githubmirror/dosfstools

src/fat.c
560 строк16 KB
Steven J. Magnani
Attempt to improve clarity of the orphan cluster reclaim code.
23 янв 2013, 15:50
23 янв 2013, 15:50bc84254
/* 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>

   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/>.

   On Debian systems, 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 "dosfsck.h"
#include "io.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,unsigned long cluster,DOS_FS *fs)
{
    unsigned char *ptr;

    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 = CF_LE_W(((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. */
	{
	    unsigned long e = CF_LE_L(((unsigned int *) fat)[cluster]);
	    entry->value = e & 0xfffffff;
	    entry->reserved = e >> 28;
	}
	break;
      default:
	die("Bad FAT entry size: %d bits.",fs->fat_bits);
    }
}


/**
 * 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
 */
void read_fat(DOS_FS *fs)
{
    int eff_size;
    unsigned long i;
    void *first,*second = NULL;
    int first_ok,second_ok;
    unsigned long total_num_clusters;

    /* Clean up from previous pass */
    free(fs->fat);
    free(fs->cluster_owner);
    fs->fat = NULL;
    fs->cluster_owner = NULL;

    total_num_clusters = fs->clusters + 2UL;
    eff_size = (total_num_clusters*fs->fat_bits+7)/8ULL;
    first = alloc(eff_size);
    fs_read(fs->fat_start,eff_size,first);
    if (fs->nfats > 1) {
	second = alloc(eff_size);
	fs_read(fs->fat_start+fs->fat_size,eff_size,second);
    }
    if (second && memcmp(first,second,eff_size) != 0) {
	FAT_ENTRY first_media, second_media;
	get_fat(&first_media,first,0,fs);
	get_fat(&second_media,second,0,fs);
	first_ok = (first_media.value & FAT_EXTD(fs)) == FAT_EXTD(fs);
	second_ok = (second_media.value & FAT_EXTD(fs)) == FAT_EXTD(fs);
	if (first_ok && !second_ok) {
	    printf("FATs differ - using first FAT.\n");
	    fs_write(fs->fat_start+fs->fat_size,eff_size,first);
	}
	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);
	}
	if (first_ok && second_ok) {
	    if (interactive) {
		printf("FATs differ but appear to be intact. Use which FAT ?\n"
		  "1) Use first FAT\n2) Use second FAT\n");
		if (get_key("12","?") == '1') {
		    fs_write(fs->fat_start+fs->fat_size,eff_size,first);
		} else {
		    fs_write(fs->fat_start,eff_size,second);
		    memcpy(first,second,eff_size);
		}
	    }
	    else {
		printf("FATs differ but appear to be intact. Using first "
		  "FAT.\n");
		fs_write(fs->fat_start+fs->fat_size,eff_size,first);
	    }
	}
	if (!first_ok && !second_ok) {
	    printf("Both FATs appear to be corrupt. Giving up.\n");
	    exit(1);
	}
    }
    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 *)));

    /* Truncate any cluster chains that link to something out of range */
    for (i = 2; i < fs->clusters+2; i++) {
        FAT_ENTRY curEntry;
        get_fat(&curEntry, fs->fat, i, fs);
	if (curEntry.value == 1) {
	    printf("Cluster %ld out of range (1). Setting to EOF.\n",
		   i-2);
	    set_fat(fs, i, -1);
	}
	if (curEntry.value >= fs->clusters+2 &&
	    (curEntry.value < FAT_MIN_BAD(fs))) {
	    printf("Cluster %ld out of range (%ld > %ld). Setting to EOF.\n",
		   i-2, curEntry.value, fs->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,unsigned long cluster,unsigned long new)
{
    unsigned char *data = NULL;
    int size;
    loff_t offs;

    if ((long)new == -1)
	new = FAT_EOF(fs);
    else if ((long)new == -2)
	new = FAT_BAD(fs);
    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;
            get_fat(&subseqEntry, fs->fat, cluster+1, fs);
	    data[0] = new & 0xff;
	    data[1] = (new >> 8) | (cluster == fs->clusters-1 ? 0 :
	      (0xff & subseqEntry.value) << 4);
	}
	size = 2;
	break;
      case 16:
        data = fs->fat + cluster*2;
	offs = fs->fat_start+cluster*2;
	*(unsigned short *) data = CT_LE_W(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. */
	    *(unsigned long *) data = CT_LE_L( (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,unsigned long 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
 */
unsigned long next_cluster(DOS_FS *fs,unsigned long cluster)
{
    unsigned long 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;
}


loff_t cluster_start(DOS_FS *fs,unsigned long cluster)
{
    return fs->data_start+((loff_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,unsigned long 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,unsigned long cluster)
{
    if (fs->cluster_owner == NULL)
        return NULL;
    else
        return fs->cluster_owner[cluster];
}


void fix_bad(DOS_FS *fs)
{
    unsigned long i;

    if (verbose)
	printf("Checking for bad clusters.\n");
    for (i = 2; i < fs->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",i);
		set_fat(fs,i,-2);
	    }
    }
}


void reclaim_free(DOS_FS *fs)
{
    int reclaimed;
    unsigned long i;

    if (verbose)
	printf("Checking for unused clusters.\n");
    reclaimed = 0;
    for (i = 2; i < fs->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",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, unsigned long *num_refs,
		     unsigned long start_cluster)
{
    int prev;
    unsigned long i,walk;

    if (start_cluster == 0)
	start_cluster = 2;

    for (i = start_cluster; i < fs->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;
    unsigned long i,next,walk;
    unsigned long *num_refs = NULL;	/* Only for orphaned clusters */
    unsigned long total_num_clusters;

    if (verbose)
	printf("Reclaiming unconnected clusters.\n");

    total_num_clusters = fs->clusters + 2UL;
    num_refs = alloc(total_num_clusters * sizeof(unsigned long));
    memset(num_refs, 0, (total_num_clusters * sizeof(unsigned long)));

    /* 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->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",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;
	    loff_t offset;
	    files++;
	    offset = alloc_rootdir_entry(fs,&de,"FSCK%04dREC");
	    de.start = CT_LE_W(i&0xffff);
	    if (fs->fat_bits == 32)
		de.starthi = CT_LE_W(i>>16);
	    for (walk = i; walk > 0 && walk != -1;
		 walk = next_cluster(fs,walk)) {
		de.size = CT_LE_L(CF_LE_L(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);
}


unsigned long update_free(DOS_FS *fs)
{
    unsigned long i;
    unsigned long free = 0;
    int do_set = 0;

    for (i = 2; i < fs->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 >= 0) {
	if (free != fs->free_clusters) {
	    printf( "Free cluster summary wrong (%ld vs. really %ld)\n",
		    fs->free_clusters,free);
	    if (interactive)
		printf( "1) Correct\n2) Don't correct\n" );
	    else printf( "  Auto-correcting.\n" );
	    if (!interactive || get_key("12","?") == '1')
		do_set = 1;
	}
    }
    else {
	printf( "Free cluster summary uninitialized (should be %ld)\n", free );
	if (interactive)
	    printf( "1) Set it\n2) Leave it uninitialized\n" );
	else printf( "  Auto-setting.\n" );
	if (!interactive || get_key("12","?") == '1')
	    do_set = 1;
    }

    if (do_set) {
	unsigned long le_free = CT_LE_L(free);
	fs->free_clusters = free;
	fs_write(fs->fsinfo_start+offsetof(struct info_sector,free_clusters),
		 sizeof(le_free), &le_free);
    }

    return free;
}

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