util-linux/mkfs_vfat.c

   1 /* vi: set sw=4 ts=4: */
   2 /*
   3  * mkfs_vfat: utility to create FAT32 filesystem
   4  * inspired by dosfstools
   5  *
   6  * Busybox'ed (2009) by Vladimir Dronnikov <dronnikov@gmail.com>
   7  *
   8  * Licensed under GPLv2, see file LICENSE in this source tree.
   9  */
  10
  11 //usage:#define mkfs_vfat_trivial_usage
  12 //usage:       "[-v] [-n LABEL] BLOCKDEV [KBYTES]"
  13 /* Accepted but ignored:
  14        "[-c] [-C] [-I] [-l bad-block-file] [-b backup-boot-sector] "
  15        "[-m boot-msg-file] [-i volume-id] "
  16        "[-s sectors-per-cluster] [-S logical-sector-size] [-f number-of-FATs] "
  17        "[-h hidden-sectors] [-F fat-size] [-r root-dir-entries] [-R reserved-sectors] "
  18 */
  19 //usage:#define mkfs_vfat_full_usage "\n\n"
  20 //usage:       "Make a FAT32 filesystem\n"
  21 /* //usage:  "\n        -c      Check device for bad blocks" */
  22 //usage:     "\n        -v      Verbose"
  23 /* //usage:  "\n        -I      Allow to use entire disk device (e.g. /dev/hda)" */
  24 //usage:     "\n        -n LBL  Volume label"
  25
  26 #include "libbb.h"
  27
  28 #include <linux/hdreg.h> /* HDIO_GETGEO */
  29 #include <linux/fd.h>    /* FDGETPRM */
  30 #include <sys/mount.h>   /* BLKSSZGET */
  31 #if !defined(BLKSSZGET)
  32 # define BLKSSZGET _IO(0x12, 104)
  33 #endif
  34 //#include <linux/msdos_fs.h>
  35
  36 #define SECTOR_SIZE             512
  37
  38 #define SECTORS_PER_BLOCK       (BLOCK_SIZE / SECTOR_SIZE)
  39
  40 // M$ says the high 4 bits of a FAT32 FAT entry are reserved
  41 #define EOF_FAT32       0x0FFFFFF8
  42 #define BAD_FAT32       0x0FFFFFF7
  43 #define MAX_CLUST_32    0x0FFFFFF0
  44
  45 #define ATTR_VOLUME     8
  46
  47 #define NUM_FATS        2
  48
  49 /* FAT32 filesystem looks like this:
  50  * sector -nn...-1: "hidden" sectors, all sectors before this partition
  51  * (-h hidden-sectors sets it. Useful only for boot loaders,
  52  *  they need to know _disk_ offset in order to be able to correctly
  53  *  address sectors relative to start of disk)
  54  * sector 0: boot sector
  55  * sector 1: info sector
  56  * sector 2: set aside for boot code which didn't fit into sector 0
  57  * ...(zero-filled sectors)...
  58  * sector B: backup copy of sector 0 [B set by -b backup-boot-sector]
  59  * sector B+1: backup copy of sector 1
  60  * sector B+2: backup copy of sector 2
  61  * ...(zero-filled sectors)...
  62  * sector R: FAT#1 [R set by -R reserved-sectors]
  63  * ...(FAT#1)...
  64  * sector R+fat_size: FAT#2
  65  * ...(FAT#2)...
  66  * sector R+fat_size*2: cluster #2
  67  * ...(cluster #2)...
  68  * sector R+fat_size*2+clust_size: cluster #3
  69  * ...(the rest is filled by clusters till the end)...
  70  */
  71
  72 enum {
  73 // Perhaps this should remain constant
  74         info_sector_number = 1,
  75 // TODO: make these cmdline options
  76 // dont forget sanity check: backup_boot_sector + 3 <= reserved_sect
  77         backup_boot_sector = 3,
  78         reserved_sect      = 6,
  79 };
  80
  81 // how many blocks we try to read while testing
  82 #define TEST_BUFFER_BLOCKS      16
  83
  84 struct msdos_dir_entry {
  85         char     name[11];       /* 000 name and extension */
  86         uint8_t  attr;           /* 00b attribute bits */
  87         uint8_t  lcase;          /* 00c case for base and extension */
  88         uint8_t  ctime_cs;       /* 00d creation time, centiseconds (0-199) */
  89         uint16_t ctime;          /* 00e creation time */
  90         uint16_t cdate;          /* 010 creation date */
  91         uint16_t adate;          /* 012 last access date */
  92         uint16_t starthi;        /* 014 high 16 bits of cluster in FAT32 */
  93         uint16_t time;           /* 016 time */
  94         uint16_t date;           /* 018 date */
  95         uint16_t start;          /* 01a first cluster */
  96         uint32_t size;           /* 01c file size in bytes */
  97 } PACKED;
  98
  99 /* Example of boot sector's beginning:
 100 0000  eb 58 90 4d 53 57 49 4e  34 2e 31 00 02 08 26 00  |...MSWIN4.1...&.|
 101 0010  02 00 00 00 00 f8 00 00  3f 00 ff 00 3f 00 00 00  |........?...?...|
 102 0020  54 9b d0 00 0d 34 00 00  00 00 00 00 02 00 00 00  |T....4..........|
 103 0030  01 00 06 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
 104 0040  80 00 29 71 df 51 e0 4e  4f 20 4e 41 4d 45 20 20  |..)q.Q.NO NAME  |
 105 0050  20 20 46 41 54 33 32 20  20 20 33 c9 8e d1 bc f4  |  FAT32   3.....|
 106 */
 107 struct msdos_volume_info { /* (offsets are relative to start of boot sector) */
 108         uint8_t  drive_number;    /* 040 BIOS drive number */
 109         uint8_t  reserved;        /* 041 unused */
 110         uint8_t  ext_boot_sign;   /* 042 0x29 if fields below exist (DOS 3.3+) */
 111         uint32_t volume_id32;     /* 043 volume ID number */
 112         char     volume_label[11];/* 047 volume label */
 113         char     fs_type[8];      /* 052 typically "FATnn" */
 114 } PACKED;                         /* 05a end. Total size 26 (0x1a) bytes */
 115
 116 struct msdos_boot_sector {
 117         /* We use strcpy to fill both, and gcc-4.4.x complains if they are separate */
 118         char     boot_jump_and_sys_id[3+8]; /* 000 short or near jump instruction */
 119         /*char   system_id[8];*/     /* 003 name - can be used to special case partition manager volumes */
 120         uint16_t bytes_per_sect;     /* 00b bytes per logical sector */
 121         uint8_t  sect_per_clust;     /* 00d sectors/cluster */
 122         uint16_t reserved_sect;      /* 00e reserved sectors (sector offset of 1st FAT relative to volume start) */
 123         uint8_t  fats;               /* 010 number of FATs */
 124         uint16_t dir_entries;        /* 011 root directory entries */
 125         uint16_t volume_size_sect;   /* 013 volume size in sectors */
 126         uint8_t  media_byte;         /* 015 media code */
 127         uint16_t sect_per_fat;       /* 016 sectors/FAT */
 128         uint16_t sect_per_track;     /* 018 sectors per track */
 129         uint16_t heads;              /* 01a number of heads */
 130         uint32_t hidden;             /* 01c hidden sectors (sector offset of volume within physical disk) */
 131         uint32_t fat32_volume_size_sect; /* 020 volume size in sectors (if volume_size_sect == 0) */
 132         uint32_t fat32_sect_per_fat; /* 024 sectors/FAT */
 133         uint16_t fat32_flags;        /* 028 bit 8: fat mirroring, low 4: active fat */
 134         uint8_t  fat32_version[2];   /* 02a major, minor filesystem version (I see 0,0) */
 135         uint32_t fat32_root_cluster; /* 02c first cluster in root directory */
 136         uint16_t fat32_info_sector;  /* 030 filesystem info sector (usually 1) */
 137         uint16_t fat32_backup_boot;  /* 032 backup boot sector (usually 6) */
 138         uint32_t reserved2[3];       /* 034 unused */
 139         struct msdos_volume_info vi; /* 040 */
 140         char     boot_code[0x200 - 0x5a - 2]; /* 05a */
 141 #define BOOT_SIGN 0xAA55
 142         uint16_t boot_sign;          /* 1fe */
 143 } PACKED;
 144
 145 #define FAT_FSINFO_SIG1 0x41615252
 146 #define FAT_FSINFO_SIG2 0x61417272
 147 struct fat32_fsinfo {
 148         uint32_t signature1;         /* 0x52,0x52,0x41,0x61, "RRaA" */
 149         uint32_t reserved1[128 - 8];
 150         uint32_t signature2;         /* 0x72,0x72,0x61,0x41, "rrAa" */
 151         uint32_t free_clusters;      /* free cluster count.  -1 if unknown */
 152         uint32_t next_cluster;       /* most recently allocated cluster */
 153         uint32_t reserved2[3];
 154         uint16_t reserved3;          /* 1fc */
 155         uint16_t boot_sign;          /* 1fe */
 156 } PACKED;
 157
 158 struct bug_check {
 159         char BUG1[sizeof(struct msdos_dir_entry  ) == 0x20 ? 1 : -1];
 160         char BUG2[sizeof(struct msdos_volume_info) == 0x1a ? 1 : -1];
 161         char BUG3[sizeof(struct msdos_boot_sector) == 0x200 ? 1 : -1];
 162         char BUG4[sizeof(struct fat32_fsinfo     ) == 0x200 ? 1 : -1];
 163 };
 164
 165 static const char boot_code[] ALIGN1 =
 166         "\x0e"          /* 05a:         push  cs */
 167         "\x1f"          /* 05b:         pop   ds */
 168         "\xbe\x77\x7c"  /*  write_msg:  mov   si, offset message_txt */
 169         "\xac"          /* 05f:         lodsb */
 170         "\x22\xc0"      /* 060:         and   al, al */
 171         "\x74\x0b"      /* 062:         jz    key_press */
 172         "\x56"          /* 064:         push  si */
 173         "\xb4\x0e"      /* 065:         mov   ah, 0eh */
 174         "\xbb\x07\x00"  /* 067:         mov   bx, 0007h */
 175         "\xcd\x10"      /* 06a:         int   10h */
 176         "\x5e"          /* 06c:         pop   si */
 177         "\xeb\xf0"      /* 06d:         jmp   write_msg */
 178         "\x32\xe4"      /*  key_press:  xor   ah, ah */
 179         "\xcd\x16"      /* 071:         int   16h */
 180         "\xcd\x19"      /* 073:         int   19h */
 181         "\xeb\xfe"      /*  foo:        jmp   foo */
 182         /* 077: message_txt: */
 183         "This is not a bootable disk\r\n";
 184
 185
 186 #define MARK_CLUSTER(cluster, value) \
 187         ((uint32_t *)fat)[cluster] = SWAP_LE32(value)
 188
 189 void BUG_unsupported_field_size(void);
 190 #define STORE_LE(field, value) \
 191 do { \
 192         if (sizeof(field) == 4) \
 193                 field = SWAP_LE32(value); \
 194         else if (sizeof(field) == 2) \
 195                 field = SWAP_LE16(value); \
 196         else if (sizeof(field) == 1) \
 197                 field = (value); \
 198         else \
 199                 BUG_unsupported_field_size(); \
 200 } while (0)
 201
 202 /* compat:
 203  * mkdosfs 2.11 (12 Mar 2005)
 204  * Usage: mkdosfs [-A] [-c] [-C] [-v] [-I] [-l bad-block-file]
 205  *        [-b backup-boot-sector]
 206  *        [-m boot-msg-file] [-n volume-name] [-i volume-id]
 207  *        [-s sectors-per-cluster] [-S logical-sector-size]
 208  *        [-f number-of-FATs]
 209  *        [-h hidden-sectors] [-F fat-size] [-r root-dir-entries]
 210  *        [-R reserved-sectors]
 211  *        /dev/name [blocks]
 212  */
 213 int mkfs_vfat_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
 214 int mkfs_vfat_main(int argc UNUSED_PARAM, char **argv)
 215 {
 216         struct stat st;
 217         const char *volume_label = "";
 218         char *buf;
 219         char *device_name;
 220         uoff_t volume_size_bytes;
 221         uoff_t volume_size_sect;
 222         uint32_t total_clust;
 223         uint32_t volume_id;
 224         int dev;
 225         unsigned bytes_per_sect;
 226         unsigned sect_per_fat;
 227         unsigned opts;
 228         uint16_t sect_per_track;
 229         uint8_t media_byte;
 230         uint8_t sect_per_clust;
 231         uint8_t heads;
 232         enum {
 233                 OPT_A = 1 << 0,  // [IGNORED] atari format
 234                 OPT_b = 1 << 1,  // [IGNORED] location of backup boot sector
 235                 OPT_c = 1 << 2,  // [IGNORED] check filesystem
 236                 OPT_C = 1 << 3,  // [IGNORED] create a new file
 237                 OPT_f = 1 << 4,  // [IGNORED] number of FATs
 238                 OPT_F = 1 << 5,  // [IGNORED, implied 32] choose FAT size
 239                 OPT_h = 1 << 6,  // [IGNORED] number of hidden sectors
 240                 OPT_I = 1 << 7,  // [IGNORED] don't bark at entire disk devices
 241                 OPT_i = 1 << 8,  // [IGNORED] volume ID
 242                 OPT_l = 1 << 9,  // [IGNORED] bad block filename
 243                 OPT_m = 1 << 10, // [IGNORED] message file
 244                 OPT_n = 1 << 11, // volume label
 245                 OPT_r = 1 << 12, // [IGNORED] root directory entries
 246                 OPT_R = 1 << 13, // [IGNORED] number of reserved sectors
 247                 OPT_s = 1 << 14, // [IGNORED] sectors per cluster
 248                 OPT_S = 1 << 15, // [IGNORED] sector size
 249                 OPT_v = 1 << 16, // verbose
 250         };
 251
 252         opt_complementary = "-1";//:b+:f+:F+:h+:r+:R+:s+:S+:vv:c--l:l--c";
 253         opts = getopt32(argv, "Ab:cCf:F:h:Ii:l:m:n:r:R:s:S:v",
 254                 NULL, NULL, NULL, NULL, NULL,
 255                 NULL, NULL, &volume_label, NULL, NULL, NULL, NULL);
 256         argv += optind;
 257
 258         // cache device name
 259         device_name = argv[0];
 260         // default volume ID = creation time
 261         volume_id = time(NULL);
 262
 263         dev = xopen(device_name, O_RDWR);
 264         xfstat(dev, &st, device_name);
 265
 266         //
 267         // Get image size and sector size
 268         //
 269         bytes_per_sect = SECTOR_SIZE;
 270         if (!S_ISBLK(st.st_mode)) {
 271                 if (!S_ISREG(st.st_mode)) {
 272                         if (!argv[1])
 273                                 bb_error_msg_and_die("image size must be specified");
 274                 }
 275                 // not a block device, skip bad sectors check
 276                 opts &= ~OPT_c;
 277         } else {
 278                 int min_bytes_per_sect;
 279 #if 0
 280                 unsigned device_num;
 281                 // for true block devices we do check sanity
 282                 device_num = st.st_rdev & 0xff3f;
 283                 // do we allow to format the whole disk device?
 284                 if (!(opts & OPT_I) && (
 285                         device_num == 0x0300 || // hda, hdb
 286                         (device_num & 0xff0f) == 0x0800 || // sd
 287                         device_num == 0x0d00 || // xd
 288                         device_num == 0x1600 )  // hdc, hdd
 289                 )
 290                         bb_error_msg_and_die("will not try to make filesystem on full-disk device (use -I if wanted)");
 291                 // can't work on mounted filesystems
 292                 if (find_mount_point(device_name, 0))
 293                         bb_error_msg_and_die("can't format mounted filesystem");
 294 #endif
 295                 // get true sector size
 296                 // (parameter must be int*, not long* or size_t*)
 297                 xioctl(dev, BLKSSZGET, &min_bytes_per_sect);
 298                 if (min_bytes_per_sect > SECTOR_SIZE) {
 299                         bytes_per_sect = min_bytes_per_sect;
 300                         bb_error_msg("for this device sector size is %u", min_bytes_per_sect);
 301                 }
 302         }
 303         volume_size_bytes = get_volume_size_in_bytes(dev, argv[1], 1024, /*extend:*/ 1);
 304         volume_size_sect = volume_size_bytes / bytes_per_sect;
 305
 306         //
 307         // Find out or guess media parameters
 308         //
 309         media_byte = 0xf8;
 310         heads = 255;
 311         sect_per_track = 63;
 312         sect_per_clust = 1;
 313         {
 314                 struct hd_geometry geometry;
 315                 // size (in sectors), sect (per track), head
 316                 struct floppy_struct param;
 317
 318                 // N.B. whether to use HDIO_GETGEO or HDIO_REQ?
 319                 if (ioctl(dev, HDIO_GETGEO, &geometry) == 0
 320                  && geometry.sectors
 321                  && geometry.heads
 322                 ) {
 323                         // hard drive
 324                         sect_per_track = geometry.sectors;
 325                         heads = geometry.heads;
 326
 327  set_cluster_size:
 328                         /* For FAT32, try to do the same as M$'s format command
 329                          * (see http://www.win.tue.nl/~aeb/linux/fs/fat/fatgen103.pdf p. 20):
 330                          * fs size <= 260M: 0.5k clusters
 331                          * fs size <=   8G: 4k clusters
 332                          * fs size <=  16G: 8k clusters
 333                          * fs size >   16G: 16k clusters
 334                          */
 335                         sect_per_clust = 1;
 336                         if (volume_size_bytes >= 260*1024*1024) {
 337                                 sect_per_clust = 8;
 338                                 /* fight gcc: */
 339                                 /* "error: integer overflow in expression" */
 340                                 /* "error: right shift count >= width of type" */
 341                                 if (sizeof(off_t) > 4) {
 342                                         unsigned t = (volume_size_bytes >> 31 >> 1);
 343                                         if (t >= 8/4)
 344                                                 sect_per_clust = 16;
 345                                         if (t >= 16/4)
 346                                                 sect_per_clust = 32;
 347                                 }
 348                         }
 349                 } else {
 350                         // floppy, loop, or regular file
 351                         int not_floppy = ioctl(dev, FDGETPRM, &param);
 352                         if (not_floppy == 0) {
 353                                 // floppy disk
 354                                 sect_per_track = param.sect;
 355                                 heads = param.head;
 356                                 volume_size_sect = param.size;
 357                                 volume_size_bytes = param.size * SECTOR_SIZE;
 358                         }
 359                         // setup the media descriptor byte
 360                         switch (volume_size_sect) {
 361                         case 2*360:     // 5.25", 2, 9, 40 - 360K
 362                                 media_byte = 0xfd;
 363                                 break;
 364                         case 2*720:     // 3.5", 2, 9, 80 - 720K
 365                         case 2*1200:    // 5.25", 2, 15, 80 - 1200K
 366                                 media_byte = 0xf9;
 367                                 break;
 368                         default:        // anything else
 369                                 if (not_floppy)
 370                                         goto set_cluster_size;
 371                         case 2*1440:    // 3.5", 2, 18, 80 - 1440K
 372                         case 2*2880:    // 3.5", 2, 36, 80 - 2880K
 373                                 media_byte = 0xf0;
 374                                 break;
 375                         }
 376                         // not floppy, but size matches floppy exactly.
 377                         // perhaps it is a floppy image.
 378                         // we already set media_byte as if it is a floppy,
 379                         // now set sect_per_track and heads.
 380                         heads = 2;
 381                         sect_per_track = (unsigned)volume_size_sect / 160;
 382                         if (sect_per_track < 9)
 383                                 sect_per_track = 9;
 384                 }
 385         }
 386
 387         //
 388         // Calculate number of clusters, sectors/cluster, sectors/FAT
 389         // (an initial guess for sect_per_clust should already be set)
 390         //
 391         // "mkdosfs -v -F 32 image5k 5" is the minimum:
 392         // 2 sectors for FATs and 2 data sectors
 393         if ((off_t)(volume_size_sect - reserved_sect) < 4)
 394                 bb_error_msg_and_die("the image is too small for FAT32");
 395         sect_per_fat = 1;
 396         while (1) {
 397                 while (1) {
 398                         int spf_adj;
 399                         uoff_t tcl = (volume_size_sect - reserved_sect - NUM_FATS * sect_per_fat) / sect_per_clust;
 400                         // tcl may be > MAX_CLUST_32 here, but it may be
 401                         // because sect_per_fat is underestimated,
 402                         // and with increased sect_per_fat it still may become
 403                         // <= MAX_CLUST_32. Therefore, we do not check
 404                         // against MAX_CLUST_32, but against a bigger const:
 405                         if (tcl > 0x80ffffff)
 406                                 goto next;
 407                         total_clust = tcl; // fits in uint32_t
 408                         // Every cluster needs 4 bytes in FAT. +2 entries since
 409                         // FAT has space for non-existent clusters 0 and 1.
 410                         // Let's see how many sectors that needs.
 411                         //May overflow at "*4":
 412                         //spf_adj = ((total_clust+2) * 4 + bytes_per_sect-1) / bytes_per_sect - sect_per_fat;
 413                         //Same in the more obscure, non-overflowing form:
 414                         spf_adj = ((total_clust+2) + (bytes_per_sect/4)-1) / (bytes_per_sect/4) - sect_per_fat;
 415 #if 0
 416                         bb_error_msg("sect_per_clust:%u sect_per_fat:%u total_clust:%u",
 417                                         sect_per_clust, sect_per_fat, (int)tcl);
 418                         bb_error_msg("adjust to sect_per_fat:%d", spf_adj);
 419 #endif
 420                         if (spf_adj <= 0) {
 421                                 // do not need to adjust sect_per_fat.
 422                                 // so, was total_clust too big after all?
 423                                 if (total_clust <= MAX_CLUST_32)
 424                                         goto found_total_clust; // no
 425                                 // yes, total_clust is _a bit_ too big
 426                                 goto next;
 427                         }
 428                         // adjust sect_per_fat, go back and recalc total_clust
 429                         // (note: just "sect_per_fat += spf_adj" isn't ok)
 430                         sect_per_fat += ((unsigned)spf_adj / 2) | 1;
 431                 }
 432  next:
 433                 if (sect_per_clust == 128)
 434                         bb_error_msg_and_die("can't make FAT32 with >128 sectors/cluster");
 435                 sect_per_clust *= 2;
 436                 sect_per_fat = (sect_per_fat / 2) | 1;
 437         }
 438  found_total_clust:
 439
 440         //
 441         // Print info
 442         //
 443         if (opts & OPT_v) {
 444                 fprintf(stderr,
 445                         "Device '%s':\n"
 446                         "heads:%u, sectors/track:%u, bytes/sector:%u\n"
 447                         "media descriptor:%02x\n"
 448                         "total sectors:%"OFF_FMT"u, clusters:%u, sectors/cluster:%u\n"
 449                         "FATs:2, sectors/FAT:%u\n"
 450                         "volumeID:%08x, label:'%s'\n",
 451                         device_name,
 452                         heads, sect_per_track, bytes_per_sect,
 453                         (int)media_byte,
 454                         volume_size_sect, (int)total_clust, (int)sect_per_clust,
 455                         sect_per_fat,
 456                         (int)volume_id, volume_label
 457                 );
 458         }
 459
 460         //
 461         // Write filesystem image sequentially (no seeking)
 462         //
 463         {
 464                 // (a | b) is poor man's max(a, b)
 465                 unsigned bufsize = reserved_sect;
 466                 //bufsize |= sect_per_fat; // can be quite large
 467                 bufsize |= 2; // use this instead
 468                 bufsize |= sect_per_clust;
 469                 buf = xzalloc(bufsize * bytes_per_sect);
 470         }
 471
 472         { // boot and fsinfo sectors, and their copies
 473                 struct msdos_boot_sector *boot_blk = (void*)buf;
 474                 struct fat32_fsinfo *info = (void*)(buf + bytes_per_sect);
 475
 476                 strcpy(boot_blk->boot_jump_and_sys_id, "\xeb\x58\x90" "mkdosfs");
 477                 STORE_LE(boot_blk->bytes_per_sect, bytes_per_sect);
 478                 STORE_LE(boot_blk->sect_per_clust, sect_per_clust);
 479                 // cast in needed on big endian to suppress a warning
 480                 STORE_LE(boot_blk->reserved_sect, (uint16_t)reserved_sect);
 481                 STORE_LE(boot_blk->fats, 2);
 482                 //STORE_LE(boot_blk->dir_entries, 0); // for FAT32, stays 0
 483                 if (volume_size_sect <= 0xffff)
 484                         STORE_LE(boot_blk->volume_size_sect, volume_size_sect);
 485                 STORE_LE(boot_blk->media_byte, media_byte);
 486                 // wrong: this would make Linux think that it's fat12/16:
 487                 //if (sect_per_fat <= 0xffff)
 488                 //      STORE_LE(boot_blk->sect_per_fat, sect_per_fat);
 489                 // works:
 490                 //STORE_LE(boot_blk->sect_per_fat, 0);
 491                 STORE_LE(boot_blk->sect_per_track, sect_per_track);
 492                 STORE_LE(boot_blk->heads, heads);
 493                 //STORE_LE(boot_blk->hidden, 0);
 494                 STORE_LE(boot_blk->fat32_volume_size_sect, volume_size_sect);
 495                 STORE_LE(boot_blk->fat32_sect_per_fat, sect_per_fat);
 496                 //STORE_LE(boot_blk->fat32_flags, 0);
 497                 //STORE_LE(boot_blk->fat32_version[2], 0,0);
 498                 STORE_LE(boot_blk->fat32_root_cluster, 2);
 499                 STORE_LE(boot_blk->fat32_info_sector, info_sector_number);
 500                 STORE_LE(boot_blk->fat32_backup_boot, backup_boot_sector);
 501                 //STORE_LE(boot_blk->reserved2[3], 0,0,0);
 502                 STORE_LE(boot_blk->vi.ext_boot_sign, 0x29);
 503                 STORE_LE(boot_blk->vi.volume_id32, volume_id);
 504                 strncpy(boot_blk->vi.fs_type, "FAT32   ", sizeof(boot_blk->vi.fs_type));
 505                 strncpy(boot_blk->vi.volume_label, volume_label, sizeof(boot_blk->vi.volume_label));
 506                 memcpy(boot_blk->boot_code, boot_code, sizeof(boot_code));
 507                 STORE_LE(boot_blk->boot_sign, BOOT_SIGN);
 508
 509                 STORE_LE(info->signature1, FAT_FSINFO_SIG1);
 510                 STORE_LE(info->signature2, FAT_FSINFO_SIG2);
 511                 // we've allocated cluster 2 for the root dir
 512                 STORE_LE(info->free_clusters, (total_clust - 1));
 513                 STORE_LE(info->next_cluster, 2);
 514                 STORE_LE(info->boot_sign, BOOT_SIGN);
 515
 516                 // 1st copy
 517                 xwrite(dev, buf, bytes_per_sect * backup_boot_sector);
 518                 // 2nd copy and possibly zero sectors
 519                 xwrite(dev, buf, bytes_per_sect * (reserved_sect - backup_boot_sector));
 520         }
 521
 522         { // file allocation tables
 523                 unsigned i,j;
 524                 unsigned char *fat = (void*)buf;
 525
 526                 memset(buf, 0, bytes_per_sect * 2);
 527                 // initial FAT entries
 528                 MARK_CLUSTER(0, 0x0fffff00 | media_byte);
 529                 MARK_CLUSTER(1, 0xffffffff);
 530                 // mark cluster 2 as EOF (used for root dir)
 531                 MARK_CLUSTER(2, EOF_FAT32);
 532                 for (i = 0; i < NUM_FATS; i++) {
 533                         xwrite(dev, buf, bytes_per_sect);
 534                         for (j = 1; j < sect_per_fat; j++)
 535                                 xwrite(dev, buf + bytes_per_sect, bytes_per_sect);
 536                 }
 537         }
 538
 539         // root directory
 540         // empty directory is just a set of zero bytes
 541         memset(buf, 0, sect_per_clust * bytes_per_sect);
 542         if (volume_label[0]) {
 543                 // create dir entry for volume_label
 544                 struct msdos_dir_entry *de;
 545 #if 0
 546                 struct tm tm_time;
 547                 uint16_t t, d;
 548 #endif
 549                 de = (void*)buf;
 550                 strncpy(de->name, volume_label, sizeof(de->name));
 551                 STORE_LE(de->attr, ATTR_VOLUME);
 552 #if 0
 553                 localtime_r(&create_time, &tm_time);
 554                 t = (tm_time.tm_sec >> 1) + (tm_time.tm_min << 5) + (tm_time.tm_hour << 11);
 555                 d = tm_time.tm_mday + ((tm_time.tm_mon+1) << 5) + ((tm_time.tm_year-80) << 9);
 556                 STORE_LE(de->time, t);
 557                 STORE_LE(de->date, d);
 558                 //STORE_LE(de->ctime_cs, 0);
 559                 de->ctime = de->time;
 560                 de->cdate = de->date;
 561                 de->adate = de->date;
 562 #endif
 563         }
 564         xwrite(dev, buf, sect_per_clust * bytes_per_sect);
 565
 566 #if 0
 567         if (opts & OPT_c) {
 568                 uoff_t volume_size_blocks;
 569                 unsigned start_data_sector;
 570                 unsigned start_data_block;
 571                 unsigned badblocks = 0;
 572                 int try, got;
 573                 off_t currently_testing;
 574                 char *blkbuf = xmalloc(BLOCK_SIZE * TEST_BUFFER_BLOCKS);
 575
 576                 volume_size_blocks = (volume_size_bytes >> BLOCK_SIZE_BITS);
 577                 // N.B. the two following vars are in hard sectors, i.e. SECTOR_SIZE byte sectors!
 578                 start_data_sector = (reserved_sect + NUM_FATS * sect_per_fat) * (bytes_per_sect / SECTOR_SIZE);
 579                 start_data_block = (start_data_sector + SECTORS_PER_BLOCK - 1) / SECTORS_PER_BLOCK;
 580
 581                 bb_info_msg("searching for bad blocks ");
 582                 currently_testing = 0;
 583                 try = TEST_BUFFER_BLOCKS;
 584                 while (currently_testing < volume_size_blocks) {
 585                         if (currently_testing + try > volume_size_blocks)
 586                                 try = volume_size_blocks - currently_testing;
 587                         // perform a test on a block. return the number of blocks
 588                         // that could be read successfully.
 589                         // seek to the correct location
 590                         xlseek(dev, currently_testing * BLOCK_SIZE, SEEK_SET);
 591                         // try reading
 592                         got = read(dev, blkbuf, try * BLOCK_SIZE);
 593                         if (got < 0)
 594                                 got = 0;
 595                         if (got & (BLOCK_SIZE - 1))
 596                                 bb_error_msg("unexpected values in do_check: probably bugs");
 597                         got /= BLOCK_SIZE;
 598                         currently_testing += got;
 599                         if (got == try) {
 600                                 try = TEST_BUFFER_BLOCKS;
 601                                 continue;
 602                         }
 603                         try = 1;
 604                         if (currently_testing < start_data_block)
 605                                 bb_error_msg_and_die("bad blocks before data-area: cannot make fs");
 606
 607                         // mark all of the sectors in the block as bad
 608                         for (i = 0; i < SECTORS_PER_BLOCK; i++) {
 609                                 int cluster = (currently_testing * SECTORS_PER_BLOCK + i - start_data_sector) / (int) (sect_per_clust) / (bytes_per_sect / SECTOR_SIZE);
 610                                 if (cluster < 0)
 611                                         bb_error_msg_and_die("invalid cluster number in mark_sector: probably bug!");
 612                                 MARK_CLUSTER(cluster, BAD_FAT32);
 613                         }
 614                         badblocks++;
 615                         currently_testing++;
 616                 }
 617                 free(blkbuf);
 618                 if (badblocks)
 619                         bb_info_msg("%d bad block(s)", badblocks);
 620         }
 621 #endif
 622
 623         // cleanup
 624         if (ENABLE_FEATURE_CLEAN_UP) {
 625                 free(buf);
 626                 close(dev);
 627         }
 628
 629         return 0;
 630 }