/*      $OpenBSD: msdosfs_fat.c,v 1.8 2024/04/23 13:34:51 jsg Exp $     */
/*      $NetBSD: msdosfs_fat.c,v 1.31 2016/05/07 16:43:02 mlelstv Exp $ */

/*-
 * Copyright (C) 1994, 1995, 1997 Wolfgang Solfrank.
 * Copyright (C) 1994, 1995, 1997 TooLs GmbH.
 * All rights reserved.
 * Original code by Paul Popelka (paulp@uts.amdahl.com) (see below).
 *
 * 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.
 * 3. All advertising materials mentioning features or use of this software
 *    must display the following acknowledgement:
 *      This product includes software developed by TooLs GmbH.
 * 4. The name of TooLs GmbH may not be used to endorse or promote products
 *    derived from this software without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY TOOLS GMBH ``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 TOOLS GMBH 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.
 */
/*
 * Written by Paul Popelka (paulp@uts.amdahl.com)
 *
 * You can do anything you want with this software, just don't say you wrote
 * it, and don't remove this notice.
 *
 * This software is provided "as is".
 *
 * The author supplies this software to be publicly redistributed on the
 * understanding that the author is not responsible for the correct
 * functioning of this software in any circumstances and is not liable for
 * any damages caused by this software.
 *
 * October 1992
 */

#include <sys/types.h>
#include <sys/time.h>

/*
 * kernel include files.
 */

#include "ffs/buf.h"

/*
 * msdosfs include files.
 */
#include <msdosfs/bpb.h>
#include "msdos/msdosfsmount.h"
#include "msdos/direntry.h"
#include "msdos/denode.h"
#include "msdos/fat.h"
#include "makefs.h"

/*
 * Fat cache stats.
 */
int fc_fileextends;             /* # of file extends                     */
int fc_lfcempty;                /* # of time last file cluster cache entry
                                 * was empty */
int fc_bmapcalls;               /* # of times pcbmap was called          */

#define LMMAX   20
int fc_lmdistance[LMMAX];       /* counters for how far off the last
                                 * cluster mapped entry was. */
int fc_largedistance;           /* off by more than LMMAX                */
int fc_wherefrom, fc_whereto, fc_lastclust;
int pm_fatblocksize;

#ifdef MSDOSFS_DEBUG
#define DPRINTF(a) printf a
#else
#define DPRINTF(a)
#endif

static void fatblock(struct msdosfsmount *, u_long, u_long *, u_long *,
                          u_long *);
void updatefats(struct msdosfsmount *, struct mkfsbuf *, u_long);
static inline void usemap_free(struct msdosfsmount *, u_long);
static inline void usemap_alloc(struct msdosfsmount *, u_long);
static int fatchain(struct msdosfsmount *, u_long, u_long, u_long);
int chainlength(struct msdosfsmount *, u_long, u_long);
int chainalloc(struct msdosfsmount *, u_long, u_long, u_long, u_long *,
                    u_long *);

static void
fatblock(struct msdosfsmount *pmp, u_long ofs, u_long *bnp, u_long *sizep, u_long *bop)
{
        u_long bn, size;

        bn = ofs / pmp->pm_fatblocksize * pmp->pm_fatblocksec;
        size = MINIMUM(pmp->pm_fatblocksec, pmp->pm_FATsecs - bn)
            * pmp->pm_BytesPerSec;
        bn += pmp->pm_fatblk + pmp->pm_curfat * pmp->pm_FATsecs;

        DPRINTF(("%s(ofs=%lu bn=%lu, size=%lu, bo=%lu)\n", __func__, ofs, bn,
            size, ofs % pmp->pm_fatblocksize));
        if (bnp)
                *bnp = bn;
        if (sizep)
                *sizep = size;
        if (bop)
                *bop = ofs % pmp->pm_fatblocksize;

        pm_fatblocksize = pmp->pm_fatblocksize;
}

/*
 * Map the logical cluster number of a file into a physical disk sector
 * that is filesystem relative.
 *
 * dep    - address of denode representing the file of interest
 * findcn - file relative cluster whose filesystem relative cluster number
 *          and/or block number are/is to be found
 * bnp    - address of where to place the file system relative block number.
 *          If this pointer is null then don't return this quantity.
 * cnp    - address of where to place the file system relative cluster number.
 *          If this pointer is null then don't return this quantity.
 *
 * NOTE: Either bnp or cnp must be non-null.
 * This function has one side effect.  If the requested file relative cluster
 * is beyond the end of file, then the actual number of clusters in the file
 * is returned in *cnp.  This is useful for determining how long a directory is.
 *  If cnp is null, nothing is returned.
 */
int
pcbmap(struct denode *dep, u_long findcn, daddr_t *bnp, u_long *cnp, int *sp)
        /* findcn:               file relative cluster to get            */
        /* bnp:          returned filesys rel sector number      */
        /* cnp:          returned cluster number                 */
        /* sp:           returned block size                     */
{
        int error;
        u_long i;
        u_long cn;
        u_long prevcn = 0; /* XXX: prevcn could be used uninitialized */
        u_long byteoffset;
        u_long bn;
        u_long bo;
        struct mkfsbuf *bp = NULL;
        u_long bp_bn = -1;
        struct msdosfsmount *pmp = dep->de_pmp;
        u_long bsize;

        fc_bmapcalls++;

        /*
         * If they don't give us someplace to return a value then don't
         * bother doing anything.
         */
        if (bnp == NULL && cnp == NULL && sp == NULL)
                return (0);

        cn = dep->de_StartCluster;
        DPRINTF(("%s(start cluster=%lu)\n", __func__, cn));
        /*
         * The "file" that makes up the root directory is contiguous,
         * permanently allocated, of fixed size, and is not made up of
         * clusters.  If the cluster number is beyond the end of the root
         * directory, then return the number of clusters in the file.
         */
        if (cn == MSDOSFSROOT) {
                if (dep->de_Attributes & ATTR_DIRECTORY) {
                        if (de_cn2off(pmp, findcn) >= dep->de_FileSize) {
                                if (cnp)
                                        *cnp = de_bn2cn(pmp, pmp->pm_rootdirsize);
                                DPRINTF(("%s(root, %lu ETOOBIG)\n", __func__,
                                    de_cn2off(pmp, findcn)));
                                return (E2BIG);
                        }
                        if (bnp)
                                *bnp = pmp->pm_rootdirblk + de_cn2bn(pmp, findcn);
                        if (cnp)
                                *cnp = MSDOSFSROOT;
                        if (sp)
                                *sp = MINIMUM(pmp->pm_bpcluster,
                                    dep->de_FileSize - de_cn2off(pmp, findcn));
                        DPRINTF(("%s(root, bn=%lu, cn=%u)\n", __func__,
                            pmp->pm_rootdirblk + de_cn2bn(pmp, findcn),
                            MSDOSFSROOT));
                        return (0);
                } else {                /* just an empty file */
                        if (cnp)
                                *cnp = 0;
                        DPRINTF(("%s(root, empty ETOOBIG)\n", __func__));
                        return (E2BIG);
                }
        }

        /*
         * All other files do I/O in cluster sized blocks
         */
        if (sp)
                *sp = pmp->pm_bpcluster;

        /*
         * Rummage around in the FAT cache, maybe we can avoid tromping
         * thru every FAT entry for the file. And, keep track of how far
         * off the cache was from where we wanted to be.
         */
        i = 0;
        fc_lookup(dep, findcn, &i, &cn);
        DPRINTF(("%s(bpcluster=%lu i=%lu cn=%lu\n", __func__, pmp->pm_bpcluster,
            i, cn));
        if ((bn = findcn - i) >= LMMAX) {
                fc_largedistance++;
                fc_wherefrom = i;
                fc_whereto = findcn;
                fc_lastclust = dep->de_fc[FC_LASTFC].fc_frcn;
        } else
                fc_lmdistance[bn]++;

        /*
         * Handle all other files or directories the normal way.
         */
        for (; i < findcn; i++) {
                /*
                 * Stop with all reserved clusters, not just with EOF.
                 */
                if (cn >= (CLUST_RSRVD & pmp->pm_fatmask))
                        goto hiteof;

                /*
                 * Also stop when cluster is not in the filesystem
                 */
                if (cn < CLUST_FIRST || cn > pmp->pm_maxcluster) {
                        DPRINTF(("%s(cn, %lu not in %lu..%lu)\n", __func__,
                                cn, (u_long)CLUST_FIRST, pmp->pm_maxcluster));
                        if (bp)
                                brelse(bp, 0);
                        return (EINVAL);
                }

                byteoffset = FATOFS(pmp, cn);
                fatblock(pmp, byteoffset, &bn, &bsize, &bo);
                if (bn != bp_bn) {
                        if (bp)
                                brelse(bp, 0);
                        error = bread(pmp->pm_devvp, de_bn2kb(pmp, bn), bsize,
                            0, &bp);
                        if (error) {
                                DPRINTF(("%s(bread, %d)\n", __func__, error));
                                return (error);
                        }
                        bp_bn = bn;
                }
                prevcn = cn;
                if (bo >= bsize) {
                        if (bp)
                                brelse(bp, 0);
                        DPRINTF(("%s(block, %lu >= %lu)\n", __func__, bo,
                            bsize));
                        return (EIO);
                }
                KASSERT(bp != NULL);
                if (FAT32(pmp))
                        cn = getulong((char *)bp->b_data + bo);
                else
                        cn = getushort((char *)bp->b_data + bo);
                if (FAT12(pmp) && (prevcn & 1))
                        cn >>= 4;
                DPRINTF(("%s(cn=%lu masked=%lu)\n", __func__, cn,
                    cn & pmp->pm_fatmask));
                cn &= pmp->pm_fatmask;
        }

        if (!MSDOSFSEOF(cn, pmp->pm_fatmask)) {
                if (bp)
                        brelse(bp, 0);
                if (bnp)
                        *bnp = cntobn(pmp, cn);
                if (cnp)
                        *cnp = cn;
                DPRINTF(("%s(bn=%lu, cn=%lu)\n", __func__, cntobn(pmp, cn),
                    cn));
                fc_setcache(dep, FC_LASTMAP, i, cn);
                return (0);
        }

hiteof:;
        if (cnp)
                *cnp = i;
        if (bp)
                brelse(bp, 0);
        /* update last file cluster entry in the FAT cache */
        fc_setcache(dep, FC_LASTFC, i - 1, prevcn);
        DPRINTF(("%s(eof, %lu)\n", __func__, i));
        return (E2BIG);
}

/*
 * Find the closest entry in the FAT cache to the cluster we are looking
 * for.
 */
void
fc_lookup(struct denode *dep, u_long findcn, u_long *frcnp, u_long *fsrcnp)
{
        int i;
        u_long cn;
        struct fatcache *closest = 0;

        for (i = 0; i < FC_SIZE; i++) {
                cn = dep->de_fc[i].fc_frcn;
                if (cn != FCE_EMPTY && cn <= findcn) {
                        if (closest == 0 || cn > closest->fc_frcn)
                                closest = &dep->de_fc[i];
                }
        }
        if (closest) {
                *frcnp = closest->fc_frcn;
                *fsrcnp = closest->fc_fsrcn;
        }
}

/*
 * Purge the FAT cache in denode dep of all entries relating to file
 * relative cluster frcn and beyond.
 */
void
fc_purge(struct denode *dep, u_int frcn)
{
        int i;
        struct fatcache *fcp;

        fcp = dep->de_fc;
        for (i = 0; i < FC_SIZE; i++, fcp++) {
                if (fcp->fc_frcn >= frcn)
                        fcp->fc_frcn = FCE_EMPTY;
        }
}

/*
 * Update the FAT.
 * If mirroring the FAT, update all copies, with the first copy as last.
 * Else update only the current FAT (ignoring the others).
 *
 * pmp   - msdosfsmount structure for filesystem to update
 * bp    - addr of modified FAT block
 * fatbn - block number relative to begin of filesystem of the modified FAT block.
 */
void
updatefats(struct msdosfsmount *pmp, struct mkfsbuf *bp, u_long fatbn)
{
        int i, error;
        struct mkfsbuf *bpn;

        DPRINTF(("%s(pmp %p, bp %p, fatbn %lu)\n", __func__, pmp, bp, fatbn));

        /*
         * If we have an FSInfo block, update it.
         */
        if (pmp->pm_fsinfo) {
                u_long cn = pmp->pm_nxtfree;

                if (pmp->pm_freeclustercount
                    && (pmp->pm_inusemap[cn / N_INUSEBITS]
                        & (1 << (cn % N_INUSEBITS)))) {
                        /*
                         * The cluster indicated in FSInfo isn't free
                         * any longer.  Got get a new free one.
                         */
                        for (cn = 0; cn < pmp->pm_maxcluster; cn++)
                                if (pmp->pm_inusemap[cn / N_INUSEBITS] != (u_int)-1)
                                        break;
                        pmp->pm_nxtfree = cn
                                + ffs(pmp->pm_inusemap[cn / N_INUSEBITS]
                                      ^ (u_int)-1) - 1;
                }
                /*
                 * XXX  If the fsinfo block is stored on media with
                 *      2KB or larger sectors, is the fsinfo structure
                 *      padded at the end or in the middle?
                 */
                if (bread(pmp->pm_devvp, de_bn2kb(pmp, pmp->pm_fsinfo),
                    pmp->pm_BytesPerSec, B_MODIFY, &bpn) != 0) {
                        /*
                         * Ignore the error, but turn off FSInfo update for the future.
                         */
                        pmp->pm_fsinfo = 0;
                } else {
                        struct fsinfo *fp = (struct fsinfo *)bpn->b_data;

                        putulong(fp->fsinfree, pmp->pm_freeclustercount);
                        putulong(fp->fsinxtfree, pmp->pm_nxtfree);
                        if (pmp->pm_flags & MSDOSFSMNT_WAITONFAT)
                                bwrite(bpn);
                        else
                                bdwrite(bpn);
                }
        }

        if (pmp->pm_flags & MSDOSFS_FATMIRROR) {
                /*
                 * Now copy the block(s) of the modified FAT to the other copies of
                 * the FAT and write them out.  This is faster than reading in the
                 * other FATs and then writing them back out.  This could tie up
                 * the FAT for quite a while. Preventing others from accessing it.
                 * To prevent us from going after the FAT quite so much we use
                 * delayed writes, unless they specified "synchronous" when the
                 * filesystem was mounted.  If synch is asked for then use
                 * bwrite()'s and really slow things down.
                 */
                for (i = 1; i < pmp->pm_FATs; i++) {
                        fatbn += pmp->pm_FATsecs;
                        /* getblk() never fails */
                        bpn = getblk(pmp->pm_devvp, de_bn2kb(pmp, fatbn),
                            bp->b_bcount, 0, 0);
                        memcpy(bpn->b_data, bp->b_data, bp->b_bcount);
                        if (pmp->pm_flags & MSDOSFSMNT_WAITONFAT) {
                                error = bwrite(bpn);
                                if (error)
                                        printf("%s: copy FAT %d (error=%d)\n",
                                                 __func__, i, error);
                        } else
                                bdwrite(bpn);
                }
        }

        /*
         * Write out the first (or current) FAT last.
         */
        if (pmp->pm_flags & MSDOSFSMNT_WAITONFAT) {
                error =  bwrite(bp);
                if (error)
                        printf("%s: write FAT (error=%d)\n",
                                __func__, error);
        } else
                bdwrite(bp);
        /*
         * Maybe update fsinfo sector here?
         */
}

/*
 * Updating entries in 12 bit FATs is a pain in the butt.
 *
 * The following picture shows where nibbles go when moving from a 12 bit
 * cluster number into the appropriate bytes in the FAT.
 *
 *      byte m        byte m+1      byte m+2
 *      +----+----+   +----+----+   +----+----+
 *      |  0    1 |   |  2    3 |   |  4    5 |   FAT bytes
 *      +----+----+   +----+----+   +----+----+
 *
 *      +----+----+----+   +----+----+----+
 *      |  3    0    1 |   |  4    5    2 |
 *      +----+----+----+   +----+----+----+
 *      cluster n          cluster n+1
 *
 * Where n is even. m = n + (n >> 2)
 *
 */
static inline void
usemap_alloc(struct msdosfsmount *pmp, u_long cn)
{

        pmp->pm_inusemap[cn / N_INUSEBITS] |= 1 << (cn % N_INUSEBITS);
        pmp->pm_freeclustercount--;
}

static inline void
usemap_free(struct msdosfsmount *pmp, u_long cn)
{

        pmp->pm_freeclustercount++;
        pmp->pm_inusemap[cn / N_INUSEBITS] &= ~(1 << (cn % N_INUSEBITS));
}

int
clusterfree(struct msdosfsmount *pmp, u_long cluster, u_long *oldcnp)
{
        int error;
        u_long oldcn;

        usemap_free(pmp, cluster);
        error = fatentry(FAT_GET_AND_SET, pmp, cluster, &oldcn, MSDOSFSFREE);
        if (error) {
                usemap_alloc(pmp, cluster);
                return (error);
        }
        /*
         * If the cluster was successfully marked free, then update
         * the count of free clusters, and turn off the "allocated"
         * bit in the "in use" cluster bit map.
         */
        if (oldcnp)
                *oldcnp = oldcn;
        return (0);
}

/*
 * Get or Set or 'Get and Set' the cluster'th entry in the FAT.
 *
 * function     - whether to get or set a fat entry
 * pmp          - address of the msdosfsmount structure for the filesystem
 *                whose FAT is to be manipulated.
 * cn           - which cluster is of interest
 * oldcontents  - address of a word that is to receive the contents of the
 *                cluster'th entry if this is a get function
 * newcontents  - the new value to be written into the cluster'th element of
 *                the FAT if this is a set function.
 *
 * This function can also be used to free a cluster by setting the FAT entry
 * for a cluster to 0.
 *
 * All copies of the FAT are updated if this is a set function. NOTE: If
 * fatentry() marks a cluster as free it does not update the inusemap in
 * the msdosfsmount structure. This is left to the caller.
 */
int
fatentry(int function, struct msdosfsmount *pmp, u_long cn, u_long *oldcontents, u_long newcontents)
{
        int error;
        u_long readcn;
        u_long bn, bo, bsize, byteoffset;
        struct mkfsbuf *bp;

        DPRINTF(("%s(func %d, pmp %p, clust %lu, oldcon %p, newcon " "%lx)\n",
            __func__, function, pmp, cn, oldcontents, newcontents));

#ifdef DIAGNOSTIC
        /*
         * Be sure they asked us to do something.
         */
        if ((function & (FAT_SET | FAT_GET)) == 0) {
                DPRINTF(("%s(): function code doesn't specify get or set\n",
                    __func__));
                return (EINVAL);
        }

        /*
         * If they asked us to return a cluster number but didn't tell us
         * where to put it, give them an error.
         */
        if ((function & FAT_GET) && oldcontents == NULL) {
                DPRINTF(("%s(): get function with no place to put result\n",
                        __func__));
                return (EINVAL);
        }
#endif

        /*
         * Be sure the requested cluster is in the filesystem.
         */
        if (cn < CLUST_FIRST || cn > pmp->pm_maxcluster)
                return (EINVAL);

        byteoffset = FATOFS(pmp, cn);
        fatblock(pmp, byteoffset, &bn, &bsize, &bo);
        if ((error = bread(pmp->pm_devvp, de_bn2kb(pmp, bn), bsize,
            0, &bp)) != 0) {
                return (error);
        }

        if (function & FAT_GET) {
                if (FAT32(pmp))
                        readcn = getulong((char *)bp->b_data + bo);
                else
                        readcn = getushort((char *)bp->b_data + bo);
                if (FAT12(pmp) & (cn & 1))
                        readcn >>= 4;
                readcn &= pmp->pm_fatmask;
                *oldcontents = readcn;
        }
        if (function & FAT_SET) {
                switch (pmp->pm_fatmask) {
                case FAT12_MASK:
                        readcn = getushort((char *)bp->b_data + bo);
                        if (cn & 1) {
                                readcn &= 0x000f;
                                readcn |= newcontents << 4;
                        } else {
                                readcn &= 0xf000;
                                readcn |= newcontents & 0xfff;
                        }
                        putushort((char *)bp->b_data + bo, readcn);
                        break;
                case FAT16_MASK:
                        putushort((char *)bp->b_data + bo, newcontents);
                        break;
                case FAT32_MASK:
                        /*
                         * According to spec we have to retain the
                         * high order bits of the FAT entry.
                         */
                        readcn = getulong((char *)bp->b_data + bo);
                        readcn &= ~FAT32_MASK;
                        readcn |= newcontents & FAT32_MASK;
                        putulong((char *)bp->b_data + bo, readcn);
                        break;
                }
                updatefats(pmp, bp, bn);
                bp = NULL;
                pmp->pm_fmod = 1;
        }
        if (bp)
                brelse(bp, 0);
        return (0);
}

/*
 * Update a contiguous cluster chain
 *
 * pmp      - mount point
 * start    - first cluster of chain
 * count    - number of clusters in chain
 * fillwith - what to write into FAT entry of last cluster
 */
static int
fatchain(struct msdosfsmount *pmp, u_long start, u_long count, u_long fillwith)
{
        int error;
        u_long bn, bo, bsize, byteoffset, readcn, newc;
        struct mkfsbuf *bp;

        DPRINTF(("%s(pmp %p, start %lu, count %lu, fillwith %lx)\n", __func__,
            pmp, start, count, fillwith));
        /*
         * Be sure the clusters are in the filesystem.
         */
        if (start < CLUST_FIRST || start + count - 1 > pmp->pm_maxcluster)
                return (EINVAL);

        while (count > 0) {
                byteoffset = FATOFS(pmp, start);
                fatblock(pmp, byteoffset, &bn, &bsize, &bo);
                error = bread(pmp->pm_devvp, de_bn2kb(pmp, bn), bsize,
                    B_MODIFY, &bp);
                if (error) {
                        return (error);
                }
                while (count > 0) {
                        start++;
                        newc = --count > 0 ? start : fillwith;
                        switch (pmp->pm_fatmask) {
                        case FAT12_MASK:
                                readcn = getushort((char *)bp->b_data + bo);
                                if (start & 1) {
                                        readcn &= 0xf000;
                                        readcn |= newc & 0xfff;
                                } else {
                                        readcn &= 0x000f;
                                        readcn |= newc << 4;
                                }
                                putushort((char *)bp->b_data + bo, readcn);
                                bo++;
                                if (!(start & 1))
                                        bo++;
                                break;
                        case FAT16_MASK:
                                putushort((char *)bp->b_data + bo, newc);
                                bo += 2;
                                break;
                        case FAT32_MASK:
                                readcn = getulong((char *)bp->b_data + bo);
                                readcn &= ~pmp->pm_fatmask;
                                readcn |= newc & pmp->pm_fatmask;
                                putulong((char *)bp->b_data + bo, readcn);
                                bo += 4;
                                break;
                        }
                        if (bo >= bsize)
                                break;
                }
                updatefats(pmp, bp, bn);
        }
        pmp->pm_fmod = 1;
        return (0);
}

/*
 * Check the length of a free cluster chain starting at start.
 *
 * pmp   - mount point
 * start - start of chain
 * count - maximum interesting length
 */
int
chainlength(struct msdosfsmount *pmp, u_long start, u_long count)
{
        u_long idx, max_idx;
        u_int map;
        u_long len;

        max_idx = pmp->pm_maxcluster / N_INUSEBITS;
        idx = start / N_INUSEBITS;
        start %= N_INUSEBITS;
        map = pmp->pm_inusemap[idx];
        map &= ~((1 << start) - 1);
        if (map) {
                len = ffs(map) - 1 - start;
                return (len > count ? count : len);
        }
        len = N_INUSEBITS - start;
        if (len >= count)
                return (count);
        while (++idx <= max_idx) {
                if (len >= count)
                        break;
                if ((map = pmp->pm_inusemap[idx]) != 0) {
                        len +=  ffs(map) - 1;
                        break;
                }
                len += N_INUSEBITS;
        }
        return (len > count ? count : len);
}

/*
 * Allocate contiguous free clusters.
 *
 * pmp        - mount point.
 * start      - start of cluster chain.
 * count      - number of clusters to allocate.
 * fillwith   - put this value into the FAT entry for the
 *              last allocated cluster.
 * retcluster - put the first allocated cluster's number here.
 * got        - how many clusters were actually allocated.
 */
int
chainalloc(struct msdosfsmount *pmp, u_long start, u_long count, u_long fillwith, u_long *retcluster, u_long *got)
{
        int error;
        u_long cl, n;

        for (cl = start, n = count; n-- > 0;)
                usemap_alloc(pmp, cl++);
        if ((error = fatchain(pmp, start, count, fillwith)) != 0)
                return (error);

        DPRINTF(("%s(): allocated cluster chain at %lu (%lu clusters)\n",
            __func__, start, count));
        if (retcluster)
                *retcluster = start;
        if (got)
                *got = count;
        return (0);
}

/*
 * Allocate contiguous free clusters.
 *
 * pmp        - mount point.
 * start      - preferred start of cluster chain.
 * count      - number of clusters requested.
 * fillwith   - put this value into the FAT entry for the
 *              last allocated cluster.
 * retcluster - put the first allocated cluster's number here.
 * got        - how many clusters were actually allocated.
 */
int
clusteralloc(struct msdosfsmount *pmp, u_long start, u_long count, u_long *retcluster, u_long *got)
{
        u_long idx;
        u_long len, newst, foundl, cn, l;
        u_long foundcn = 0; /* XXX: foundcn could be used uninitialized */
        u_long fillwith = CLUST_EOFE;
        u_int map;

        DPRINTF(("%s(): find %lu clusters\n", __func__, count));
        if (start) {
                if ((len = chainlength(pmp, start, count)) >= count)
                        return (chainalloc(pmp, start, count, fillwith, retcluster, got));
        } else {
                /*
                 * This is a new file, initialize start
                 */
                struct timeval tv;

                microtime(&tv);
                start = (tv.tv_usec >> 10) | tv.tv_usec;
                len = 0;
        }

        /*
         * Start at a (pseudo) random place to maximize cluster runs
         * under multiple writers.
         */
        newst = (start * 1103515245 + 12345) % (pmp->pm_maxcluster + 1);
        foundl = 0;

        for (cn = newst; cn <= pmp->pm_maxcluster;) {
                idx = cn / N_INUSEBITS;
                map = pmp->pm_inusemap[idx];
                map |= (1 << (cn % N_INUSEBITS)) - 1;
                if (map != (u_int)-1) {
                        cn = idx * N_INUSEBITS + ffs(map^(u_int)-1) - 1;
                        if ((l = chainlength(pmp, cn, count)) >= count)
                                return (chainalloc(pmp, cn, count, fillwith, retcluster, got));
                        if (l > foundl) {
                                foundcn = cn;
                                foundl = l;
                        }
                        cn += l + 1;
                        continue;
                }
                cn += N_INUSEBITS - cn % N_INUSEBITS;
        }
        for (cn = 0; cn < newst;) {
                idx = cn / N_INUSEBITS;
                map = pmp->pm_inusemap[idx];
                map |= (1 << (cn % N_INUSEBITS)) - 1;
                if (map != (u_int)-1) {
                        cn = idx * N_INUSEBITS + ffs(map^(u_int)-1) - 1;
                        if ((l = chainlength(pmp, cn, count)) >= count)
                                return (chainalloc(pmp, cn, count, fillwith, retcluster, got));
                        if (l > foundl) {
                                foundcn = cn;
                                foundl = l;
                        }
                        cn += l + 1;
                        continue;
                }
                cn += N_INUSEBITS - cn % N_INUSEBITS;
        }

        if (!foundl)
                return (ENOSPC);

        if (len)
                return (chainalloc(pmp, start, len, fillwith, retcluster, got));
        else
                return (chainalloc(pmp, foundcn, foundl, fillwith, retcluster, got));
}


/*
 * Free a chain of clusters.
 *
 * pmp          - address of the msdosfs mount structure for the filesystem
 *                containing the cluster chain to be freed.
 * startcluster - number of the 1st cluster in the chain of clusters to be
 *                freed.
 */
int
freeclusterchain(struct msdosfsmount *pmp, u_long cluster)
{
        int error;
        struct mkfsbuf *bp = NULL;
        u_long bn, bo, bsize, byteoffset;
        u_long readcn, lbn = -1;

        while (cluster >= CLUST_FIRST && cluster <= pmp->pm_maxcluster) {
                byteoffset = FATOFS(pmp, cluster);
                fatblock(pmp, byteoffset, &bn, &bsize, &bo);
                if (lbn != bn) {
                        if (bp)
                                updatefats(pmp, bp, lbn);
                        error = bread(pmp->pm_devvp, de_bn2kb(pmp, bn), bsize,
                            B_MODIFY, &bp);
                        if (error) {
                                return (error);
                        }
                        lbn = bn;
                }
                usemap_free(pmp, cluster);
                KASSERT(bp != NULL);
                switch (pmp->pm_fatmask) {
                case FAT12_MASK:
                        readcn = getushort((char *)bp->b_data + bo);
                        if (cluster & 1) {
                                cluster = readcn >> 4;
                                readcn &= 0x000f;
                                readcn |= MSDOSFSFREE << 4;
                        } else {
                                cluster = readcn;
                                readcn &= 0xf000;
                                readcn |= MSDOSFSFREE & 0xfff;
                        }
                        putushort((char *)bp->b_data + bo, readcn);
                        break;
                case FAT16_MASK:
                        cluster = getushort((char *)bp->b_data + bo);
                        putushort((char *)bp->b_data + bo, MSDOSFSFREE);
                        break;
                case FAT32_MASK:
                        cluster = getulong((char *)bp->b_data + bo);
                        putulong((char *)bp->b_data + bo,
                                 (MSDOSFSFREE & FAT32_MASK) | (cluster & ~FAT32_MASK));
                        break;
                }
                cluster &= pmp->pm_fatmask;
        }
        if (bp)
                updatefats(pmp, bp, bn);
        return (0);
}

/*
 * Read in FAT blocks looking for free clusters. For every free cluster
 * found turn off its corresponding bit in the pm_inusemap.
 */
int
fillinusemap(struct msdosfsmount *pmp)
{
        struct mkfsbuf *bp = NULL;
        u_long cn, readcn;
        int error;
        u_long bn, bo, bsize, byteoffset;

        /*
         * Mark all clusters in use, we mark the free ones in the FAT scan
         * loop further down.
         */
        for (cn = 0; cn < (pmp->pm_maxcluster + N_INUSEBITS) / N_INUSEBITS; cn++)
                pmp->pm_inusemap[cn] = (u_int)-1;

        /*
         * Figure how many free clusters are in the filesystem by ripping
         * through the FAT counting the number of entries whose content is
         * zero.  These represent free clusters.
         */
        pmp->pm_freeclustercount = 0;
        for (cn = CLUST_FIRST; cn <= pmp->pm_maxcluster; cn++) {
                byteoffset = FATOFS(pmp, cn);
                bo = byteoffset % pmp->pm_fatblocksize;
                if (!bo || !bp) {
                        /* Read new FAT block */
                        if (bp)
                                brelse(bp, 0);
                        fatblock(pmp, byteoffset, &bn, &bsize, NULL);
                        error = bread(pmp->pm_devvp, de_bn2kb(pmp, bn), bsize,
                            0, &bp);
                        if (error) {
                                return (error);
                        }
                }
                if (FAT32(pmp))
                        readcn = getulong((char *)bp->b_data + bo);
                else
                        readcn = getushort((char *)bp->b_data + bo);
                if (FAT12(pmp) && (cn & 1))
                        readcn >>= 4;
                readcn &= pmp->pm_fatmask;

                if (readcn == 0)
                        usemap_free(pmp, cn);
        }
        if (bp)
                brelse(bp, 0);
        return (0);
}

/*
 * Allocate a new cluster and chain it onto the end of the file.
 *
 * dep   - the file to extend
 * count - number of clusters to allocate
 * bpp   - where to return the address of the buf header for the first new
 *         file block
 * ncp   - where to put cluster number of the first newly allocated cluster
 *         If this pointer is 0, do not return the cluster number.
 * flags - see fat.h
 *
 * NOTE: This function is not responsible for turning on the DE_UPDATE bit of
 * the de_flag field of the denode and it does not change the de_FileSize
 * field.  This is left for the caller to do.
 */

int
extendfile(struct denode *dep, u_long count, struct mkfsbuf **bpp, u_long *ncp, int flags)
{
        int error;
        u_long frcn = 0, cn, got;
        struct msdosfsmount *pmp = dep->de_pmp;
        struct mkfsbuf *bp;

        /*
         * Don't try to extend the root directory
         */
        if (dep->de_StartCluster == MSDOSFSROOT
            && (dep->de_Attributes & ATTR_DIRECTORY)) {
                DPRINTF(("%s(): attempt to extend root directory\n", __func__));
                return (ENOSPC);
        }

        /*
         * If the "file's last cluster" cache entry is empty, and the file
         * is not empty, then fill the cache entry by calling pcbmap().
         */
        fc_fileextends++;
        if (dep->de_fc[FC_LASTFC].fc_frcn == FCE_EMPTY &&
            dep->de_StartCluster != 0) {
                fc_lfcempty++;
                error = pcbmap(dep, CLUST_END, 0, &cn, 0);
                /* we expect it to return E2BIG */
                if (error != E2BIG)
                        return (error);
        }

        fc_last_to_nexttolast(dep);

        while (count > 0) {

                /*
                 * Allocate a new cluster chain and cat onto the end of the
                 * file.  If the file is empty we make de_StartCluster point
                 * to the new block.  Note that de_StartCluster being 0 is
                 * sufficient to be sure the file is empty since we exclude
                 * attempts to extend the root directory above, and the root
                 * dir is the only file with a startcluster of 0 that has
                 * blocks allocated (sort of).
                 */

                if (dep->de_StartCluster == 0)
                        cn = 0;
                else
                        cn = dep->de_fc[FC_LASTFC].fc_fsrcn + 1;
                error = clusteralloc(pmp, cn, count, &cn, &got);
                if (error)
                        return (error);

                count -= got;

                /*
                 * Give them the filesystem relative cluster number if they want
                 * it.
                 */
                if (ncp) {
                        *ncp = cn;
                        ncp = NULL;
                }

                if (dep->de_StartCluster == 0) {
                        dep->de_StartCluster = cn;
                        frcn = 0;
                } else {
                        error = fatentry(FAT_SET, pmp,
                                         dep->de_fc[FC_LASTFC].fc_fsrcn,
                                         0, cn);
                        if (error) {
                                clusterfree(pmp, cn, NULL);
                                return (error);
                        }
                        frcn = dep->de_fc[FC_LASTFC].fc_frcn + 1;
                }

                /*
                 * Update the "last cluster of the file" entry in the
                 * denode's FAT cache.
                 */

                fc_setcache(dep, FC_LASTFC, frcn + got - 1, cn + got - 1);
                if ((flags & DE_CLEAR) &&
                    (dep->de_Attributes & ATTR_DIRECTORY)) {
                        while (got-- > 0) {
                                bp = getblk(pmp->pm_devvp,
                                    de_bn2kb(pmp, cntobn(pmp, cn++)),
                                    pmp->pm_bpcluster, 0, 0);
                                clrbuf(bp);
                                if (bpp) {
                                        *bpp = bp;
                                        bpp = NULL;
                                } else {
                                        bdwrite(bp);
                                }
                        }
                }
        }

        return (0);
}