boot.c - platform/external/fsck_msdos - Git at Google

 /*-
  * SPDX-License-Identifier: BSD-2-Clause-FreeBSD
  *
  * Copyright (C) 1995, 1997 Wolfgang Solfrank
  * Copyright (c) 1995 Martin Husemann
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
  *    documentation and/or other materials provided with the distribution.
  *
  * THIS SOFTWARE IS PROVIDED BY THE AUTHORS ``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 AUTHORS 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.
  */


 #include <sys/cdefs.h>
 #ifndef lint
 __RCSID("$NetBSD: boot.c,v 1.22 2020/01/11 16:29:07 christos Exp $");
 static const char rcsid[] =
   "$FreeBSD$";
 #endif /* not lint */

 #include <sys/param.h>

 #include <stdint.h>
 #include <stdlib.h>
 #include <string.h>
 #include <stdio.h>
 #include <unistd.h>

 #include "ext.h"
 #include "fsutil.h"

 int
 readboot(int dosfs, struct bootblock *boot)
 {
 	u_char block[DOSBOOTBLOCKSIZE];
 	u_char fsinfo[2 * DOSBOOTBLOCKSIZE];
 	int ret = FSOK;

 	if ((size_t)read(dosfs, block, sizeof block) != sizeof block) {
 		perr("could not read boot block");
 		return FSFATAL;
 	}

 	if (block[510] != 0x55 || block[511] != 0xaa) {
 		pfatal("Invalid signature in boot block: %02x%02x",
 		    block[511], block[510]);
 		return FSFATAL;
 	}

 	memset(boot, 0, sizeof *boot);
 	boot->ValidFat = -1;

 	/* Decode BIOS Parameter Block */

 	/* Bytes per sector: can only be  512, 1024, 2048 and 4096. */
 	boot->bpbBytesPerSec = block[11] + (block[12] << 8);
 	if (boot->bpbBytesPerSec < DOSBOOTBLOCKSIZE_REAL ||
 	    boot->bpbBytesPerSec > DOSBOOTBLOCKSIZE ||
 	    !powerof2(boot->bpbBytesPerSec)) {
 		pfatal("Invalid sector size: %u", boot->bpbBytesPerSec);
 		return FSFATAL;
 	}

 	/* Sectors per cluster: can only be: 1, 2, 4, 8, 16, 32, 64, 128. */
 	boot->bpbSecPerClust = block[13];
 	if (boot->bpbSecPerClust == 0 || !powerof2(boot->bpbSecPerClust)) {
 		pfatal("Invalid cluster size: %u", boot->bpbSecPerClust);
 		return FSFATAL;
 	}

 	/* Reserved sectors: must be non-zero */
 	boot->bpbResSectors = block[14] + (block[15] << 8);
 	if (boot->bpbResSectors < 1) {
 		pfatal("Invalid reserved sectors: %u",
 		    boot->bpbResSectors);
 		return FSFATAL;
 	}

 	/* Number of FATs */
 	boot->bpbFATs = block[16];
 	if (boot->bpbFATs == 0) {
 		pfatal("Invalid number of FATs: %u", boot->bpbFATs);
 		return FSFATAL;
 	}

 	/* Root directory entries for FAT12 and FAT16 */
 	boot->bpbRootDirEnts = block[17] + (block[18] << 8);
 	if (!boot->bpbRootDirEnts) {
 		/* bpbRootDirEnts = 0 suggests that we are FAT32 */
 		boot->flags |= FAT32;
 	}

 	/* Total sectors (16 bits) */
 	boot->bpbSectors = block[19] + (block[20] << 8);
 	if (boot->bpbSectors != 0 && (boot->flags & FAT32)) {
 		pfatal("Invalid 16-bit total sector count on FAT32: %u",
 		    boot->bpbSectors);
 		return FSFATAL;
 	}

 	/* Media type: ignored */
 	boot->bpbMedia = block[21];

 	/* FAT12/FAT16: 16-bit count of sectors per FAT */
 	boot->bpbFATsmall = block[22] + (block[23] << 8);
 	if (boot->bpbFATsmall != 0 && (boot->flags & FAT32)) {
 		pfatal("Invalid 16-bit FAT sector count on FAT32: %u",
 		    boot->bpbFATsmall);
 		return FSFATAL;
 	}

 	/* Legacy CHS geometry numbers: ignored */
 	boot->SecPerTrack = block[24] + (block[25] << 8);
 	boot->bpbHeads = block[26] + (block[27] << 8);

 	/* Hidden sectors: ignored */
 	boot->bpbHiddenSecs = block[28] + (block[29] << 8) +
 	    (block[30] << 16) + (block[31] << 24);

 	/* Total sectors (32 bits) */
 	boot->bpbHugeSectors = block[32] + (block[33] << 8) +
 	    (block[34] << 16) + (block[35] << 24);
 	if (boot->bpbHugeSectors == 0) {
 		if (boot->flags & FAT32) {
 			pfatal("FAT32 with sector count of zero");
 			return FSFATAL;
 		} else if (boot->bpbSectors == 0) {
 			pfatal("FAT with sector count of zero");
 			return FSFATAL;
 		}
 		boot->NumSectors = boot->bpbSectors;
 	} else {
 		if (boot->bpbSectors != 0) {
 			pfatal("Invalid FAT sector count");
 			return FSFATAL;
 		}
 		boot->NumSectors = boot->bpbHugeSectors;
 	}

 	if (boot->flags & FAT32) {
 		/* If the OEM Name field is EXFAT, it's not FAT32, so bail */
 		if (!memcmp(&block[3], "EXFAT   ", 8)) {
 			pfatal("exFAT filesystem is not supported.");
 			return FSFATAL;
 		}

 		/* 32-bit count of sectors per FAT */
 		boot->FATsecs = block[36] + (block[37] << 8)
 				+ (block[38] << 16) + (block[39] << 24);

 		if (block[40] & 0x80)
 			boot->ValidFat = block[40] & 0x0f;

 		/* FAT32 version, bail out if not 0.0 */
 		if (block[42] || block[43]) {
 			pfatal("Unknown file system version: %x.%x",
 			       block[43], block[42]);
 			return FSFATAL;
 		}

 		/*
 		 * Cluster number of the first cluster of root directory.
 		 *
 		 * Should be 2 but do not require it.
 		 */
 		boot->bpbRootClust = block[44] + (block[45] << 8)
 			       + (block[46] << 16) + (block[47] << 24);

 		/* Sector number of the FSInfo structure, usually 1 */
 		boot->bpbFSInfo = block[48] + (block[49] << 8);

 		/* Sector number of the backup boot block, ignored */
 		boot->bpbBackup = block[50] + (block[51] << 8);

 		/* Check basic parameters */
 		if (boot->bpbFSInfo == 0) {
 			/*
 			 * Either the BIOS Parameter Block has been corrupted,
 			 * or this is not a FAT32 filesystem, most likely an
 			 * exFAT filesystem.
 			 */
 			pfatal("Invalid FAT32 Extended BIOS Parameter Block");
 			return FSFATAL;
 		}

 		/* Read in and verify the FSInfo block */
 		if (lseek(dosfs, boot->bpbFSInfo * boot->bpbBytesPerSec,
 		    SEEK_SET) != boot->bpbFSInfo * boot->bpbBytesPerSec
 		    || read(dosfs, fsinfo, sizeof fsinfo) != sizeof fsinfo) {
 			perr("could not read fsinfo block");
 			return FSFATAL;
 		}
 		if (memcmp(fsinfo, "RRaA", 4)
 		    || memcmp(fsinfo + 0x1e4, "rrAa", 4)
 		    || fsinfo[0x1fc]
 		    || fsinfo[0x1fd]
 		    || fsinfo[0x1fe] != 0x55
 		    || fsinfo[0x1ff] != 0xaa
 		    || fsinfo[0x3fc]
 		    || fsinfo[0x3fd]
 		    || fsinfo[0x3fe] != 0x55
 		    || fsinfo[0x3ff] != 0xaa) {
 			pwarn("Invalid signature in fsinfo block\n");
 			if (ask(0, "Fix")) {
 				memcpy(fsinfo, "RRaA", 4);
 				memcpy(fsinfo + 0x1e4, "rrAa", 4);
 				fsinfo[0x1fc] = fsinfo[0x1fd] = 0;
 				fsinfo[0x1fe] = 0x55;
 				fsinfo[0x1ff] = 0xaa;
 				fsinfo[0x3fc] = fsinfo[0x3fd] = 0;
 				fsinfo[0x3fe] = 0x55;
 				fsinfo[0x3ff] = 0xaa;
 				if (lseek(dosfs, boot->bpbFSInfo *
 				    boot->bpbBytesPerSec, SEEK_SET)
 				    != boot->bpbFSInfo * boot->bpbBytesPerSec
 				    || write(dosfs, fsinfo, sizeof fsinfo)
 				    != sizeof fsinfo) {
 					perr("Unable to write bpbFSInfo");
 					return FSFATAL;
 				}
 				ret = FSBOOTMOD;
 			} else
 				boot->bpbFSInfo = 0;
 		} else {
 			/* We appear to have a valid FSInfo block, decode */
 			boot->FSFree = fsinfo[0x1e8] + (fsinfo[0x1e9] << 8)
 				       + (fsinfo[0x1ea] << 16)
 				       + (fsinfo[0x1eb] << 24);
 			boot->FSNext = fsinfo[0x1ec] + (fsinfo[0x1ed] << 8)
 				       + (fsinfo[0x1ee] << 16)
 				       + (fsinfo[0x1ef] << 24);
 		}
 	} else {
 		/* !FAT32: FAT12/FAT16 */
 		boot->FATsecs = boot->bpbFATsmall;
 	}

 	if (boot->FATsecs < 1 || boot->FATsecs > UINT32_MAX / boot->bpbFATs) {
 		pfatal("Invalid FATs(%u) with FATsecs(%zu)",
 			boot->bpbFATs, (size_t)boot->FATsecs);
 		return FSFATAL;
 	}

 	boot->FirstCluster = (boot->bpbRootDirEnts * 32 +
 	    boot->bpbBytesPerSec - 1) / boot->bpbBytesPerSec +
 	    boot->bpbResSectors + boot->bpbFATs * boot->FATsecs;

 	if (boot->FirstCluster + boot->bpbSecPerClust > boot->NumSectors) {
 		pfatal("Cluster offset too large (%u clusters)\n",
 		    boot->FirstCluster);
 		return FSFATAL;
 	}

 	/*
 	 * The number of clusters is derived from available data sectors,
 	 * divided by sectors per cluster.
 	 */
 	boot->NumClusters =
 	    (boot->NumSectors - boot->FirstCluster) / boot->bpbSecPerClust;

 	if (boot->flags & FAT32) {
 		if (boot->NumClusters > (CLUST_RSRVD & CLUST32_MASK)) {
 			pfatal("Filesystem too big (%u clusters) for FAT32 partition",
 			    boot->NumClusters);
 			return FSFATAL;
 		}
 		if (boot->NumClusters < (CLUST_RSRVD & CLUST16_MASK)) {
 			pfatal("Filesystem too small (%u clusters) for FAT32 partition",
 			    boot->NumClusters);
 			return FSFATAL;
 		}
 		boot->ClustMask = CLUST32_MASK;

 		if (boot->bpbRootClust < CLUST_FIRST ||
 		    boot->bpbRootClust >= boot->NumClusters) {
 			pfatal("Root directory starts with cluster out of range(%u)",
 			       boot->bpbRootClust);
 			return FSFATAL;
 		}
 	} else if (boot->NumClusters < (CLUST_RSRVD&CLUST12_MASK)) {
 		boot->ClustMask = CLUST12_MASK;
 	} else if (boot->NumClusters < (CLUST_RSRVD&CLUST16_MASK)) {
 		boot->ClustMask = CLUST16_MASK;
 	} else {
 		pfatal("Filesystem too big (%u clusters) for non-FAT32 partition",
 		       boot->NumClusters);
 		return FSFATAL;
 	}

 	switch (boot->ClustMask) {
 	case CLUST32_MASK:
 		boot->NumFatEntries = (boot->FATsecs * boot->bpbBytesPerSec) / 4;
 		break;
 	case CLUST16_MASK:
 		boot->NumFatEntries = (boot->FATsecs * boot->bpbBytesPerSec) / 2;
 		break;
 	default:
 		boot->NumFatEntries = (boot->FATsecs * boot->bpbBytesPerSec * 2) / 3;
 		break;
 	}

 	if (boot->NumFatEntries < boot->NumClusters) {
 		pfatal("FAT size too small, %u entries won't fit into %u sectors\n",
 		       boot->NumClusters, boot->FATsecs);
 		return FSFATAL;
 	}

 	/*
 	 * There are two reserved clusters. To avoid adding CLUST_FIRST every
 	 * time we perform boundary checks, we increment the NumClusters by 2,
 	 * which is CLUST_FIRST to denote the first out-of-range cluster number.
 	 */
 	boot->NumClusters += CLUST_FIRST;

 	boot->ClusterSize = boot->bpbBytesPerSec * boot->bpbSecPerClust;

 	boot->NumFiles = 1;
 	boot->NumFree = 0;

 	return ret;
 }

 int
 writefsinfo(int dosfs, struct bootblock *boot)
 {
 	u_char fsinfo[2 * DOSBOOTBLOCKSIZE];

 	if (lseek(dosfs, boot->bpbFSInfo * boot->bpbBytesPerSec, SEEK_SET)
 	    != boot->bpbFSInfo * boot->bpbBytesPerSec
 	    || read(dosfs, fsinfo, sizeof fsinfo) != sizeof fsinfo) {
 		perr("could not read fsinfo block");
 		return FSFATAL;
 	}
 	fsinfo[0x1e8] = (u_char)boot->FSFree;
 	fsinfo[0x1e9] = (u_char)(boot->FSFree >> 8);
 	fsinfo[0x1ea] = (u_char)(boot->FSFree >> 16);
 	fsinfo[0x1eb] = (u_char)(boot->FSFree >> 24);
 	fsinfo[0x1ec] = (u_char)boot->FSNext;
 	fsinfo[0x1ed] = (u_char)(boot->FSNext >> 8);
 	fsinfo[0x1ee] = (u_char)(boot->FSNext >> 16);
 	fsinfo[0x1ef] = (u_char)(boot->FSNext >> 24);
 	if (lseek(dosfs, boot->bpbFSInfo * boot->bpbBytesPerSec, SEEK_SET)
 	    != boot->bpbFSInfo * boot->bpbBytesPerSec
 	    || write(dosfs, fsinfo, sizeof fsinfo)
 	    != sizeof fsinfo) {
 		perr("Unable to write bpbFSInfo");
 		return FSFATAL;
 	}
 	/*
 	 * Technically, we should return FSBOOTMOD here.
 	 *
 	 * However, since Win95 OSR2 (the first M$ OS that has
 	 * support for FAT32) doesn't maintain the FSINFO block
 	 * correctly, it has to be fixed pretty often.
 	 *
 	 * Therefore, we handle the FSINFO block only informally,
 	 * fixing it if necessary, but otherwise ignoring the
 	 * fact that it was incorrect.
 	 */
 	return 0;
 }
	/*-
	* SPDX-License-Identifier: BSD-2-Clause-FreeBSD
	*
	* Copyright (C) 1995, 1997 Wolfgang Solfrank
	* Copyright (c) 1995 Martin Husemann
	*
	* Redistribution and use in source and binary forms, with or without
	* modification, are permitted provided that the following conditions
	* are met:
	* 1. Redistributions of source code must retain the above copyright
	* notice, this list of conditions and the following disclaimer.
	* 2. Redistributions in binary form must reproduce the above copyright
	* notice, this list of conditions and the following disclaimer in the
	* documentation and/or other materials provided with the distribution.
	*
	* THIS SOFTWARE IS PROVIDED BY THE AUTHORS ``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 AUTHORS 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.
	*/


	#include <sys/cdefs.h>
	#ifndef lint
	__RCSID("$NetBSD: boot.c,v 1.22 2020/01/11 16:29:07 christos Exp $");
	static const char rcsid[] =
	"$FreeBSD$";
	#endif /* not lint */

	#include <sys/param.h>

	#include <stdint.h>
	#include <stdlib.h>
	#include <string.h>
	#include <stdio.h>
	#include <unistd.h>

	#include "ext.h"
	#include "fsutil.h"

	int
	readboot(int dosfs, struct bootblock *boot)
	{
	u_char block[DOSBOOTBLOCKSIZE];
	u_char fsinfo[2 * DOSBOOTBLOCKSIZE];
	int ret = FSOK;

	if ((size_t)read(dosfs, block, sizeof block) != sizeof block) {
	perr("could not read boot block");
	return FSFATAL;
	}

	if (block[510] != 0x55 \|\| block[511] != 0xaa) {
	pfatal("Invalid signature in boot block: %02x%02x",
	block[511], block[510]);
	return FSFATAL;
	}

	memset(boot, 0, sizeof *boot);
	boot->ValidFat = -1;

	/* Decode BIOS Parameter Block */

	/* Bytes per sector: can only be 512, 1024, 2048 and 4096. */
	boot->bpbBytesPerSec = block[11] + (block[12] << 8);
	if (boot->bpbBytesPerSec < DOSBOOTBLOCKSIZE_REAL \|\|
	boot->bpbBytesPerSec > DOSBOOTBLOCKSIZE \|\|
	!powerof2(boot->bpbBytesPerSec)) {
	pfatal("Invalid sector size: %u", boot->bpbBytesPerSec);
	return FSFATAL;
	}

	/* Sectors per cluster: can only be: 1, 2, 4, 8, 16, 32, 64, 128. */
	boot->bpbSecPerClust = block[13];
	if (boot->bpbSecPerClust == 0 \|\| !powerof2(boot->bpbSecPerClust)) {
	pfatal("Invalid cluster size: %u", boot->bpbSecPerClust);
	return FSFATAL;
	}

	/* Reserved sectors: must be non-zero */
	boot->bpbResSectors = block[14] + (block[15] << 8);
	if (boot->bpbResSectors < 1) {
	pfatal("Invalid reserved sectors: %u",
	boot->bpbResSectors);
	return FSFATAL;
	}

	/* Number of FATs */
	boot->bpbFATs = block[16];
	if (boot->bpbFATs == 0) {
	pfatal("Invalid number of FATs: %u", boot->bpbFATs);
	return FSFATAL;
	}

	/* Root directory entries for FAT12 and FAT16 */
	boot->bpbRootDirEnts = block[17] + (block[18] << 8);
	if (!boot->bpbRootDirEnts) {
	/* bpbRootDirEnts = 0 suggests that we are FAT32 */
	boot->flags \|= FAT32;
	}

	/* Total sectors (16 bits) */
	boot->bpbSectors = block[19] + (block[20] << 8);
	if (boot->bpbSectors != 0 && (boot->flags & FAT32)) {
	pfatal("Invalid 16-bit total sector count on FAT32: %u",
	boot->bpbSectors);
	return FSFATAL;
	}

	/* Media type: ignored */
	boot->bpbMedia = block[21];

	/* FAT12/FAT16: 16-bit count of sectors per FAT */
	boot->bpbFATsmall = block[22] + (block[23] << 8);
	if (boot->bpbFATsmall != 0 && (boot->flags & FAT32)) {
	pfatal("Invalid 16-bit FAT sector count on FAT32: %u",
	boot->bpbFATsmall);
	return FSFATAL;
	}

	/* Legacy CHS geometry numbers: ignored */
	boot->SecPerTrack = block[24] + (block[25] << 8);
	boot->bpbHeads = block[26] + (block[27] << 8);

	/* Hidden sectors: ignored */
	boot->bpbHiddenSecs = block[28] + (block[29] << 8) +
	(block[30] << 16) + (block[31] << 24);

	/* Total sectors (32 bits) */
	boot->bpbHugeSectors = block[32] + (block[33] << 8) +
	(block[34] << 16) + (block[35] << 24);
	if (boot->bpbHugeSectors == 0) {
	if (boot->flags & FAT32) {
	pfatal("FAT32 with sector count of zero");
	return FSFATAL;
	} else if (boot->bpbSectors == 0) {
	pfatal("FAT with sector count of zero");
	return FSFATAL;
	}
	boot->NumSectors = boot->bpbSectors;
	} else {
	if (boot->bpbSectors != 0) {
	pfatal("Invalid FAT sector count");
	return FSFATAL;
	}
	boot->NumSectors = boot->bpbHugeSectors;
	}

	if (boot->flags & FAT32) {
	/* If the OEM Name field is EXFAT, it's not FAT32, so bail */
	if (!memcmp(&block[3], "EXFAT ", 8)) {
	pfatal("exFAT filesystem is not supported.");
	return FSFATAL;
	}

	/* 32-bit count of sectors per FAT */
	boot->FATsecs = block[36] + (block[37] << 8)
	+ (block[38] << 16) + (block[39] << 24);

	if (block[40] & 0x80)
	boot->ValidFat = block[40] & 0x0f;

	/* FAT32 version, bail out if not 0.0 */
	if (block[42] \|\| block[43]) {
	pfatal("Unknown file system version: %x.%x",
	block[43], block[42]);
	return FSFATAL;
	}

	/*
	* Cluster number of the first cluster of root directory.
	*
	* Should be 2 but do not require it.
	*/
	boot->bpbRootClust = block[44] + (block[45] << 8)
	+ (block[46] << 16) + (block[47] << 24);

	/* Sector number of the FSInfo structure, usually 1 */
	boot->bpbFSInfo = block[48] + (block[49] << 8);

	/* Sector number of the backup boot block, ignored */
	boot->bpbBackup = block[50] + (block[51] << 8);

	/* Check basic parameters */
	if (boot->bpbFSInfo == 0) {
	/*
	* Either the BIOS Parameter Block has been corrupted,
	* or this is not a FAT32 filesystem, most likely an
	* exFAT filesystem.
	*/
	pfatal("Invalid FAT32 Extended BIOS Parameter Block");
	return FSFATAL;
	}

	/* Read in and verify the FSInfo block */
	if (lseek(dosfs, boot->bpbFSInfo * boot->bpbBytesPerSec,
	SEEK_SET) != boot->bpbFSInfo * boot->bpbBytesPerSec
	\|\| read(dosfs, fsinfo, sizeof fsinfo) != sizeof fsinfo) {
	perr("could not read fsinfo block");
	return FSFATAL;
	}
	if (memcmp(fsinfo, "RRaA", 4)
	\|\| memcmp(fsinfo + 0x1e4, "rrAa", 4)
	\|\| fsinfo[0x1fc]
	\|\| fsinfo[0x1fd]
	\|\| fsinfo[0x1fe] != 0x55
	\|\| fsinfo[0x1ff] != 0xaa
	\|\| fsinfo[0x3fc]
	\|\| fsinfo[0x3fd]
	\|\| fsinfo[0x3fe] != 0x55
	\|\| fsinfo[0x3ff] != 0xaa) {
	pwarn("Invalid signature in fsinfo block\n");
	if (ask(0, "Fix")) {
	memcpy(fsinfo, "RRaA", 4);
	memcpy(fsinfo + 0x1e4, "rrAa", 4);
	fsinfo[0x1fc] = fsinfo[0x1fd] = 0;
	fsinfo[0x1fe] = 0x55;
	fsinfo[0x1ff] = 0xaa;
	fsinfo[0x3fc] = fsinfo[0x3fd] = 0;
	fsinfo[0x3fe] = 0x55;
	fsinfo[0x3ff] = 0xaa;
	if (lseek(dosfs, boot->bpbFSInfo *
	boot->bpbBytesPerSec, SEEK_SET)
	!= boot->bpbFSInfo * boot->bpbBytesPerSec
	\|\| write(dosfs, fsinfo, sizeof fsinfo)
	!= sizeof fsinfo) {
	perr("Unable to write bpbFSInfo");
	return FSFATAL;
	}
	ret = FSBOOTMOD;
	} else
	boot->bpbFSInfo = 0;
	} else {
	/* We appear to have a valid FSInfo block, decode */
	boot->FSFree = fsinfo[0x1e8] + (fsinfo[0x1e9] << 8)
	+ (fsinfo[0x1ea] << 16)
	+ (fsinfo[0x1eb] << 24);
	boot->FSNext = fsinfo[0x1ec] + (fsinfo[0x1ed] << 8)
	+ (fsinfo[0x1ee] << 16)
	+ (fsinfo[0x1ef] << 24);
	}
	} else {
	/* !FAT32: FAT12/FAT16 */
	boot->FATsecs = boot->bpbFATsmall;
	}

	if (boot->FATsecs < 1 \|\| boot->FATsecs > UINT32_MAX / boot->bpbFATs) {
	pfatal("Invalid FATs(%u) with FATsecs(%zu)",
	boot->bpbFATs, (size_t)boot->FATsecs);
	return FSFATAL;
	}

	boot->FirstCluster = (boot->bpbRootDirEnts * 32 +
	boot->bpbBytesPerSec - 1) / boot->bpbBytesPerSec +
	boot->bpbResSectors + boot->bpbFATs * boot->FATsecs;

	if (boot->FirstCluster + boot->bpbSecPerClust > boot->NumSectors) {
	pfatal("Cluster offset too large (%u clusters)\n",
	boot->FirstCluster);
	return FSFATAL;
	}

	/*
	* The number of clusters is derived from available data sectors,
	* divided by sectors per cluster.
	*/
	boot->NumClusters =
	(boot->NumSectors - boot->FirstCluster) / boot->bpbSecPerClust;

	if (boot->flags & FAT32) {
	if (boot->NumClusters > (CLUST_RSRVD & CLUST32_MASK)) {
	pfatal("Filesystem too big (%u clusters) for FAT32 partition",
	boot->NumClusters);
	return FSFATAL;
	}
	if (boot->NumClusters < (CLUST_RSRVD & CLUST16_MASK)) {
	pfatal("Filesystem too small (%u clusters) for FAT32 partition",
	boot->NumClusters);
	return FSFATAL;
	}
	boot->ClustMask = CLUST32_MASK;

	if (boot->bpbRootClust < CLUST_FIRST \|\|
	boot->bpbRootClust >= boot->NumClusters) {
	pfatal("Root directory starts with cluster out of range(%u)",
	boot->bpbRootClust);
	return FSFATAL;
	}
	} else if (boot->NumClusters < (CLUST_RSRVD&CLUST12_MASK)) {
	boot->ClustMask = CLUST12_MASK;
	} else if (boot->NumClusters < (CLUST_RSRVD&CLUST16_MASK)) {
	boot->ClustMask = CLUST16_MASK;
	} else {
	pfatal("Filesystem too big (%u clusters) for non-FAT32 partition",
	boot->NumClusters);
	return FSFATAL;
	}

	switch (boot->ClustMask) {
	case CLUST32_MASK:
	boot->NumFatEntries = (boot->FATsecs * boot->bpbBytesPerSec) / 4;
	break;
	case CLUST16_MASK:
	boot->NumFatEntries = (boot->FATsecs * boot->bpbBytesPerSec) / 2;
	break;
	default:
	boot->NumFatEntries = (boot->FATsecs * boot->bpbBytesPerSec * 2) / 3;
	break;
	}

	if (boot->NumFatEntries < boot->NumClusters) {
	pfatal("FAT size too small, %u entries won't fit into %u sectors\n",
	boot->NumClusters, boot->FATsecs);
	return FSFATAL;
	}

	/*
	* There are two reserved clusters. To avoid adding CLUST_FIRST every
	* time we perform boundary checks, we increment the NumClusters by 2,
	* which is CLUST_FIRST to denote the first out-of-range cluster number.
	*/
	boot->NumClusters += CLUST_FIRST;

	boot->ClusterSize = boot->bpbBytesPerSec * boot->bpbSecPerClust;

	boot->NumFiles = 1;
	boot->NumFree = 0;

	return ret;
	}

	int
	writefsinfo(int dosfs, struct bootblock *boot)
	{
	u_char fsinfo[2 * DOSBOOTBLOCKSIZE];

	if (lseek(dosfs, boot->bpbFSInfo * boot->bpbBytesPerSec, SEEK_SET)
	!= boot->bpbFSInfo * boot->bpbBytesPerSec
	\|\| read(dosfs, fsinfo, sizeof fsinfo) != sizeof fsinfo) {
	perr("could not read fsinfo block");
	return FSFATAL;
	}
	fsinfo[0x1e8] = (u_char)boot->FSFree;
	fsinfo[0x1e9] = (u_char)(boot->FSFree >> 8);
	fsinfo[0x1ea] = (u_char)(boot->FSFree >> 16);
	fsinfo[0x1eb] = (u_char)(boot->FSFree >> 24);
	fsinfo[0x1ec] = (u_char)boot->FSNext;
	fsinfo[0x1ed] = (u_char)(boot->FSNext >> 8);
	fsinfo[0x1ee] = (u_char)(boot->FSNext >> 16);
	fsinfo[0x1ef] = (u_char)(boot->FSNext >> 24);
	if (lseek(dosfs, boot->bpbFSInfo * boot->bpbBytesPerSec, SEEK_SET)
	!= boot->bpbFSInfo * boot->bpbBytesPerSec
	\|\| write(dosfs, fsinfo, sizeof fsinfo)
	!= sizeof fsinfo) {
	perr("Unable to write bpbFSInfo");
	return FSFATAL;
	}
	/*
	* Technically, we should return FSBOOTMOD here.
	*
	* However, since Win95 OSR2 (the first M$ OS that has
	* support for FAT32) doesn't maintain the FSINFO block
	* correctly, it has to be fixed pretty often.
	*
	* Therefore, we handle the FSINFO block only informally,
	* fixing it if necessary, but otherwise ignoring the
	* fact that it was incorrect.
	*/
	return 0;
	}