fat.c

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/* fat.c - Read/write access to the FAT
 
   Copyright (C) 1993 Werner Almesberger <werner.almesberger@lrc.di.epfl.ch>
   Copyright (C) 1998 Roman Hodek <Roman.Hodek@informatik.uni-erlangen.de>
   Copyright (C) 2008-2014 Daniel Baumann <mail@daniel-baumann.ch>
   Copyright (C) 2021 Pali Rohár <pali.rohar@gmail.com>
 
   This program is free software: you can redistribute it and/or modify
   it under the terms of the GNU General Public License as published by
   the Free Software Foundation, either version 3 of the License, or
   (at your option) any later version.
 
   This program is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
   GNU General Public License for more details.
 
   You should have received a copy of the GNU General Public License
   along with this program. If not, see <http://www.gnu.org/licenses/>.
 
   The complete text of the GNU General Public License
   can be found in /usr/share/common-licenses/GPL-3 file.
*/
 
/* FAT32, VFAT, Atari format support, and various fixes additions May 1998
 * by Roman Hodek <Roman.Hodek@informatik.uni-erlangen.de> */
 
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
 
#include "common.h"
#include "fsck.fat.h"
#include "io.h"
#include "boot.h"
#include "check.h"
#include "fat.h"
 
/**
 * Fetch the FAT entry for a specified cluster.
 *
 * @param[out]  entry       Cluster to which cluster of interest is linked
 * @param[in]   fat     FAT table for the partition
 * @param[in]   cluster     Cluster of interest
 * @param[in]   fs          Information from the FAT boot sectors (bits per FAT entry)
 */
void get_fat(FAT_ENTRY * entry, void *fat, uint32_t cluster, DOS_FS * fs)
{
    unsigned char *ptr;
 
    if (cluster > fs->data_clusters + 1) {
    die("Internal error: cluster out of range in get_fat() (%lu > %lu).",
        (unsigned long)cluster, (unsigned long)(fs->data_clusters + 1));
    }
 
    switch (fs->fat_bits) {
    case 12:
    ptr = &((unsigned char *)fat)[cluster * 3 / 2];
    entry->value = 0xfff & (cluster & 1 ? (ptr[0] >> 4) | (ptr[1] << 4) :
                (ptr[0] | ptr[1] << 8));
    break;
    case 16:
    entry->value = le16toh(((unsigned short *)fat)[cluster]);
    break;
    case 32:
    /* According to M$, the high 4 bits of a FAT32 entry are reserved and
     * are not part of the cluster number. So we cut them off. */
    {
        uint32_t e = le32toh(((unsigned int *)fat)[cluster]);
        entry->value = e & 0xfffffff;
        entry->reserved = e >> 28;
    }
    break;
    default:
    die("Bad FAT entry size: %d bits.", fs->fat_bits);
    }
}
 
void release_fat(DOS_FS * fs)
{
    if (fs->fat)
    free(fs->fat);
    if (fs->cluster_owner)
    free(fs->cluster_owner);
    fs->fat = NULL;
    fs->cluster_owner = NULL;
}
 
static void fix_first_cluster(DOS_FS * fs, void * first_cluster)
{
    struct boot_sector b;
 
    fs_read(0, sizeof(b), &b);
 
    printf("Fixing first cluster in FAT.\n");
    if (fs->fat_bits == 12)
        *(uint16_t *)first_cluster = htole16((le16toh(*(uint16_t *)first_cluster) & 0xf000) | FAT_EXTD(fs) | b.media);
    else if (fs->fat_bits == 16)
        *(uint16_t *)first_cluster = htole16(FAT_EXTD(fs) | b.media);
    else
        *(uint32_t *)first_cluster = htole32(FAT_EXTD(fs) | b.media);
}
 
/**
 * Build a bookkeeping structure from the partition's FAT table.
 * If the partition has multiple FATs and they don't agree, try to pick a winner,
 * and queue a command to overwrite the loser.
 * One error that is fixed here is a cluster that links to something out of range.
 *
 * @param[inout]    fs      Information about the filesystem
 * @param[in]       mode    0 - read-only, 1 - read-write (no repair), 2 - repair
 */
void read_fat(DOS_FS * fs, int mode)
{
    int eff_size, alloc_size;
    uint32_t i;
    void *first, *second = NULL;
    int first_ok, second_ok = 0;
    FAT_ENTRY first_media, second_media;
    uint32_t total_num_clusters;
 
    if (fat_table > fs->nfats)
        die("Requested FAT table %ld does not exist.", fat_table);
    if (fat_table > 2)
        die("Reading FAT table greather than 2 is implemented yet.");
 
    /* Clean up from previous pass */
    release_fat(fs);
 
    total_num_clusters = fs->data_clusters + 2;
    eff_size = (total_num_clusters * fs->fat_bits + 7) / 8ULL;
 
    if (fs->fat_bits != 12)
        alloc_size = eff_size;
    else
        /* round up to an even number of FAT entries to avoid special
         * casing the last entry in get_fat() */
        alloc_size = (total_num_clusters * 12 + 23) / 24 * 3;
 
    first = alloc(alloc_size);
    fs_read(fs->fat_start, eff_size, first);
    get_fat(&first_media, first, 0, fs);
    first_ok = (first_media.value & FAT_EXTD(fs)) == FAT_EXTD(fs);
    if (fs->nfats > 1) {
    second = alloc(alloc_size);
    fs_read(fs->fat_start + fs->fat_size, eff_size, second);
    get_fat(&second_media, second, 0, fs);
    second_ok = (second_media.value & FAT_EXTD(fs)) == FAT_EXTD(fs);
    }
    if (mode != 0 && fat_table == 0) {
        if (!first_ok && second && !second_ok)
            die("Both FATs appear to be corrupt. Giving up. Run fsck.fat with non-zero -F option.");
        if (!first_ok && !second)
            die("First FAT appears to be corrupt and second FAT does not exist. Giving up. Run fsck.fat with -F 1 option.");
    }
    if (mode == 0 && !first_ok && second && second_ok) {
        /* In read-only mode if first FAT is corrupted and second is OK then use second FAT */
        void *first_backup = first;
        first = second;
        second = first_backup;
    }
    if (mode != 0 && fat_table == 0 && second && memcmp(first, second, eff_size) != 0) {
    if (mode != 2)
        die("FATs differ, please run fsck.fat");
    if (first_ok && !second_ok) {
        printf("FATs differ - using first FAT.\n");
        fs_write(fs->fat_start + fs->fat_size, eff_size, first);
    } else if (!first_ok && second_ok) {
        printf("FATs differ - using second FAT.\n");
        fs_write(fs->fat_start, eff_size, second);
        memcpy(first, second, eff_size);
    } else {
        if (first_ok && second_ok)
        printf("FATs differ but appear to be intact.\n");
        else
        printf("FATs differ and both appear to be corrupt.\n");
        if (get_choice(1, "  Using first FAT.",
               2,
               1, "Use first FAT",
               2, "Use second FAT") == 1) {
        if (!first_ok) {
            fix_first_cluster(fs, first);
            fs_write(fs->fat_start, (fs->fat_bits + 7) / 8, first);
        }
        fs_write(fs->fat_start + fs->fat_size, eff_size, first);
        } else {
        if (!second_ok) {
            fix_first_cluster(fs, second);
            fs_write(fs->fat_start + fs->fat_size, (fs->fat_bits + 7) / 8, second);
        }
        fs_write(fs->fat_start, eff_size, second);
        memcpy(first, second, eff_size);
        }
    }
    }
    if (mode != 0 && fat_table != 0) {
        if (fat_table == 1) {
            printf("Using first FAT.\n");
            if (!first_ok) {
                fix_first_cluster(fs, first);
                fs_write(fs->fat_start, (fs->fat_bits + 7) / 8, first);
            }
            if (second && memcmp(first, second, eff_size) != 0)
                fs_write(fs->fat_start + fs->fat_size, eff_size, first);
        } else if (fat_table == 2) {
            printf("Using second FAT.\n");
            if (!second_ok) {
                fix_first_cluster(fs, second);
                fs_write(fs->fat_start + fs->fat_size, (fs->fat_bits + 7) / 8, second);
            }
            if (memcmp(first, second, eff_size) != 0) {
                fs_write(fs->fat_start, eff_size, second);
                memcpy(first, second, eff_size);
            }
        }
    }
    if (second) {
    free(second);
    }
    fs->fat = (unsigned char *)first;
 
    fs->cluster_owner = alloc(total_num_clusters * sizeof(DOS_FILE *));
    memset(fs->cluster_owner, 0, (total_num_clusters * sizeof(DOS_FILE *)));
 
    if (mode == 0)
        return;
 
    /* Truncate any cluster chains that link to something out of range */
    for (i = 2; i < fs->data_clusters + 2; i++) {
    FAT_ENTRY curEntry;
    get_fat(&curEntry, fs->fat, i, fs);
    if (curEntry.value == 1) {
        if (mode != 2)
        die("Cluster %ld out of range (1), please run fsck.fat",
            (long)(i - 2));
        printf("Cluster %ld out of range (1). Setting to EOF.\n",
           (long)(i - 2));
        set_fat(fs, i, -1);
    }
    if (curEntry.value >= fs->data_clusters + 2 &&
        (curEntry.value < FAT_MIN_BAD(fs))) {
        if (mode != 2)
        die("Cluster %ld out of range (%ld > %ld), please run fsck.fat",
            (long)(i - 2), (long)curEntry.value,
            (long)(fs->data_clusters + 2 - 1));
        printf("Cluster %ld out of range (%ld > %ld). Setting to EOF.\n",
           (long)(i - 2), (long)curEntry.value,
           (long)(fs->data_clusters + 2 - 1));
        set_fat(fs, i, -1);
    }
    }
}
 
/**
 * Update the FAT entry for a specified cluster
 * (i.e., change the cluster it links to).
 * Queue a command to write out this change.
 *
 * @param[in,out]   fs          Information about the filesystem
 * @param[in]       cluster     Cluster to change
 * @param[in]       new         Cluster to link to
 *              Special values:
 *                 0 == free cluster
 *                -1 == end-of-chain
 *                -2 == bad cluster
 */
void set_fat(DOS_FS * fs, uint32_t cluster, int32_t new)
{
    unsigned char *data = NULL;
    int size;
    off_t offs;
 
    if (cluster > fs->data_clusters + 1) {
    die("Internal error: cluster out of range in set_fat() (%lu > %lu).",
        (unsigned long)cluster, (unsigned long)(fs->data_clusters + 1));
    }
 
    if (new == -1)
    new = FAT_EOF(fs);
    else if ((long)new == -2)
    new = FAT_BAD(fs);
    else if (new > fs->data_clusters + 1) {
    die("Internal error: new cluster out of range in set_fat() (%lu > %lu).",
        (unsigned long)new, (unsigned long)(fs->data_clusters + 1));
    }
 
    switch (fs->fat_bits) {
    case 12:
    data = fs->fat + cluster * 3 / 2;
    offs = fs->fat_start + cluster * 3 / 2;
    if (cluster & 1) {
        FAT_ENTRY prevEntry;
        get_fat(&prevEntry, fs->fat, cluster - 1, fs);
        data[0] = ((new & 0xf) << 4) | (prevEntry.value >> 8);
        data[1] = new >> 4;
    } else {
        FAT_ENTRY subseqEntry;
        if (cluster != fs->data_clusters + 1)
        get_fat(&subseqEntry, fs->fat, cluster + 1, fs);
        else
        subseqEntry.value = 0;
        data[0] = new & 0xff;
        data[1] = (new >> 8) | ((0xff & subseqEntry.value) << 4);
    }
    size = 2;
    break;
    case 16:
    data = fs->fat + cluster * 2;
    offs = fs->fat_start + cluster * 2;
    *(unsigned short *)data = htole16(new);
    size = 2;
    break;
    case 32:
    {
        FAT_ENTRY curEntry;
        get_fat(&curEntry, fs->fat, cluster, fs);
 
        data = fs->fat + cluster * 4;
        offs = fs->fat_start + cluster * 4;
        /* According to M$, the high 4 bits of a FAT32 entry are reserved and
         * are not part of the cluster number. So we never touch them. */
        *(uint32_t *)data = htole32((new & 0xfffffff) |
                         (curEntry.reserved << 28));
        size = 4;
    }
    break;
    default:
    die("Bad FAT entry size: %d bits.", fs->fat_bits);
    }
    fs_write(offs, size, data);
    if (fs->nfats > 1) {
    fs_write(offs + fs->fat_size, size, data);
    }
}
 
int bad_cluster(DOS_FS * fs, uint32_t cluster)
{
    FAT_ENTRY curEntry;
    get_fat(&curEntry, fs->fat, cluster, fs);
 
    return FAT_IS_BAD(fs, curEntry.value);
}
 
/**
 * Get the cluster to which the specified cluster is linked.
 * If the linked cluster is marked bad, abort.
 *
 * @param[in]   fs          Information about the filesystem
 * @param[in]   cluster     Cluster to follow
 *
 * @return  -1              'cluster' is at the end of the chain
 * @return  Other values    Next cluster in this chain
 */
uint32_t next_cluster(DOS_FS * fs, uint32_t cluster)
{
    uint32_t value;
    FAT_ENTRY curEntry;
 
    get_fat(&curEntry, fs->fat, cluster, fs);
 
    value = curEntry.value;
    if (FAT_IS_BAD(fs, value))
    die("Internal error: next_cluster on bad cluster");
    return FAT_IS_EOF(fs, value) ? -1 : value;
}
 
off_t cluster_start(DOS_FS * fs, uint32_t cluster)
{
    /* TODO: check overflow */
    return fs->data_start + ((off_t)cluster - 2) * (unsigned long long)fs->cluster_size;
}
 
/**
 * Update internal bookkeeping to show that the specified cluster belongs
 * to the specified dentry.
 *
 * @param[in,out]   fs          Information about the filesystem
 * @param[in]       cluster     Cluster being assigned
 * @param[in]       owner       Information on dentry that owns this cluster
 *                              (may be NULL)
 */
void set_owner(DOS_FS * fs, uint32_t cluster, DOS_FILE * owner)
{
    if (fs->cluster_owner == NULL)
    die("Internal error: attempt to set owner in non-existent table");
 
    if (owner && fs->cluster_owner[cluster]
    && (fs->cluster_owner[cluster] != owner))
    die("Internal error: attempt to change file owner");
    fs->cluster_owner[cluster] = owner;
}
 
DOS_FILE *get_owner(DOS_FS * fs, uint32_t cluster)
{
    if (fs->cluster_owner == NULL)
    return NULL;
    else
    return fs->cluster_owner[cluster];
}
 
void fix_bad(DOS_FS * fs)
{
    uint32_t i;
 
    if (verbose)
    printf("Checking for bad clusters.\n");
    for (i = 2; i < fs->data_clusters + 2; i++) {
    FAT_ENTRY curEntry;
    get_fat(&curEntry, fs->fat, i, fs);
 
    if (!get_owner(fs, i) && !FAT_IS_BAD(fs, curEntry.value))
        if (!fs_test(cluster_start(fs, i), fs->cluster_size)) {
        printf("Cluster %lu is unreadable.\n", (unsigned long)i);
        set_fat(fs, i, -2);
        }
    }
}
 
void reclaim_free(DOS_FS * fs)
{
    int reclaimed;
    uint32_t i;
 
    if (verbose)
    printf("Checking for unused clusters.\n");
    reclaimed = 0;
    for (i = 2; i < fs->data_clusters + 2; i++) {
    FAT_ENTRY curEntry;
    get_fat(&curEntry, fs->fat, i, fs);
 
    if (!get_owner(fs, i) && curEntry.value &&
        !FAT_IS_BAD(fs, curEntry.value)) {
        set_fat(fs, i, 0);
        reclaimed++;
    }
    }
    if (reclaimed)
    printf("Reclaimed %d unused cluster%s (%llu bytes).\n", (int)reclaimed,
           reclaimed == 1 ? "" : "s",
           (unsigned long long)reclaimed * fs->cluster_size);
}
 
/**
 * Assign the specified owner to all orphan chains (except cycles).
 * Break cross-links between orphan chains.
 *
 * @param[in,out]   fs             Information about the filesystem
 * @param[in]       owner          dentry to be assigned ownership of orphans
 * @param[in,out]   num_refs       For each orphan cluster [index], how many
 *                 clusters link to it.
 * @param[in]       start_cluster  Where to start scanning for orphans
 */
static void tag_free(DOS_FS * fs, DOS_FILE * owner, uint32_t *num_refs,
             uint32_t start_cluster)
{
    int prev;
    uint32_t i, walk;
 
    if (start_cluster == 0)
    start_cluster = 2;
 
    for (i = start_cluster; i < fs->data_clusters + 2; i++) {
    FAT_ENTRY curEntry;
    get_fat(&curEntry, fs->fat, i, fs);
 
    /* If the current entry is the head of an un-owned chain... */
    if (curEntry.value && !FAT_IS_BAD(fs, curEntry.value) &&
        !get_owner(fs, i) && !num_refs[i]) {
        prev = 0;
        /* Walk the chain, claiming ownership as we go */
        for (walk = i; walk != -1; walk = next_cluster(fs, walk)) {
        if (!get_owner(fs, walk)) {
            set_owner(fs, walk, owner);
        } else {
            /* We've run into cross-links between orphaned chains,
             * or a cycle with a tail.
             * Terminate this orphan chain (break the link)
             */
            set_fat(fs, prev, -1);
 
            /* This is not necessary because 'walk' is owned and thus
             * will never become the head of a chain (the only case
             * that would matter during reclaim to files).
             * It's easier to decrement than to prove that it's
             * unnecessary.
             */
            num_refs[walk]--;
            break;
        }
        prev = walk;
        }
    }
    }
}
 
/**
 * Recover orphan chains to files, handling any cycles or cross-links.
 *
 * @param[in,out]   fs             Information about the filesystem
 */
void reclaim_file(DOS_FS * fs)
{
    DOS_FILE orphan;
    int reclaimed, files;
    int changed = 0;
    uint32_t i, next, walk;
    uint32_t *num_refs = NULL;  /* Only for orphaned clusters */
    uint32_t total_num_clusters;
 
    if (verbose)
    printf("Reclaiming unconnected clusters.\n");
 
    total_num_clusters = fs->data_clusters + 2;
    num_refs = alloc(total_num_clusters * sizeof(uint32_t));
    memset(num_refs, 0, (total_num_clusters * sizeof(uint32_t)));
 
    /* Guarantee that all orphan chains (except cycles) end cleanly
     * with an end-of-chain mark.
     */
 
    for (i = 2; i < total_num_clusters; i++) {
    FAT_ENTRY curEntry;
    get_fat(&curEntry, fs->fat, i, fs);
 
    next = curEntry.value;
    if (!get_owner(fs, i) && next && next < fs->data_clusters + 2) {
        /* Cluster is linked, but not owned (orphan) */
        FAT_ENTRY nextEntry;
        get_fat(&nextEntry, fs->fat, next, fs);
 
        /* Mark it end-of-chain if it links into an owned cluster,
         * a free cluster, or a bad cluster.
         */
        if (get_owner(fs, next) || !nextEntry.value ||
        FAT_IS_BAD(fs, nextEntry.value))
        set_fat(fs, i, -1);
        else
        num_refs[next]++;
    }
    }
 
    /* Scan until all the orphans are accounted for,
     * and all cycles and cross-links are broken
     */
    do {
    tag_free(fs, &orphan, num_refs, changed);
    changed = 0;
 
    /* Any unaccounted-for orphans must be part of a cycle */
    for (i = 2; i < total_num_clusters; i++) {
        FAT_ENTRY curEntry;
        get_fat(&curEntry, fs->fat, i, fs);
 
        if (curEntry.value && !FAT_IS_BAD(fs, curEntry.value) &&
        !get_owner(fs, i)) {
        if (!num_refs[curEntry.value]--)
            die("Internal error: num_refs going below zero");
        set_fat(fs, i, -1);
        changed = curEntry.value;
        printf("Broke cycle at cluster %lu in free chain.\n", (unsigned long)i);
 
        /* If we've created a new chain head,
         * tag_free() can claim it
         */
        if (num_refs[curEntry.value] == 0)
            break;
        }
    }
    }
    while (changed);
 
    /* Now we can start recovery */
    files = reclaimed = 0;
    for (i = 2; i < total_num_clusters; i++)
    /* If this cluster is the head of an orphan chain... */
    if (get_owner(fs, i) == &orphan && !num_refs[i]) {
        DIR_ENT de;
        off_t offset;
        files++;
        offset = alloc_rootdir_entry(fs, &de, "FSCK%04dREC", 1);
        de.start = htole16(i & 0xffff);
        if (fs->fat_bits == 32)
        de.starthi = htole16(i >> 16);
        for (walk = i; walk > 0 && walk != -1;
         walk = next_cluster(fs, walk)) {
        de.size = htole32(le32toh(de.size) + fs->cluster_size);
        reclaimed++;
        }
        fs_write(offset, sizeof(DIR_ENT), &de);
    }
    if (reclaimed)
    printf("Reclaimed %d unused cluster%s (%llu bytes) in %d chain%s.\n",
           reclaimed, reclaimed == 1 ? "" : "s",
           (unsigned long long)reclaimed * fs->cluster_size, files,
           files == 1 ? "" : "s");
 
    free(num_refs);
}
 
uint32_t update_free(DOS_FS * fs)
{
    uint32_t i;
    uint32_t free = 0;
    int do_set = 0;
 
    for (i = 2; i < fs->data_clusters + 2; i++) {
    FAT_ENTRY curEntry;
    get_fat(&curEntry, fs->fat, i, fs);
 
    if (!get_owner(fs, i) && !FAT_IS_BAD(fs, curEntry.value))
        ++free;
    }
 
    if (!fs->fsinfo_start)
    return free;
 
    if (verbose)
    printf("Checking free cluster summary.\n");
    if (fs->free_clusters != 0xFFFFFFFF) {
    if (free != fs->free_clusters) {
        printf("Free cluster summary wrong (%ld vs. really %ld)\n",
           (long)fs->free_clusters, (long)free);
        if (get_choice(1, "  Auto-correcting.",
               2,
               1, "Correct",
               2, "Don't correct") == 1)
        do_set = 1;
    }
    } else {
    printf("Free cluster summary uninitialized (should be %ld)\n", (long)free);
    if (rw) {
        if (get_choice(1, "  Auto-setting.",
               2,
               1, "Set it",
               2, "Leave it uninitialized") == 1)
        do_set = 1;
    }
    }
 
    if (do_set) {
    uint32_t le_free = htole32(free);
    fs->free_clusters = free;
    fs_write(fs->fsinfo_start + offsetof(struct info_sector, free_clusters),
         sizeof(le_free), &le_free);
    }
 
    return free;
}