source: mainline/uspace/srv/fs/fat/fat_fat.c@ d260a95

/*
 * Copyright (c) 2008 Jakub Jermar
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 *
 * - Redistributions of source code must retain the above copyright
 *   notice, this list of conditions and the following disclaimer.
 * - 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.
 * - The name of the author may not be used to endorse or promote products
 *   derived from this software without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``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 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.
 */

/** @addtogroup fs
 * @{
 */

/**
 * @file        fat_fat.c
 * @brief       Functions that manipulate the File Allocation Tables.
 */

#include "fat_fat.h"
#include "fat_dentry.h"
#include "fat.h"
#include "../../vfs/vfs.h"
#include <libfs.h>
#include <libblock.h>
#include <errno.h>
#include <byteorder.h>
#include <align.h>
#include <assert.h>
#include <fibril_synch.h>
#include <mem.h>

/*
 * Convenience macros for computing some frequently used values from the
 * primitive boot sector members.
 */
#define CLBN2PBN(bs, cl, bn) \
        (SSA((bs)) + ((cl) - FAT_CLST_FIRST) * SPC((bs)) + (bn) % SPC((bs)))

/**
 * The fat_alloc_lock mutex protects all copies of the File Allocation Table
 * during allocation of clusters. The lock does not have to be held durring
 * deallocation of clusters.
 */
static FIBRIL_MUTEX_INITIALIZE(fat_alloc_lock);

/** Walk the cluster chain.
 *
 * @param bs            Buffer holding the boot sector for the file.
 * @param devmap_handle Device handle of the device with the file.
 * @param firstc        First cluster to start the walk with.
 * @param lastc         If non-NULL, output argument hodling the last cluster
 *                      number visited.
 * @param numc          If non-NULL, output argument holding the number of
 *                      clusters seen during the walk.
 * @param max_clusters  Maximum number of clusters to visit.
 *
 * @return              EOK on success or a negative error code.
 */
int
fat_cluster_walk(fat_bs_t *bs, devmap_handle_t devmap_handle, fat_cluster_t firstc,
    fat_cluster_t *lastc, uint16_t *numc, uint16_t max_clusters)
{
        uint16_t clusters = 0;
        fat_cluster_t clst = firstc, clst_last1 = FAT_CLST_LAST1(bs);
        fat_cluster_t clst_bad = FAT_CLST_BAD(bs);
        int rc;

        if (firstc == FAT_CLST_RES0) {
                /* No space allocated to the file. */
                if (lastc)
                        *lastc = firstc;
                if (numc)
                        *numc = 0;
                return EOK;
        }

        while (clst < clst_last1 && clusters < max_clusters) {
                assert(clst >= FAT_CLST_FIRST);
                if (lastc)
                        *lastc = clst;  /* remember the last cluster number */

                /* read FAT1 */
                rc = fat_get_cluster(bs, devmap_handle, FAT1, clst, &clst);
                if (rc != EOK)
                        return rc;

                assert(clst != clst_bad);
                clusters++;
        }

        if (lastc && clst < clst_last1)
                *lastc = clst;
        if (numc)
                *numc = clusters;

        return EOK;
}

/** Read block from file located on a FAT file system.
 *
 * @param block         Pointer to a block pointer for storing result.
 * @param bs            Buffer holding the boot sector of the file system.
 * @param nodep         FAT node.
 * @param bn            Block number.
 * @param flags         Flags passed to libblock.
 *
 * @return              EOK on success or a negative error code.
 */
int
fat_block_get(block_t **block, struct fat_bs *bs, fat_node_t *nodep,
    aoff64_t bn, int flags)
{
        fat_cluster_t firstc = nodep->firstc;
        fat_cluster_t currc;
        aoff64_t relbn = bn;
        int rc;

        if (!nodep->size)
                return ELIMIT;

        if (nodep->firstc == FAT_CLST_ROOT)
                goto fall_through;

        if (((((nodep->size - 1) / BPS(bs)) / SPC(bs)) == bn / SPC(bs)) &&
            nodep->lastc_cached_valid) {
                /*
                 * This is a request to read a block within the last cluster
                 * when fortunately we have the last cluster number cached.
                 */
                return block_get(block, nodep->idx->devmap_handle,
                    CLBN2PBN(bs, nodep->lastc_cached_value, bn), flags);
        }

        if (nodep->currc_cached_valid && bn >= nodep->currc_cached_bn) {
                /*
                 * We can start with the cluster cached by the previous call to
                 * fat_block_get().
                 */
                firstc = nodep->currc_cached_value;
                relbn -= (nodep->currc_cached_bn / SPC(bs)) * SPC(bs);
        }

fall_through:
        rc = _fat_block_get(block, bs, nodep->idx->devmap_handle, firstc,
            &currc, relbn, flags);
        if (rc != EOK)
                return rc;

        /*
         * Update the "current" cluster cache.
         */
        nodep->currc_cached_valid = true;
        nodep->currc_cached_bn = bn;
        nodep->currc_cached_value = currc;

        return rc;
}

/** Read block from file located on a FAT file system.
 *
 * @param block         Pointer to a block pointer for storing result.
 * @param bs            Buffer holding the boot sector of the file system.
 * @param devmap_handle Device handle of the file system.
 * @param fcl           First cluster used by the file. Can be zero if the file
 *                      is empty.
 * @param clp           If not NULL, address where the cluster containing bn
 *                      will be stored.
 *                      stored
 * @param bn            Block number.
 * @param flags         Flags passed to libblock.
 *
 * @return              EOK on success or a negative error code.
 */
int
_fat_block_get(block_t **block, fat_bs_t *bs, devmap_handle_t devmap_handle,
    fat_cluster_t fcl, fat_cluster_t *clp, aoff64_t bn, int flags)
{
        uint16_t clusters;
        unsigned max_clusters;
        fat_cluster_t c;
        int rc;

        /*
         * This function can only operate on non-zero length files.
         */
        if (fcl == FAT_CLST_RES0)
                return ELIMIT;

        if (fcl == FAT_CLST_ROOT) {
                /* root directory special case */
                assert(bn < RDS(bs));
                rc = block_get(block, devmap_handle,
                    RSCNT(bs) + FATCNT(bs) * SF(bs) + bn, flags);
                return rc;
        }

        max_clusters = bn / SPC(bs);
        rc = fat_cluster_walk(bs, devmap_handle, fcl, &c, &clusters, max_clusters);
        if (rc != EOK)
                return rc;
        assert(clusters == max_clusters);

        rc = block_get(block, devmap_handle, CLBN2PBN(bs, c, bn), flags);

        if (clp)
                *clp = c;

        return rc;
}

/** Fill the gap between EOF and a new file position.
 *
 * @param bs            Buffer holding the boot sector for nodep.
 * @param nodep         FAT node with the gap.
 * @param mcl           First cluster in an independent cluster chain that will
 *                      be later appended to the end of the node's own cluster
 *                      chain. If pos is still in the last allocated cluster,
 *                      this argument is ignored.
 * @param pos           Position in the last node block.
 *
 * @return              EOK on success or a negative error code.
 */
int fat_fill_gap(fat_bs_t *bs, fat_node_t *nodep, fat_cluster_t mcl, aoff64_t pos)
{
        block_t *b;
        aoff64_t o, boundary;
        int rc;

        boundary = ROUND_UP(nodep->size, BPS(bs) * SPC(bs));

        /* zero out already allocated space */
        for (o = nodep->size; o < pos && o < boundary;
            o = ALIGN_DOWN(o + BPS(bs), BPS(bs))) {
                int flags = (o % BPS(bs) == 0) ?
                    BLOCK_FLAGS_NOREAD : BLOCK_FLAGS_NONE;
                rc = fat_block_get(&b, bs, nodep, o / BPS(bs), flags);
                if (rc != EOK)
                        return rc;
                memset(b->data + o % BPS(bs), 0, BPS(bs) - o % BPS(bs));
                b->dirty = true;                /* need to sync node */
                rc = block_put(b);
                if (rc != EOK)
                        return rc;
        }

        if (o >= pos)
                return EOK;

        /* zero out the initial part of the new cluster chain */
        for (o = boundary; o < pos; o += BPS(bs)) {
                rc = _fat_block_get(&b, bs, nodep->idx->devmap_handle, mcl,
                    NULL, (o - boundary) / BPS(bs), BLOCK_FLAGS_NOREAD);
                if (rc != EOK)
                        return rc;
                memset(b->data, 0, min(BPS(bs), pos - o));
                b->dirty = true;                /* need to sync node */
                rc = block_put(b);
                if (rc != EOK)
                        return rc;
        }

        return EOK;
}

/** Get cluster from the first FAT.
 *
 * @param bs            Buffer holding the boot sector for the file system.
 * @param devmap_handle Device handle for the file system.
 * @param clst          Cluster which to get.
 * @param value         Output argument holding the value of the cluster.
 *
 * @return              EOK or a negative error code.
 */
int
fat_get_cluster(fat_bs_t *bs, devmap_handle_t devmap_handle, unsigned fatno,
    fat_cluster_t clst, fat_cluster_t *value)
{
        block_t *b, *b1;
        aoff64_t offset;
        int rc;

        assert(fatno < FATCNT(bs));

        if (FAT_IS_FAT12(bs))
                offset = (clst + clst/2);
        else
                offset = (clst * sizeof(fat_cluster_t));

        rc = block_get(&b, devmap_handle, RSCNT(bs) + SF(bs) * fatno +
            offset / BPS(bs), BLOCK_FLAGS_NONE);
        if (rc != EOK)
                return rc;

        /* This cluster access spans a sector boundary. Check only for FAT12 */
        if (FAT_IS_FAT12(bs) && (offset % BPS(bs)+1 == BPS(bs))) {
                /* Is it last sector of FAT? */
                if (offset / BPS(bs) < SF(bs)) {
                        /* No. Reading next sector */
                        rc = block_get(&b1, devmap_handle, 1 + RSCNT(bs) +
                            SF(bs)*fatno + offset / BPS(bs), BLOCK_FLAGS_NONE);
                        if (rc != EOK) {
                                block_put(b);
                                return rc;
                        }
                        /*
                         * Combining value with last byte of current sector and
                         * first byte of next sector
                         */
                        *value  = *(uint8_t *)(b->data + BPS(bs) - 1);
                        *value |= *(uint8_t *)(b1->data);

                        rc = block_put(b1);
                        if (rc != EOK) {
                                block_put(b);
                                return rc;
                        }
                }
                else {
                        /* Yes. It is last sector of FAT */
                        block_put(b);
                        return ERANGE;
                }
        }
        else
                *value = *(fat_cluster_t *)(b->data + offset % BPS(bs));

        if (FAT_IS_FAT12(bs)) {
                if (clst & 0x0001)
                *value = (*value) >> 4;
                else
                *value = (*value) & 0x0fff;
        }

        *value = uint16_t_le2host(*value);
        rc = block_put(b);

        return rc;
}

/** Set cluster in one instance of FAT.
 *
 * @param bs            Buffer holding the boot sector for the file system.
 * @param devmap_handle Device handle for the file system.
 * @param fatno         Number of the FAT instance where to make the change.
 * @param clst          Cluster which is to be set.
 * @param value         Value to set the cluster with.
 *
 * @return              EOK on success or a negative error code.
 */
int
fat_set_cluster(fat_bs_t *bs, devmap_handle_t devmap_handle, unsigned fatno,
    fat_cluster_t clst, fat_cluster_t value)
{
        block_t *b, *b1;
        aoff64_t offset;
        fat_cluster_t *cp, temp;
        int rc;
        int spans = 0;

        assert(fatno < FATCNT(bs));

        if (FAT_IS_FAT12(bs))
                offset = (clst + clst/2);
        else
                offset = (clst * sizeof(fat_cluster_t));

        rc = block_get(&b, devmap_handle, RSCNT(bs) + SF(bs) * fatno +
            offset / BPS(bs), BLOCK_FLAGS_NONE);
        if (rc != EOK)
                return rc;

        /* This cluster access spans a sector boundary. Check only for FAT12 */
        if (FAT_IS_FAT12(bs) && (offset % BPS(bs)+1 == BPS(bs))) {
                /* Is it last sector of FAT? */
                if (offset / BPS(bs) < SF(bs)) {
                        /* No. Reading next sector */
                        rc = block_get(&b1, devmap_handle, 1 + RSCNT(bs) +
                            SF(bs)*fatno + offset / BPS(bs), BLOCK_FLAGS_NONE);
                        if (rc != EOK) {
                                block_put(b);
                                return rc;
                        }
                        /*
                         * Combining value with last byte of current sector and
                         * first byte of next sector
                         */
                        spans=1;
                        cp = &temp;
                        *cp  = *(uint8_t *)(b->data + BPS(bs) - 1);
                        *cp |= *(uint8_t *)(b1->data);
                }
                else {
                        /* Yes. It is last sector of fat */
                        block_put(b);
                        return ERANGE;
                }
        }
        else
                cp = (fat_cluster_t *)(b->data + offset % BPS(bs));

        value = host2uint16_t_le(value);
        if (FAT_IS_FAT12(bs)) {
                if (clst & 0x0001) {
                        *cp &= 0x000f;
                        *cp |= value << 4;
                }
                else {
                        *cp &= 0xf000;
                        *cp |= value & 0x0fff;
                }

                if (spans)
                {
                        *(uint8_t *)(b->data + BPS(bs) - 1) = cp[0];
                        *(uint8_t *)(b1->data) = cp[1];

                        b1->dirty = true;
                        rc = block_put(b1);
                        if (rc != EOK) {
                                block_put(b);
                                return rc;
                        }
                }
        }
        else
                *cp = value;

        b->dirty = true;        /* need to sync block */
        rc = block_put(b);
        return rc;
}

/** Replay the allocatoin of clusters in all shadow instances of FAT.
 *
 * @param bs            Buffer holding the boot sector of the file system.
 * @param devmap_handle Device handle of the file system.
 * @param lifo          Chain of allocated clusters.
 * @param nclsts        Number of clusters in the lifo chain.
 *
 * @return              EOK on success or a negative error code.
 */
int fat_alloc_shadow_clusters(fat_bs_t *bs, devmap_handle_t devmap_handle,
    fat_cluster_t *lifo, unsigned nclsts)
{
        uint8_t fatno;
        unsigned c;
        fat_cluster_t clst_last1 = FAT_CLST_LAST1(bs);
        int rc;

        for (fatno = FAT1 + 1; fatno < bs->fatcnt; fatno++) {
                for (c = 0; c < nclsts; c++) {
                        rc = fat_set_cluster(bs, devmap_handle, fatno, lifo[c],
                            c == 0 ? clst_last1 : lifo[c - 1]);
                        if (rc != EOK)
                                return rc;
                }
        }

        return EOK;
}

/** Allocate clusters in all copies of FAT.
 *
 * This function will attempt to allocate the requested number of clusters in
 * all instances of the FAT.  The FAT will be altered so that the allocated
 * clusters form an independent chain (i.e. a chain which does not belong to any
 * file yet).
 *
 * @param bs            Buffer holding the boot sector of the file system.
 * @param devmap_handle Device handle of the file system.
 * @param nclsts        Number of clusters to allocate.
 * @param mcl           Output parameter where the first cluster in the chain
 *                      will be returned.
 * @param lcl           Output parameter where the last cluster in the chain
 *                      will be returned.
 *
 * @return              EOK on success, a negative error code otherwise.
 */
int
fat_alloc_clusters(fat_bs_t *bs, devmap_handle_t devmap_handle, unsigned nclsts,
    fat_cluster_t *mcl, fat_cluster_t *lcl)
{
        fat_cluster_t *lifo;    /* stack for storing free cluster numbers */
        unsigned found = 0;     /* top of the free cluster number stack */
        fat_cluster_t clst, value, clst_last1 = FAT_CLST_LAST1(bs);
        int rc = EOK;

        lifo = (fat_cluster_t *) malloc(nclsts * sizeof(fat_cluster_t));
        if (!lifo)
                return ENOMEM;
        /*
         * Search FAT1 for unused clusters.
         */
        fibril_mutex_lock(&fat_alloc_lock);
        for (clst=FAT_CLST_FIRST; clst < CC(bs)+2 && found < nclsts; clst++) {
                rc = fat_get_cluster(bs, devmap_handle, FAT1, clst, &value);
                if (rc != EOK)
                break;

                if (value == FAT_CLST_RES0) {
                /*
                 * The cluster is free. Put it into our stack
                 * of found clusters and mark it as non-free.
                 */
                lifo[found] = clst;
                rc = fat_set_cluster(bs, devmap_handle, FAT1, clst,
                    (found == 0) ?  clst_last1 : lifo[found - 1]);
                if (rc != EOK)
                        break;

                found++;
                }
        }

        if (rc == EOK && found == nclsts) {
                rc = fat_alloc_shadow_clusters(bs, devmap_handle, lifo, nclsts);
                if (rc == EOK) {
                        *mcl = lifo[found - 1];
                        *lcl = lifo[0];
                        free(lifo);
                        fibril_mutex_unlock(&fat_alloc_lock);
                        return EOK;
                }
        }

        /* If something wrong - free the clusters */
        if (found > 0) {
                while (found--) {
                rc = fat_set_cluster(bs, devmap_handle, FAT1, lifo[found],
                    FAT_CLST_RES0);
                }
        }

        free(lifo);
        fibril_mutex_unlock(&fat_alloc_lock);
        return ENOSPC;
}

/** Free clusters forming a cluster chain in all copies of FAT.
 *
 * @param bs            Buffer hodling the boot sector of the file system.
 * @param devmap_handle Device handle of the file system.
 * @param firstc        First cluster in the chain which is to be freed.
 *
 * @return              EOK on success or a negative return code.
 */
int
fat_free_clusters(fat_bs_t *bs, devmap_handle_t devmap_handle, fat_cluster_t firstc)
{
        unsigned fatno;
        fat_cluster_t nextc, clst_bad = FAT_CLST_BAD(bs);
        int rc;

        /* Mark all clusters in the chain as free in all copies of FAT. */
        while (firstc < FAT_CLST_LAST1(bs)) {
                assert(firstc >= FAT_CLST_FIRST && firstc < clst_bad);
                rc = fat_get_cluster(bs, devmap_handle, FAT1, firstc, &nextc);
                if (rc != EOK)
                        return rc;
                for (fatno = FAT1; fatno < bs->fatcnt; fatno++) {
                        rc = fat_set_cluster(bs, devmap_handle, fatno, firstc,
                            FAT_CLST_RES0);
                        if (rc != EOK)
                                return rc;
                }

                firstc = nextc;
        }

        return EOK;
}

/** Append a cluster chain to the last file cluster in all FATs.
 *
 * @param bs            Buffer holding the boot sector of the file system.
 * @param nodep         Node representing the file.
 * @param mcl           First cluster of the cluster chain to append.
 * @param lcl           Last cluster of the cluster chain to append.
 *
 * @return              EOK on success or a negative error code.
 */
int
fat_append_clusters(fat_bs_t *bs, fat_node_t *nodep, fat_cluster_t mcl,
    fat_cluster_t lcl)
{
        devmap_handle_t devmap_handle = nodep->idx->devmap_handle;
        fat_cluster_t lastc;
        uint8_t fatno;
        int rc;

        if (nodep->firstc == FAT_CLST_RES0) {
                /* No clusters allocated to the node yet. */
                nodep->firstc = mcl;
                nodep->dirty = true;    /* need to sync node */
        } else {
                if (nodep->lastc_cached_valid) {
                        lastc = nodep->lastc_cached_value;
                        nodep->lastc_cached_valid = false;
                } else {
                        rc = fat_cluster_walk(bs, devmap_handle, nodep->firstc,
                            &lastc, NULL, (uint16_t) -1);
                        if (rc != EOK)
                                return rc;
                }

                for (fatno = FAT1; fatno < bs->fatcnt; fatno++) {
                        rc = fat_set_cluster(bs, nodep->idx->devmap_handle,
                            fatno, lastc, mcl);
                        if (rc != EOK)
                                return rc;
                }
        }

        nodep->lastc_cached_valid = true;
        nodep->lastc_cached_value = lcl;

        return EOK;
}

/** Chop off node clusters in all copies of FAT.
 *
 * @param bs            Buffer holding the boot sector of the file system.
 * @param nodep         FAT node where the chopping will take place.
 * @param lcl           Last cluster which will remain in the node. If this
 *                      argument is FAT_CLST_RES0, then all clusters will
 *                      be chopped off.
 *
 * @return              EOK on success or a negative return code.
 */
int fat_chop_clusters(fat_bs_t *bs, fat_node_t *nodep, fat_cluster_t lcl)
{
        fat_cluster_t clst_last1 = FAT_CLST_LAST1(bs);
        int rc;
        devmap_handle_t devmap_handle = nodep->idx->devmap_handle;

        /*
         * Invalidate cached cluster numbers.
         */
        nodep->lastc_cached_valid = false;
        if (nodep->currc_cached_value != lcl)
                nodep->currc_cached_valid = false;

        if (lcl == FAT_CLST_RES0) {
                /* The node will have zero size and no clusters allocated. */
                rc = fat_free_clusters(bs, devmap_handle, nodep->firstc);
                if (rc != EOK)
                        return rc;
                nodep->firstc = FAT_CLST_RES0;
                nodep->dirty = true;            /* need to sync node */
        } else {
                fat_cluster_t nextc;
                unsigned fatno;

                rc = fat_get_cluster(bs, devmap_handle, FAT1, lcl, &nextc);
                if (rc != EOK)
                        return rc;

                /* Terminate the cluster chain in all copies of FAT. */
                for (fatno = FAT1; fatno < bs->fatcnt; fatno++) {
                        rc = fat_set_cluster(bs, devmap_handle, fatno, lcl,
                            clst_last1);
                        if (rc != EOK)
                                return rc;
                }

                /* Free all following clusters. */
                rc = fat_free_clusters(bs, devmap_handle, nextc);
                if (rc != EOK)
                        return rc;
        }

        /*
         * Update and re-enable the last cluster cache.
         */
        nodep->lastc_cached_valid = true;
        nodep->lastc_cached_value = lcl;

        return EOK;
}

int
fat_zero_cluster(struct fat_bs *bs, devmap_handle_t devmap_handle, fat_cluster_t c)
{
        int i;
        block_t *b;
        int rc;

        for (i = 0; i < SPC(bs); i++) {
                rc = _fat_block_get(&b, bs, devmap_handle, c, NULL, i,
                    BLOCK_FLAGS_NOREAD);
                if (rc != EOK)
                        return rc;
                memset(b->data, 0, BPS(bs));
                b->dirty = true;
                rc = block_put(b);
                if (rc != EOK)
                        return rc;
        }

        return EOK;
}

/** Perform basic sanity checks on the file system.
 *
 * Verify if values of boot sector fields are sane. Also verify media
 * descriptor. This is used to rule out cases when a device obviously
 * does not contain a fat file system.
 */
int fat_sanity_check(fat_bs_t *bs, devmap_handle_t devmap_handle)
{
        fat_cluster_t e0, e1;
        unsigned fat_no;
        int rc;

        /* Check number of FATs. */
        if (bs->fatcnt == 0)
                return ENOTSUP;

        /* Check total number of sectors. */
        if (bs->totsec16 == 0 && bs->totsec32 == 0)
                return ENOTSUP;

        if (bs->totsec16 != 0 && bs->totsec32 != 0 &&
            bs->totsec16 != bs->totsec32)
                return ENOTSUP;

        /* Check media descriptor. Must be between 0xf0 and 0xff. */
        if ((bs->mdesc & 0xf0) != 0xf0)
                return ENOTSUP;

        /* Check number of sectors per FAT. */
        if (bs->sec_per_fat == 0)
                return ENOTSUP;

        /*
         * Check that the root directory entries take up whole blocks.
         * This check is rather strict, but it allows us to treat the root
         * directory and non-root directories uniformly in some places.
         * It can be removed provided that functions such as fat_read() are
         * sanitized to support file systems with this property.
         */
        if ((uint16_t_le2host(bs->root_ent_max) * sizeof(fat_dentry_t)) %
            uint16_t_le2host(bs->bps) != 0)
                return ENOTSUP;

        /* Check signature of each FAT. */
        for (fat_no = 0; fat_no < bs->fatcnt; fat_no++) {
                rc = fat_get_cluster(bs, devmap_handle, fat_no, 0, &e0);
                if (rc != EOK)
                        return EIO;

                rc = fat_get_cluster(bs, devmap_handle, fat_no, 1, &e1);
                if (rc != EOK)
                        return EIO;

                /* Check that first byte of FAT contains the media descriptor. */
                if ((e0 & 0xff) != bs->mdesc)
                        return ENOTSUP;

                /*
                 * Check that remaining bits of the first two entries are
                 * set to one.
                 */
                if (!FAT_IS_FAT12(bs) && ((e0 >> 8) != 0xff || e1 != 0xffff))
                        return ENOTSUP;
        }

        return EOK;
}

/**
 * @}
 */