(u_char *, DOS_XDE *);
140static void cp_sfn(u_char *, DOS_DE *);
141static off_t fsize(DOS_FS *, DOS_DE *);
142static int fatcnt(DOS_FS *, u_int);
143static int fatget(DOS_FS *, u_int *);
144static int fatend(u_int, u_int);
145static int ioread(DOS_FS *, u_int, void *, u_int);
146static int iobuf(DOS_FS *, u_int);
147static int ioget(struct open_file *, u_int, void *, u_int);
148
149/*
150 * Mount DOS filesystem
151 */
152static int
153dos_mount(DOS_FS *fs, struct open_file *fd)
154{
155    int err;
156
157    bzero(fs, sizeof(DOS_FS));
158    fs->fd = fd;
159    if ((err = !(fs->buf = malloc(SECSIZ)) ? errno : 0) ||
160        (err = ioget(fs->fd, 0, fs->buf, 1)) ||
161        (err = parsebs(fs, (DOS_BS *)fs->buf))) {
162        (void)dosunmount(fs);
163        return(err);
164    }
165    return 0;
166}
167
168/*
169 * Unmount mounted filesystem
170 */
171static int
172dos_unmount(DOS_FS *fs)
173{
174    int err;
175
176    if (fs->links)
177        return(EBUSY);
178    if ((err = dosunmount(fs)))
179        return(err);
180    return 0;
181}
182
183/*
184 * Common code shared by dos_mount() and dos_unmount()
185 */
186static int
187dosunmount(DOS_FS *fs)
188{
189    if (fs->buf)
190        free(fs->buf);
191    free(fs);
192    return(0);
193}
194
195/*
196 * Open DOS file
197 */
198static int
199dos_open(const char *path, struct open_file *fd)
200{
201    DOS_DE *de;
202    DOS_FILE *f;
203    DOS_FS *fs;
204    u_int size, clus;
205    int err = 0;
206
207    /* Allocate mount structure, associate with open */
208    fs = malloc(sizeof(DOS_FS));
209
210    if ((err = dos_mount(fs, fd)))
211	goto out;
212
213    if ((err = namede(fs, path, &de)))
214	goto out;
215
216    clus = stclus(fs->fatsz, de);
217    size = cv4(de->size);
218
219    if ((!(de->attr & FA_DIR) && (!clus != !size)) ||
220	((de->attr & FA_DIR) && size) ||
221	(clus && !okclus(fs, clus))) {
222        err = EINVAL;
223	goto out;
224    }
225    f = malloc(sizeof(DOS_FILE));
226    bzero(f, sizeof(DOS_FILE));
227    f->fs = fs;
228    fs->links++;
229    f->de = *de;
230    fd->f_fsdata = (void *)f;
231
232 out:
233    return(err);
234}
235
236/*
237 * Read from file
238 */
239static int
240dos_read(struct open_file *fd, void *buf, size_t nbyte, size_t *resid)
241{
242    off_t size;
243    u_int nb, off, clus, c, cnt, n;
244    DOS_FILE *f = (DOS_FILE *)fd->f_fsdata;
245    int err = 0;
246
247    nb = (u_int)nbyte;
248    if ((size = fsize(f->fs, &f->de)) == -1)
249	return EINVAL;
250    if (nb > (n = size - f->offset))
251        nb = n;
252    off = f->offset;
253    if ((clus = stclus(f->fs->fatsz, &f->de)))
254        off &= f->fs->bsize - 1;
255    c = f->c;
256    cnt = nb;
257    while (cnt) {
258        n = 0;
259        if (!c) {
260            if ((c = clus))
261                n = bytblk(f->fs, f->offset);
262        } else if (!off)
263            n++;
264        while (n--) {
265            if ((err = fatget(f->fs, &c)))
266		goto out;
267            if (!okclus(f->fs, c)) {
268		err = EINVAL;
269		goto out;
270	    }
271        }
272        if (!clus || (n = f->fs->bsize - off) > cnt)
273            n = cnt;
274        if ((err = ioread(f->fs, (c ? blkoff(f->fs, c) :
275				      secbyt(f->fs->lsndir)) + off,
276			  buf, n)))
277	    goto out;
278        f->offset += n;
279        f->c = c;
280        off = 0;
281        buf = (char *)buf + n;
282        cnt -= n;
283    }
284 out:
285    if (resid)
286	*resid = nbyte - nb + cnt;
287    return(err);
288}
289
290/*
291 * Reposition within file
292 */
293static off_t
294dos_seek(struct open_file *fd, off_t offset, int whence)
295{
296    off_t off;
297    u_int size;
298    DOS_FILE *f = (DOS_FILE *)fd->f_fsdata;
299
300    size = cv4(f->de.size);
301    switch (whence) {
302    case SEEK_SET:
303        off = 0;
304        break;
305    case SEEK_CUR:
306        off = f->offset;
307        break;
308    case SEEK_END:
309        off = size;
310        break;
311    default:
312	errno = EINVAL;
313	return(-1);
314    }
315    off += offset;
316    if (off < 0 || off > size) {
317	errno = EINVAL;
318        return(-1);
319    }
320    f->offset = (u_int)off;
321    f->c = 0;
322    return(off);
323}
324
325/*
326 * Close open file
327 */
328static int
329dos_close(struct open_file *fd)
330{
331    DOS_FILE *f = (DOS_FILE *)fd->f_fsdata;
332    DOS_FS *fs = f->fs;
333
334    f->fs->links--;
335    free(f);
336    dos_unmount(fs);
337    return 0;
338}
339
340/*
341 * Return some stat information on a file.
342 */
343static int
344dos_stat(struct open_file *fd, struct stat *sb)
345{
346    DOS_FILE *f = (DOS_FILE *)fd->f_fsdata;
347
348    /* only important stuff */
349    sb->st_mode = f->de.attr & FA_DIR ? S_IFDIR | 0555 : S_IFREG | 0444;
350    sb->st_nlink = 1;
351    sb->st_uid = 0;
352    sb->st_gid = 0;
353    if ((sb->st_size = fsize(f->fs, &f->de)) == -1)
354	return EINVAL;
355    return (0);
356}
357
358static int
359dos_readdir(struct open_file *fd, struct dirent *d)
360{
361    /* DOS_FILE *f = (DOS_FILE *)fd->f_fsdata; */
362    u_char fn[261];
363    DOS_DIR dd;
364    size_t res;
365    u_int chk, i, x, xdn;
366    int err;
367
368    x = chk = 0;
369    while (1) {
370	xdn = x;
371	x = 0;
372	err = dos_read(fd, &dd, sizeof(dd), &res);
373	if (err)
374	    return (err);
375	if (res == sizeof(dd))
376	    return (ENOENT);
377	if (dd.de.name[0] == 0)
378	    return (ENOENT);
379
380	/* Skip deleted entries */
381	if (dd.de.name[0] == 0xe5)
382	    continue;
383
384	/* Skip volume labels */
385	if (dd.de.attr & FA_LABEL)
386	    continue;
387
388	if ((dd.de.attr & FA_MASK) == FA_XDE) {
389	    if (dd.xde.seq & 0x40)
390		chk = dd.xde.chk;
391	    else if (dd.xde.seq != xdn - 1 || dd.xde.chk != chk)
392		continue;
393	    x = dd.xde.seq & ~0x40;
394	    if (x < 1 || x > 20) {
395		x = 0;
396		continue;
397	    }
398	    cp_xdnm(fn, &dd.xde);
399	} else {
400	    if (xdn == 1) {
401		x = 0;
402		for (i = 0; i < 11; i++) {
403		    x = ((x & 1) << 7) | (x >> 1);
404		    x += dd.de.name[i];
405		    x &= 0xff;
406		}
407		if (x == chk)
408		    break;
409	    } else {
410		cp_sfn(fn, &dd.de);
411		break;
412	    }
413	    x = 0;
414	}
415    }
416
417    d->d_fileno = (dd.de.clus[1] << 8) + dd.de.clus[0];
418    d->d_reclen = sizeof(*d);
419    d->d_type = (dd.de.attr & FA_DIR) ? DT_DIR : DT_REG;
420    memcpy(d->d_name, fn, sizeof(d->d_name));
421    return(0);
422}
423
424/*
425 * Parse DOS boot sector
426 */
427static int
428parsebs(DOS_FS *fs, DOS_BS *bs)
429{
430    u_int sc;
431
432    if ((bs->jmp[0] != 0x69 &&
433         bs->jmp[0] != 0xe9 &&
434         (bs->jmp[0] != 0xeb || bs->jmp[2] != 0x90)) ||
435        bs->bpb.media < 0xf0)
436        return EINVAL;
437    if (cv2(bs->bpb.secsiz) != SECSIZ)
438        return EINVAL;
439    if (!(fs->spc = bs->bpb.spc) || fs->spc & (fs->spc - 1))
440        return EINVAL;
441    fs->bsize = secbyt(fs->spc);
442    fs->bshift = ffs(fs->bsize) - 1;
443    if ((fs->spf = cv2(bs->bpb.spf))) {
444        if (bs->bpb.fats != 2)
445            return EINVAL;
446        if (!(fs->dirents = cv2(bs->bpb.dirents)))
447            return EINVAL;
448    } else {
449        if (!(fs->spf = cv4(bs->bpb.lspf)))
450            return EINVAL;
451        if (!bs->bpb.fats || bs->bpb.fats > 16)
452            return EINVAL;
453        if ((fs->rdcl = cv4(bs->bpb.rdcl)) < LOCLUS)
454            return EINVAL;
455    }
456    if (!(fs->lsnfat = cv2(bs->bpb.ressec)))
457        return EINVAL;
458    fs->lsndir = fs->lsnfat + fs->spf * bs->bpb.fats;
459    fs->lsndta = fs->lsndir + entsec(fs->dirents);
460    if (!(sc = cv2(bs->bpb.secs)) && !(sc = cv4(bs->bpb.lsecs)))
461        return EINVAL;
462    if (fs->lsndta > sc)
463        return EINVAL;
464    if ((fs->xclus = secblk(fs, sc - fs->lsndta) + 1) < LOCLUS)
465        return EINVAL;
466    fs->fatsz = fs->dirents ? fs->xclus < 0xff6 ? 12 : 16 : 32;
467    sc = (secbyt(fs->spf) << 1) / (fs->fatsz >> 2) - 1;
468    if (fs->xclus > sc)
469        fs->xclus = sc;
470    return 0;
471}
472
473/*
474 * Return directory entry from path
475 */
476static int
477namede(DOS_FS *fs, const char *path, DOS_DE **dep)
478{
479    char name[256];
480    DOS_DE *de;
481    char *s;
482    size_t n;
483    int err;
484
485    err = 0;
486    de = dot;
487    if (*path == '/')
488        path++;
489    while (*path) {
490        if (!(s = strchr(path, '/')))
491            s = strchr(path, 0);
492        if ((n = s - path) > 255)
493            return ENAMETOOLONG;
494        memcpy(name, path, n);
495        name[n] = 0;
496        path = s;
497        if (!(de->attr & FA_DIR))
498            return ENOTDIR;
499        if ((err = lookup(fs, stclus(fs->fatsz, de), name, &de)))
500            return err;
501        if (*path == '/')
502            path++;
503    }
504    *dep = de;
505    return 0;
506}
507
508/*
509 * Lookup path segment
510 */
511static int
512lookup(DOS_FS *fs, u_int clus, const char *name, DOS_DE **dep)
513{
514    static DOS_DIR dir[DEPSEC];
515    u_char lfn[261];
516    u_char sfn[13];
517    u_int nsec, lsec, xdn, chk, sec, ent, x;
518    int err, ok, i;
519
520    if (!clus)
521        for (ent = 0; ent < 2; ent++)
522            if (!strcasecmp(name, dotstr[ent])) {
523                *dep = dot + ent;
524                return 0;
525            }
526    if (!clus && fs->fatsz == 32)
527        clus = fs->rdcl;
528    nsec = !clus ? entsec(fs->dirents) : fs->spc;
529    lsec = 0;
530    xdn = chk = 0;
531    for (;;) {
532        if (!clus && !lsec)
533            lsec = fs->lsndir;
534        else if (okclus(fs, clus))
535            lsec = blklsn(fs, clus);
536        else
537            return EINVAL;
538        for (sec = 0; sec < nsec; sec++) {
539            if ((err = ioget(fs->fd, lsec + sec, dir, 1)))
540                return err;
541            for (ent = 0; ent < DEPSEC; ent++) {
542                if (!*dir[ent].de.name)
543                    return ENOENT;
544                if (*dir[ent].de.name != 0xe5) {
545                    if ((dir[ent].de.attr & FA_MASK) == FA_XDE) {
546                        x = dir[ent].xde.seq;
547                        if (x & 0x40 || (x + 1 == xdn &&
548                                         dir[ent].xde.chk == chk)) {
549                            if (x & 0x40) {
550                                chk = dir[ent].xde.chk;
551                                x &= ~0x40;
552                            }
553                            if (x >= 1 && x <= 20) {
554                                cp_xdnm(lfn, &dir[ent].xde);
555                                xdn = x;
556                                continue;
557                            }
558                        }
559                    } else if (!(dir[ent].de.attr & FA_LABEL)) {
560                        if ((ok = xdn == 1)) {
561                            for (x = 0, i = 0; i < 11; i++)
562                                x = ((((x & 1) << 7) | (x >> 1)) +
563                                     dir[ent].de.name[i]) & 0xff;
564                            ok = chk == x &&
565                                !strcasecmp(name, (const char *)lfn);
566                        }
567                        if (!ok) {
568                            cp_sfn(sfn, &dir[ent].de);
569                            ok = !strcasecmp(name, (const char *)sfn);
570                        }
571                        if (ok) {
572                            *dep = &dir[ent].de;
573                            return 0;
574                        }
575                    }
576		}
577                xdn = 0;
578            }
579        }
580        if (!clus)
581            break;
582        if ((err = fatget(fs, &clus)))
583            return err;
584        if (fatend(fs->fatsz, clus))
585            break;
586    }
587    return ENOENT;
588}
589
590/*
591 * Copy name from extended directory entry
592 */
593static void
594cp_xdnm(u_char *lfn, DOS_XDE *xde)
595{
596    static struct {
597        u_int off;
598        u_int dim;
599    } ix[3] = {
600        {offsetof(DOS_XDE, name1), sizeof(xde->name1) / 2},
601        {offsetof(DOS_XDE, name2), sizeof(xde->name2) / 2},
602        {offsetof(DOS_XDE, name3), sizeof(xde->name3) / 2}
603    };
604    u_char *p;
605    u_int n, x, c;
606
607    lfn += 13 * ((xde->seq & ~0x40) - 1);
608    for (n = 0; n < 3; n++)
609        for (p = (u_char *)xde + ix[n].off, x = ix[n].dim; x;
610	     p += 2, x--) {
611            if ((c = cv2(p)) && (c < 32 || c > 127))
612                c = '?';
613            if (!(*lfn++ = c))
614                return;
615        }
616    if (xde->seq & 0x40)
617        *lfn = 0;
618}
619
620/*
621 * Copy short filename
622 */
623static void
624cp_sfn(u_char *sfn, DOS_DE *de)
625{
626    u_char *p;
627    int j, i;
628
629    p = sfn;
630    if (*de->name != ' ') {
631        for (j = 7; de->name[j] == ' '; j--);
632        for (i = 0; i <= j; i++)
633            *p++ = de->name[i];
634        if (*de->ext != ' ') {
635            *p++ = '.';
636            for (j = 2; de->ext[j] == ' '; j--);
637            for (i = 0; i <= j; i++)
638                *p++ = de->ext[i];
639        }
640    }
641    *p = 0;
642    if (*sfn == 5)
643        *sfn = 0xe5;
644}
645
646/*
647 * Return size of file in bytes
648 */
649static off_t
650fsize(DOS_FS *fs, DOS_DE *de)
651{
652   u_long size;
653   u_int c;
654   int n;
655
656   if (!(size = cv4(de->size)) && de->attr & FA_DIR) {
657      if (!(c = cv2(de->clus)))
658         size = fs->dirents * sizeof(DOS_DE);
659      else {
660         if ((n = fatcnt(fs, c)) == -1)
661            return n;
662         size = blkbyt(fs, n);
663      }
664   }
665   return size;
666}
667
668/*
669 * Count number of clusters in chain
670 */
671static int
672fatcnt(DOS_FS *fs, u_int c)
673{
674   int n;
675
676   for (n = 0; okclus(fs, c); n++)
677      if (fatget(fs, &c))
678	  return -1;
679   return fatend(fs->fatsz, c) ? n : -1;
680}
681
682/*
683 * Get next cluster in cluster chain
684 */
685static int
686fatget(DOS_FS *fs, u_int *c)
687{
688    u_char buf[4];
689    u_int x;
690    int err;
691
692    err = ioread(fs, secbyt(fs->lsnfat) + fatoff(fs->fatsz, *c), buf,
693                 fs->fatsz != 32 ? 2 : 4);
694    if (err)
695        return err;
696    x = fs->fatsz != 32 ? cv2(buf) : cv4(buf);
697    *c = fs->fatsz == 12 ? *c & 1 ? x >> 4 : x & 0xfff : x;
698    return 0;
699}
700
701/*
702 * Is cluster an end-of-chain marker?
703 */
704static int
705fatend(u_int sz, u_int c)
706{
707    return c > (sz == 12 ? 0xff7U : sz == 16 ? 0xfff7U : 0xffffff7);
708}
709
710/*
711 * Offset-based I/O primitive
712 */
713static int
714ioread(DOS_FS *fs, u_int offset, void *buf, u_int nbyte)
715{
716    char *s;
717    u_int off, n;
718    int err;
719
720    s = buf;
721    if ((off = offset & (SECSIZ - 1))) {
722        offset -= off;
723        if ((err = iobuf(fs, bytsec(offset))))
724            return err;
725        offset += SECSIZ;
726        if ((n = SECSIZ - off) > nbyte)
727            n = nbyte;
728        memcpy(s, fs->buf + off, n);
729        s += n;
730        nbyte -= n;
731    }
732    n = nbyte & (SECSIZ - 1);
733    if (nbyte -= n) {
734        if ((err = ioget(fs->fd, bytsec(offset), s, bytsec(nbyte))))
735            return err;
736        offset += nbyte;
737        s += nbyte;
738    }
739    if (n) {
740        if ((err = iobuf(fs, bytsec(offset))))
741            return err;
742        memcpy(s, fs->buf, n);
743    }
744    return 0;
745}
746
747/*
748 * Buffered sector-based I/O primitive
749 */
750static int
751iobuf(DOS_FS *fs, u_int lsec)
752{
753    int err;
754
755    if (fs->bufsec != lsec) {
756        if ((err = ioget(fs->fd, lsec, fs->buf, 1)))
757            return err;
758        fs->bufsec = lsec;
759    }
760    return 0;
761}
762
763/*
764 * Sector-based I/O primitive
765 */
766static int
767ioget(struct open_file *fd, u_int lsec, void *buf, u_int nsec)
768{
769    int	err;
770
771    if ((err = (fd->f_dev->dv_strategy)(fd->f_devdata, F_READ, lsec,
772					secbyt(nsec), buf, NULL)))
773	return(err);
774    return(0);
775}
776