/*      $NetBSD: dosfs.c,v 1.23 2020/01/22 06:11:28 martin Exp $        */

/*
 * Copyright (c) 1996, 1998 Robert Nordier
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in
 *    the documentation and/or other materials provided with the
 *    distribution.
 *
 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR(S) ``AS IS'' AND ANY EXPRESS
 * OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
 * WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR(S) BE LIABLE FOR ANY
 * DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE
 * GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
 * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER
 * IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR
 * OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN
 * IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */

/*
 * Readonly filesystem for Microsoft FAT12/FAT16/FAT32 filesystems,
 * also supports VFAT.
 */

/*
 * XXX DOES NOT SUPPORT:
 *
 *      LIBSA_FS_SINGLECOMPONENT
 */

#include <sys/param.h>

#include <fs/msdosfs/bpb.h>
#include <fs/msdosfs/direntry.h>

#ifdef _STANDALONE
#include <lib/libkern/libkern.h>
#else
#include <string.h>
#include <stddef.h>
#endif

#include "stand.h"
#include "dosfs.h"

#define SECSIZ  512             /* sector size */
#define SSHIFT    9             /* SECSIZ shift */
#define DEPSEC   16             /* directory entries per sector */
#define DSHIFT    4             /* DEPSEC shift */
#define LOCLUS    2             /* lowest cluster number */

typedef union {
        struct direntry de;     /* standard directory entry */
        struct winentry xde;    /* extended directory entry */
} DOS_DIR;

typedef struct {
        struct open_file *fd;   /* file descriptor */
        u_char *buf;            /* buffer */
        u_int   bufsec;         /* buffered sector */
        u_int   links;          /* active links to structure */
        u_int   spc;            /* sectors per cluster */
        u_int   bsize;          /* cluster size in bytes */
        u_int   bshift;         /* cluster conversion shift */
        u_int   dirents;        /* root directory entries */
        u_int   spf;            /* sectors per fat */
        u_int   rdcl;           /* root directory start cluster */
        u_int   lsnfat;         /* start of fat */
        u_int   lsndir;         /* start of root dir */
        u_int   lsndta;         /* start of data area */
        u_int   fatsz;          /* FAT entry size */
        u_int   xclus;          /* maximum cluster number */
} DOS_FS;

typedef struct {
        DOS_FS *fs;             /* associated filesystem */
        struct direntry de;     /* directory entry */
        u_int   offset;         /* current offset */
        u_int   c;              /* last cluster read */
} DOS_FILE;

/* Initial portion of DOS boot sector */
typedef struct {
        u_char  jmp[3];         /* usually 80x86 'jmp' opcode */
        u_char  oem[8];         /* OEM name and version */
        struct byte_bpb710 bpb; /* BPB */
} DOS_BS;

/* Supply missing "." and ".." root directory entries */
static const char *const dotstr[2] = {".", ".."};
static const struct direntry dot[2] = {
        {".       ", "   ", ATTR_DIRECTORY,
                0, 0, {0, 0}, {0, 0}, {0, 0}, {0, 0},
                {0, 0}, {0x21, 0}, {0, 0}, {0, 0, 0, 0}},

        {"..      ", "   ", ATTR_DIRECTORY,
                0, 0, {0, 0}, {0, 0}, {0, 0}, {0, 0},
                {0, 0}, {0x21, 0}, {0, 0}, {0, 0, 0, 0}}
};

#ifdef SA_DOSFS_NO_BIG_PART_SUPPORT
#define BYTE_OFF_T      u_int
#else
#define BYTE_OFF_T      uint64_t
#endif

/* The usual conversion macros to avoid multiplication and division */
#define bytsec(n)      ((n) >> SSHIFT)
#define secbyt(s)      ((BYTE_OFF_T)(s) << SSHIFT)
#define entsec(e)      ((e) >> DSHIFT)
#define bytblk(fs, n)  ((n) >> (fs)->bshift)
#define blkbyt(fs, b)  ((BYTE_OFF_T)(b) << (fs)->bshift)
#define secblk(fs, s)  ((s) >> ((fs)->bshift - SSHIFT))
#define blksec(fs, b)  ((b) << ((fs)->bshift - SSHIFT))

/* Convert cluster number to offset within filesystem */
#define blkoff(fs, b) (secbyt((fs)->lsndta) + blkbyt(fs, (b) - LOCLUS))

/* Convert cluster number to logical sector number */
#define blklsn(fs, b)  ((fs)->lsndta + blksec(fs, (b) - LOCLUS))

/* Convert cluster number to offset within FAT */
#define fatoff(sz, c)  ((sz) == 12 ? (c) + ((c) >> 1) :  \
                        (sz) == 16 ? (c) << 1 :          \
                        (c) << 2)

/* Does cluster number reference a valid data cluster? */
#define okclus(fs, c)  ((c) >= LOCLUS && (c) <= (fs)->xclus)

/* Get start cluster from directory entry */
#define stclus(sz, de)  ((sz) != 32 ? (u_int)getushort((de)->deStartCluster) : \
                         ((u_int)getushort((de)->deHighClust) << 16) |  \
                         (u_int)getushort((de)->deStartCluster))

static int dosunmount(DOS_FS *);
static int parsebs(DOS_FS *, DOS_BS *);
static int namede(DOS_FS *, const char *, const struct direntry **);
static int lookup(DOS_FS *, u_int, const char *, const struct direntry **);
static void cp_xdnm(u_char *, struct winentry *);
static void cp_sfn(u_char *, struct direntry *);
static off_t fsize(DOS_FS *, struct direntry *);
static int fatcnt(DOS_FS *, u_int);
static int fatget(DOS_FS *, u_int *);
static int fatend(u_int, u_int);
static int ioread(DOS_FS *, BYTE_OFF_T, void *, u_int);
static int iobuf(DOS_FS *, u_int);
static int ioget(struct open_file *, u_int, void *, u_int);

#define strcasecmp(s1, s2) dos_strcasecmp(s1, s2)
static int
strcasecmp(const char *s1, const char *s2)
{
        char c1, c2;
        #define TO_UPPER(c) ((c) >= 'a' && (c) <= 'z' ? (c) - ('a' - 'A') : (c))
        for (;;) {
                c1 = *s1++;
                c2 = *s2++;
                if (TO_UPPER(c1) != TO_UPPER(c2))
                        return 1;
                if (c1 == 0)
                        return 0;
        }
        #undef TO_UPPER
}

/*
 * Mount DOS filesystem
 */
static int
dos_mount(DOS_FS *fs, struct open_file *fd)
{
        int     err;

        (void)memset(fs, 0, sizeof(DOS_FS));
        fs->fd = fd;
        if ((err = !(fs->buf = alloc(SECSIZ)) ? errno : 0) ||
            (err = ioget(fs->fd, 0, fs->buf, 1)) ||
            (err = parsebs(fs, (DOS_BS *)fs->buf))) {
                (void) dosunmount(fs);
                return err;
        }
        return 0;
}

#ifndef LIBSA_NO_FS_CLOSE
/*
 * Unmount mounted filesystem
 */
static int
dos_unmount(DOS_FS *fs)
{
        int     err;

        if (fs->links)
                return EBUSY;
        if ((err = dosunmount(fs)))
                return err;
        return 0;
}
#endif

/*
 * Common code shared by dos_mount() and dos_unmount()
 */
static int
dosunmount(DOS_FS *fs)
{
        if (fs->buf)
                dealloc(fs->buf, SECSIZ);
        dealloc(fs, sizeof(DOS_FS));
        return 0;
}

/*
 * Open DOS file
 */
__compactcall int
dosfs_open(const char *path, struct open_file *fd)
{
        const struct direntry *de;
        DOS_FILE *f;
        DOS_FS *fs;
        u_int   size, clus;
        int     err = 0;

        /* Allocate mount structure, associate with open */
        fs = alloc(sizeof(DOS_FS));

        if ((err = dos_mount(fs, fd)))
                goto out;

        if ((err = namede(fs, path, &de)))
                goto out;

        clus = stclus(fs->fatsz, de);
        size = getulong(de->deFileSize);

        if ((!(de->deAttributes & ATTR_DIRECTORY) && (!clus != !size)) ||
            ((de->deAttributes & ATTR_DIRECTORY) && size) ||
            (clus && !okclus(fs, clus))) {
                err = EINVAL;
                goto out;
        }

        f = alloc(sizeof(DOS_FILE));
#ifdef BOOTXX
        /* due to __internal_memset_ causing all sorts of register spillage
           (and being completely unoptimized for zeroing small amounts of
           memory), if we hand-initialize the remaining members of f to zero,
           the code size drops 68 bytes. This makes no sense, admittedly. */
        f->offset = 0;
        f->c = 0;
#else
        (void)memset(f, 0, sizeof(DOS_FILE));
#endif
        f->fs = fs;
        fs->links++;
        f->de = *de;
        fd->f_fsdata = (void *)f;
        fsmod = "msdos";

out:
        return err;
}

/*
 * Read from file
 */
__compactcall int
dosfs_read(struct open_file *fd, void *vbuf, size_t nbyte, size_t *resid)
{
        off_t   size;
        u_int8_t *buf = vbuf;
        u_int   nb, off, clus, c, cnt, n;
        DOS_FILE *f = (DOS_FILE *)fd->f_fsdata;
        int     err = 0;

        nb = (u_int) nbyte;
        if ((size = fsize(f->fs, &f->de)) == -1)
                return EINVAL;
        n = (u_int)(size - f->offset);
        if (nb > n)
                nb = n;
        off = f->offset;
        if ((clus = stclus(f->fs->fatsz, &f->de)))
                off &= f->fs->bsize - 1;
        c = f->c;
        cnt = nb;
        while (cnt) {
                n = 0;
                if (!c) {
                        if ((c = clus))
                                n = bytblk(f->fs, f->offset);
                } else if (!off) {
                        n++;
                }
                while (n--) {
                        if ((err = fatget(f->fs, &c)))
                                goto out;
                        if (!okclus(f->fs, c)) {
                                err = EINVAL;
                                goto out;
                        }
                }
                if (!clus || (n = f->fs->bsize - off) > cnt)
                        n = cnt;
                if ((err = ioread(f->fs, (c ? blkoff(f->fs, c) :
                                secbyt(f->fs->lsndir)) + off,
                            buf, n)))
                        goto out;
                f->offset += n;
                f->c = c;
                off = 0;
                buf += n;
                cnt -= n;
        }
out:
        if (resid)
                *resid = nbyte - nb + cnt;
        return err;
}

#ifndef LIBSA_NO_FS_WRITE
/*
 * Not implemented.
 */
__compactcall int
dosfs_write(struct open_file *fd, void *start, size_t size, size_t *resid)
{

        return EROFS;
}
#endif /* !LIBSA_NO_FS_WRITE */

#ifndef LIBSA_NO_FS_SEEK
/*
 * Reposition within file
 */
__compactcall off_t
dosfs_seek(struct open_file *fd, off_t offset, int whence)
{
        off_t   off;
        u_int   size;
        DOS_FILE *f = (DOS_FILE *)fd->f_fsdata;

        size = getulong(f->de.deFileSize);
        switch (whence) {
        case SEEK_SET:
                off = 0;
                break;
        case SEEK_CUR:
                off = f->offset;
                break;
        case SEEK_END:
                off = size;
                break;
        default:
                return -1;
        }
        off += offset;
        if (off < 0 || off > size)
                return -1;
        f->offset = (u_int) off;
        f->c = 0;
        return off;
}
#endif /* !LIBSA_NO_FS_SEEK */

#ifndef LIBSA_NO_FS_CLOSE
/*
 * Close open file
 */
__compactcall int
dosfs_close(struct open_file *fd)
{
        DOS_FILE *f = (DOS_FILE *)fd->f_fsdata;
        DOS_FS *fs = f->fs;

        f->fs->links--;
        dealloc(f, sizeof(DOS_FILE));
        dos_unmount(fs);
        return 0;
}
#endif /* !LIBSA_NO_FS_CLOSE */

/*
 * Return some stat information on a file.
 */
__compactcall int
dosfs_stat(struct open_file *fd, struct stat *sb)
{
        DOS_FILE *f = (DOS_FILE *)fd->f_fsdata;

        /* only important stuff */
        sb->st_mode = (f->de.deAttributes & ATTR_DIRECTORY) ?
            (S_IFDIR | 0555) : (S_IFREG | 0444);
        sb->st_nlink = 1;
        sb->st_uid = 0;
        sb->st_gid = 0;
        if ((sb->st_size = fsize(f->fs, &f->de)) == -1)
                return EINVAL;
        return 0;
}

#if defined(LIBSA_ENABLE_LS_OP)
#include "ls.h"
__compactcall void
dosfs_ls(struct open_file *f, const char *pattern)
{
        lsunsup("dosfs");
}
#endif

/*
 * Parse DOS boot sector
 */
static int
parsebs(DOS_FS *fs, DOS_BS *bs)
{
        u_int   sc;

        if ((bs->jmp[0] != 0x69 &&
                bs->jmp[0] != 0xe9 &&
                (bs->jmp[0] != 0xeb || bs->jmp[2] != 0x90)) ||
            bs->bpb.bpbMedia < 0xf0)
                return EINVAL;
        if (getushort(bs->bpb.bpbBytesPerSec) != SECSIZ)
                return EINVAL;
        if (!(fs->spc = bs->bpb.bpbSecPerClust) || fs->spc & (fs->spc - 1))
                return EINVAL;
        fs->bsize = secbyt(fs->spc);
        fs->bshift = (u_int)ffs((int)fs->bsize) - 1;
        if ((fs->spf = getushort(bs->bpb.bpbFATsecs))) {
                if (bs->bpb.bpbFATs != 2)
                        return EINVAL;
                if (!(fs->dirents = getushort(bs->bpb.bpbRootDirEnts)))
                        return EINVAL;
        } else {
                if (!(fs->spf = getulong(bs->bpb.bpbBigFATsecs)))
                        return EINVAL;
                if (!bs->bpb.bpbFATs || bs->bpb.bpbFATs > 16)
                        return EINVAL;
                if ((fs->rdcl = getulong(bs->bpb.bpbRootClust)) < LOCLUS)
                        return EINVAL;
        }
        if (!(fs->lsnfat = getushort(bs->bpb.bpbResSectors)))
                return EINVAL;
        fs->lsndir = fs->lsnfat + fs->spf * bs->bpb.bpbFATs;
        fs->lsndta = fs->lsndir + entsec(fs->dirents);
        if (!(sc = getushort(bs->bpb.bpbSectors)) &&
            !(sc = getulong(bs->bpb.bpbHugeSectors)))
                return EINVAL;
        if (fs->lsndta > sc)
                return EINVAL;
        if ((fs->xclus = secblk(fs, sc - fs->lsndta) + 1) < LOCLUS)
                return EINVAL;
        fs->fatsz = fs->dirents ? fs->xclus < 0xff6 ? 12 : 16 : 32;
        sc = (secbyt(fs->spf) << 1) / (fs->fatsz >> 2) - 1;
        if (fs->xclus > sc)
                fs->xclus = sc;
        return 0;
}

/*
 * Return directory entry from path
 */
static int
namede(DOS_FS *fs, const char *path, const struct direntry **dep)
{
        char    name[256];
        const struct direntry *de;
        char   *s;
        size_t  n;
        int     err;

        err = 0;
        de = dot;
        if (*path == '/')
                path++;
        while (*path) {
                if (!(s = strchr(path, '/')))
                        s = strchr(path, 0);
                n = (size_t)(s - path);
                if (n > 255)
                        return ENAMETOOLONG;
                memcpy(name, path, n);
                name[n] = 0;
                path = s;
                if (!(de->deAttributes & ATTR_DIRECTORY))
                        return ENOTDIR;
                if ((err = lookup(fs, stclus(fs->fatsz, de), name, &de)))
                        return err;
                if (*path == '/')
                        path++;
        }
        *dep = de;
        return 0;
}

/*
 * Lookup path segment
 */
static int
lookup(DOS_FS *fs, u_int clus, const char *name, const struct direntry **dep)
{
        static DOS_DIR *dir = NULL;
        u_char  lfn[261];
        u_char  sfn[13];
        u_int   nsec, lsec, xdn, chk, sec, ent, x;
        int     err = 0, ok, i;

        if (!clus)
                for (ent = 0; ent < 2; ent++)
                        if (!strcasecmp(name, dotstr[ent])) {
                                *dep = dot + ent;
                                return 0;
                        }

        if (dir == NULL) {
                dir = alloc(sizeof(DOS_DIR) * DEPSEC);
                if (dir == NULL)
                        return ENOMEM;
        }

        if (!clus && fs->fatsz == 32)
                clus = fs->rdcl;
        nsec = !clus ? entsec(fs->dirents) : fs->spc;
        lsec = 0;
        xdn = chk = 0;
        for (;;) {
                if (!clus && !lsec)
                        lsec = fs->lsndir;
                else if (okclus(fs, clus))
                        lsec = blklsn(fs, clus);
                else {
                        err = EINVAL;
                        goto out;
                }
                for (sec = 0; sec < nsec; sec++) {
                        if ((err = ioget(fs->fd, lsec + sec, dir, 1)))
                                goto out;
                        for (ent = 0; ent < DEPSEC; ent++) {
                                if (!*dir[ent].de.deName) {
                                        err = ENOENT;
                                        goto out;
                                }
                                if (*dir[ent].de.deName != 0xe5) {
                                        if (dir[ent].de.deAttributes ==
                                            ATTR_WIN95) {
                                                x = dir[ent].xde.weCnt;
                                                if (x & WIN_LAST ||
                                                    (x + 1 == xdn &&
                                                     dir[ent].xde.weChksum ==
                                                     chk)) {
                                                        if (x & WIN_LAST) {
                                                                chk = dir[ent].xde.weChksum;
                                                                x &= WIN_CNT;
                                                        }
                                                        if (x >= 1 && x <= 20) {
                                                                cp_xdnm(lfn, &dir[ent].xde);
                                                                xdn = x;
                                                                continue;
                                                        }
                                                }
                                        } else if (!(dir[ent].de.deAttributes &
                                                     ATTR_VOLUME)) {
                                                ok = xdn == 1;
                                                if (ok) {
                                                        for (x = 0, i = 0;
                                                             i < 11; i++)
                                                                x = ((((x & 1) << 7) | (x >> 1)) +
                                                                    (size_t)msdos_dirchar(&dir[ent].de,(size_t)i)) & 0xff;
                                                        ok = chk == x &&
                                                            !strcasecmp(name, (const char *)lfn);
                                                }
                                                if (!ok) {
                                                        cp_sfn(sfn, &dir[ent].de);
                                                        ok = !strcasecmp(name, (const char *)sfn);
                                                }
                                                if (ok) {
                                                        *dep = &dir[ent].de;
                                                        goto out2;
                                                }
                                        }
                                }
                                xdn = 0;
                        }
                }
                if (!clus)
                        break;
                if ((err = fatget(fs, &clus)))
                        goto out;
                if (fatend(fs->fatsz, clus))
                        break;
        }
        err = ENOENT;
 out:
        dealloc(dir, sizeof(DOS_DIR) * DEPSEC);
        dir = NULL;
 out2:
        return err;
}

/*
 * Copy name from extended directory entry
 */
static void
cp_xdnm(u_char *lfn, struct winentry *xde)
{
        static const struct {
                u_int   off;
                u_int   dim;
        } ix[3] = {
                { offsetof(struct winentry, wePart1),
                    sizeof(xde->wePart1) / 2 },
                { offsetof(struct winentry, wePart2),
                    sizeof(xde->wePart2) / 2 },
                { offsetof(struct winentry, wePart3),
                    sizeof(xde->wePart3) / 2 }
        };
        u_char *p;
        u_int   n, x, c;

        lfn += 13 * ((xde->weCnt & WIN_CNT) - 1);
        for (n = 0; n < 3; n++)
                for (p = (u_char *)xde + ix[n].off, x = ix[n].dim; x;
                    p += 2, x--) {
                        if ((c = getushort(p)) && (c < 32 || c > 127))
                                c = '?';
                        if (!(*lfn++ = (u_char)c))
                                return;
                }
        if (xde->weCnt & WIN_LAST)
                *lfn = 0;
}

/*
 * Copy short filename
 */
static void
cp_sfn(u_char *sfn, struct direntry *de)
{
        u_char *p;
        int     j, i;

        p = sfn;
        if (*de->deName != ' ') {
                for (j = 7; de->deName[j] == ' '; j--);
                for (i = 0; i <= j; i++)
                        *p++ = de->deName[i];
                if (*de->deExtension != ' ') {
                        *p++ = '.';
                        for (j = 2; de->deExtension[j] == ' '; j--);
                        for (i = 0; i <= j; i++)
                                *p++ = de->deExtension[i];
                }
        }
        *p = 0;
        if (*sfn == 5)
                *sfn = 0xe5;
}

/*
 * Return size of file in bytes
 */
static  off_t
fsize(DOS_FS *fs, struct direntry *de)
{
        size_t  size;
        u_int   c;
        int     n;

        if (!(size = getulong(de->deFileSize)) &&
            de->deAttributes & ATTR_DIRECTORY) {
                if (!(c = getushort(de->deStartCluster))) {
                        size = fs->dirents * sizeof(struct direntry);
                } else {
                        if ((n = fatcnt(fs, c)) == -1)
                                return n;
                        size = (size_t)blkbyt(fs, n);
                }
        }
        return (off_t)size;
}

/*
 * Count number of clusters in chain
 */
static int
fatcnt(DOS_FS *fs, u_int c)
{
        int     n;

        for (n = 0; okclus(fs, c); n++)
                if (fatget(fs, &c))
                        return -1;
        return fatend(fs->fatsz, c) ? n : -1;
}

/*
 * Get next cluster in cluster chain
 */
static int
fatget(DOS_FS *fs, u_int *c)
{
        u_char  buf[4];
        u_int   x;
        int     err;

        err = ioread(fs, secbyt(fs->lsnfat) + fatoff(fs->fatsz, *c), buf,
            fs->fatsz != 32 ? 2 : 4);
        if (err)
                return err;
        x = fs->fatsz != 32 ? getushort(buf) : getulong(buf);
        *c = fs->fatsz == 12 ? *c & 1 ? x >> 4 : x & 0xfff : x;
        return 0;
}

/*
 * Is cluster an end-of-chain marker?
 */
static int
fatend(u_int sz, u_int c)
{
        return c > (sz == 12 ? 0xff7U : sz == 16 ? 0xfff7U : 0xffffff7);
}

/*
 * Offset-based I/O primitive
 */
static int
ioread(DOS_FS *fs, BYTE_OFF_T offset, void *buf, u_int nbyte)
{
        char   *s;
        u_int   off, n;
        int     err;

        s = buf;
        if ((off = offset & (SECSIZ - 1))) {
                offset -= off;
                if ((err = iobuf(fs, bytsec(offset))))
                        return err;
                offset += SECSIZ;
                if ((n = SECSIZ - off) > nbyte)
                        n = nbyte;
                memcpy(s, fs->buf + off, n);
                s += n;
                nbyte -= n;
        }
        n = nbyte & (SECSIZ - 1);
        if (nbyte -= n) {
                if ((err = ioget(fs->fd, bytsec(offset), s, bytsec(nbyte))))
                        return err;
                offset += nbyte;
                s += nbyte;
        }
        if (n) {
                if ((err = iobuf(fs, bytsec(offset))))
                        return err;
                memcpy(s, fs->buf, n);
        }
        return 0;
}

/*
 * Buffered sector-based I/O primitive
 */
static int
iobuf(DOS_FS *fs, u_int lsec)
{
        int     err;

        if (fs->bufsec != lsec) {
                if ((err = ioget(fs->fd, lsec, fs->buf, 1)))
                        return err;
                fs->bufsec = lsec;
        }
        return 0;
}

/*
 * Sector-based I/O primitive
 */
static int
ioget(struct open_file *fd, u_int lsec, void *buf, u_int nsec)
{
        size_t rsize;
        int err;

#ifndef LIBSA_NO_TWIDDLE
        twiddle();
#endif
        err = DEV_STRATEGY(fd->f_dev)(fd->f_devdata, F_READ, lsec,
            secbyt(nsec), buf, &rsize);
        return err;
}