/* vi: set sw=4 ts=4: */
/*
 * mkfs_ext2: utility to create EXT2 filesystem
 * inspired by genext2fs
 *
 * Busybox'ed (2009) by Vladimir Dronnikov <dronnikov@gmail.com>
 *
 * Licensed under GPLv2, see file LICENSE in this tarball for details.
 */
#include "libbb.h"
#include <linux/fs.h>
#include <linux/ext2_fs.h>
#include "volume_id/volume_id_internal.h"

#define ENABLE_FEATURE_MKFS_EXT2_RESERVED_GDT 0
#define ENABLE_FEATURE_MKFS_EXT2_DIR_INDEX    1

// from e2fsprogs
#define s_reserved_gdt_blocks s_padding1
#define s_mkfs_time           s_reserved[0]
#define s_flags               s_reserved[22]

#define EXT2_HASH_HALF_MD4       1
#define EXT2_FLAGS_SIGNED_HASH   0x0001
#define EXT2_FLAGS_UNSIGNED_HASH 0x0002

// storage helpers
char BUG_wrong_field_size(void);
#define STORE_LE(field, value) \
do { \
        if (sizeof(field) == 4) \
                field = cpu_to_le32(value); \
        else if (sizeof(field) == 2) \
                field = cpu_to_le16(value); \
        else if (sizeof(field) == 1) \
                field = (value); \
        else \
                BUG_wrong_field_size(); \
} while (0)

#define FETCH_LE32(field) \
        (sizeof(field) == 4 ? cpu_to_le32(field) : BUG_wrong_field_size())

// All fields are little-endian
struct ext2_dir {
        uint32_t inode1;
        uint16_t rec_len1;
        uint8_t  name_len1;
        uint8_t  file_type1;
        char     name1[4];
        uint32_t inode2;
        uint16_t rec_len2;
        uint8_t  name_len2;
        uint8_t  file_type2;
        char     name2[4];
        uint32_t inode3;
        uint16_t rec_len3;
        uint8_t  name_len3;
        uint8_t  file_type3;
        char     name3[12];
};

static unsigned int_log2(unsigned arg)
{
        unsigned r = 0;
        while ((arg >>= 1) != 0)
                r++;
        return r;
}

// taken from mkfs_minix.c. libbb candidate?
// "uint32_t size", since we never use it for anything >32 bits
static uint32_t div_roundup(uint32_t size, uint32_t n)
{
        // Overflow-resistant
        uint32_t res = size / n;
        if (res * n != size)
                res++;
        return res;
}

static void allocate(uint8_t *bitmap, uint32_t blocksize, uint32_t start, uint32_t end)
{
        uint32_t i;

//bb_info_msg("ALLOC: [%u][%u][%u]: [%u-%u]:=[%x],[%x]", blocksize, start, end, start/8, blocksize - end/8 - 1, (1 << (start & 7)) - 1, (uint8_t)(0xFF00 >> (end & 7)));
        memset(bitmap, 0, blocksize);
        i = start / 8;
        memset(bitmap, 0xFF, i);
        bitmap[i] = (1 << (start & 7)) - 1; //0..7 => 00000000..01111111
        i = end / 8;
        bitmap[blocksize - i - 1] |= 0x7F00 >> (end & 7); //0..7 => 00000000..11111110
        memset(bitmap + blocksize - i, 0xFF, i); // N.B. no overflow here!
}

static uint32_t has_super(uint32_t x)
{
        // 0, 1 and powers of 3, 5, 7 up to 2^32 limit
        static const uint32_t supers[] = {
                0, 1, 3, 5, 7, 9, 25, 27, 49, 81, 125, 243, 343, 625, 729,
                2187, 2401, 3125, 6561, 15625, 16807, 19683, 59049, 78125,
                117649, 177147, 390625, 531441, 823543, 1594323, 1953125,
                4782969, 5764801, 9765625, 14348907, 40353607, 43046721,
                48828125, 129140163, 244140625, 282475249, 387420489,
                1162261467, 1220703125, 1977326743, 3486784401/* >2^31 */,
        };
        const uint32_t *sp = supers + ARRAY_SIZE(supers);
        while (1) {
                sp--;
                if (x == *sp)
                        return 1;
                if (x > *sp)
                        return 0;
        }
}

#define fd 3    /* predefined output descriptor */

static void PUT(uint64_t off, void *buf, uint32_t size)
{
//      bb_info_msg("PUT[%llu]:[%u]", off, size);
        xlseek(fd, off, SEEK_SET);
        xwrite(fd, buf, size);
}

// 128 and 256-byte inodes:
// 128-byte inode is described by struct ext2_inode.
// 256-byte one just has these fields appended:
//      __u16   i_extra_isize;
//      __u16   i_pad1;
//      __u32   i_ctime_extra;  /* extra Change time (nsec << 2 | epoch) */
//      __u32   i_mtime_extra;  /* extra Modification time (nsec << 2 | epoch) */
//      __u32   i_atime_extra;  /* extra Access time (nsec << 2 | epoch) */
//      __u32   i_crtime;       /* File creation time */
//      __u32   i_crtime_extra; /* extra File creation time (nsec << 2 | epoch)*/
//      __u32   i_version_hi;   /* high 32 bits for 64-bit version */
// the rest is padding.
//
// linux/ext2_fs.h has "#define i_size_high i_dir_acl" which suggests that even
// 128-byte inode is capable of describing large files (i_dir_acl is meaningful
// only for directories, which never need i_size_high).
//
// Standard mke2fs creates a filesystem with 256-byte inodes if it is
// bigger than 0.5GB. So far, we do not do this.

// Standard mke2fs 1.41.9:
// Usage: mke2fs [-c|-l filename] [-b block-size] [-f fragment-size]
//      [-i bytes-per-inode] [-I inode-size] [-J journal-options]
//      [-G meta group size] [-N number-of-inodes]
//      [-m reserved-blocks-percentage] [-o creator-os]
//      [-g blocks-per-group] [-L volume-label] [-M last-mounted-directory]
//      [-O feature[,...]] [-r fs-revision] [-E extended-option[,...]]
//      [-T fs-type] [-U UUID] [-jnqvFSV] device [blocks-count]
//
// Options not commented below are taken but silently ignored:
enum {
        OPT_c = 1 << 0,
        OPT_l = 1 << 1,
        OPT_b = 1 << 2,         // block size, in bytes
        OPT_f = 1 << 3,
        OPT_i = 1 << 4,         // bytes per inode
        OPT_I = 1 << 5,         // custom inode size, in bytes
        OPT_J = 1 << 6,
        OPT_G = 1 << 7,
        OPT_N = 1 << 8,
        OPT_m = 1 << 9,         // percentage of blocks reserved for superuser
        OPT_o = 1 << 10,
        OPT_g = 1 << 11,
        OPT_L = 1 << 12,        // label
        OPT_M = 1 << 13,
        OPT_O = 1 << 14,
        OPT_r = 1 << 15,
        OPT_E = 1 << 16,
        OPT_T = 1 << 17,
        OPT_U = 1 << 18,
        OPT_j = 1 << 19,
        OPT_n = 1 << 20,        // dry run: do not write anything
        OPT_q = 1 << 21,
        OPT_v = 1 << 22,
        OPT_F = 1 << 23,
        OPT_S = 1 << 24,
        //OPT_V = 1 << 25,      // -V version. bbox applets don't support that
};

int mkfs_ext2_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
int mkfs_ext2_main(int argc UNUSED_PARAM, char **argv)
{
        unsigned i, pos, n;
        unsigned bs, bpi;
        unsigned blocksize, blocksize_log2;
        unsigned inodesize, user_inodesize;
        unsigned reserved_percent = 5;
        unsigned long long kilobytes;
        uint32_t nblocks, nblocks_full;
        uint32_t nreserved;
        uint32_t ngroups;
        uint32_t bytes_per_inode;
        uint32_t first_block;
        uint32_t inodes_per_group;
        uint32_t group_desc_blocks;
        uint32_t inode_table_blocks;
        uint32_t lost_and_found_blocks;
        time_t timestamp;
        const char *label = "";
        struct stat st;
        struct ext2_super_block *sb; // superblock
        struct ext2_group_desc *gd; // group descriptors
        struct ext2_inode *inode;
        struct ext2_dir *dir;
        uint8_t *buf;

        // using global "option_mask32" instead of local "opts":
        // we are register starved here
        opt_complementary = "-1:b+:m+:i+";
        /*opts =*/ getopt32(argv, "cl:b:f:i:I:J:G:N:m:o:g:L:M:O:r:E:T:U:jnqvFS",
                NULL, &bs, NULL, &bpi, &user_inodesize, NULL, NULL, NULL,
                &reserved_percent, NULL, NULL, &label, NULL, NULL, NULL, NULL, NULL, NULL);
        argv += optind; // argv[0] -- device

        // check the device is a block device
        xmove_fd(xopen(argv[0], O_WRONLY), fd);
        fstat(fd, &st);
        if (!S_ISBLK(st.st_mode) && !(option_mask32 & OPT_F))
                bb_error_msg_and_die("not a block device");

        // check if it is mounted
        // N.B. what if we format a file? find_mount_point will return false negative since
        // it is loop block device which mounted!
        if (find_mount_point(argv[0], 0))
                bb_error_msg_and_die("can't format mounted filesystem");

        // open the device, get size in kbytes
        if (argv[1]) {
                kilobytes = xatoull(argv[1]);
                // seek past end fails on block devices but works on files
                if (lseek(fd, kilobytes * 1024 - 1, SEEK_SET) != (off_t)-1) {
                        if (!(option_mask32 & OPT_n))
                                xwrite(fd, "", 1); // file grows if needed
                }
                //else {
                //      bb_error_msg("warning, block device is smaller");
                //}
        } else {
                kilobytes = (uoff_t)xlseek(fd, 0, SEEK_END) / 1024;
        }

        bytes_per_inode = 16384;
        if (kilobytes < 512*1024)
                bytes_per_inode = 4096;
        if (kilobytes < 3*1024)
                bytes_per_inode = 8192;
        if (option_mask32 & OPT_i)
                bytes_per_inode = bpi;

        // Determine block size and inode size
        // block size is a multiple of 1024
        // inode size is a multiple of 128
        blocksize = 1024;
        inodesize = sizeof(struct ext2_inode); // 128
        if (kilobytes >= 512*1024) { // mke2fs 1.41.9 compat
                blocksize = 4096;
                inodesize = 256;
        }
        if (EXT2_MAX_BLOCK_SIZE > 4096) {
                // kilobytes >> 22 == size in 4gigabyte chunks.
                // if size >= 16k gigs, blocksize must be increased.
                // Try "mke2fs -F image $((16 * 1024*1024*1024))"
                while ((kilobytes >> 22) >= blocksize)
                        blocksize *= 2;
        }
        if (option_mask32 & OPT_b)
                blocksize = bs;
        if (blocksize < EXT2_MIN_BLOCK_SIZE
         || blocksize > EXT2_MAX_BLOCK_SIZE
         || (blocksize & (blocksize - 1)) // not power of 2
        ) {
                bb_error_msg_and_die("blocksize %u is bad", blocksize);
        }
        // Do we have custom inode size?
        if (option_mask32 & OPT_I) {
                if (user_inodesize < sizeof(*inode)
                 || user_inodesize > blocksize
                 || (user_inodesize & (user_inodesize - 1)) // not power of 2
                ) {
                        bb_error_msg("-%c is bad", 'I');
                } else {
                        inodesize = user_inodesize;
                }
        }

        if ((int32_t)bytes_per_inode < blocksize)
                bb_error_msg_and_die("-%c is bad", 'i');
        // number of bits in one block, i.e. 8*blocksize
#define blocks_per_group (8 * blocksize)
        first_block = (EXT2_MIN_BLOCK_SIZE == blocksize);
        blocksize_log2 = int_log2(blocksize);

        // Determine number of blocks
        kilobytes >>= (blocksize_log2 - EXT2_MIN_BLOCK_LOG_SIZE);
        nblocks = kilobytes;
        if (nblocks != kilobytes)
                bb_error_msg_and_die("block count doesn't fit in 32 bits");
#define kilobytes kilobytes_unused_after_this
        // Experimentally, standard mke2fs won't work on images smaller than 60k
        if (nblocks < 60)
                bb_error_msg_and_die("need >= 60 blocks");

        // How many reserved blocks?
        if (reserved_percent > 50)
                bb_error_msg_and_die("-%c is bad", 'm');
        nreserved = (uint64_t)nblocks * reserved_percent / 100;

        // N.B. killing e2fsprogs feature! Unused blocks don't account in calculations
        nblocks_full = nblocks;

        // If last block group is too small, nblocks may be decreased in order
        // to discard it, and control returns here to recalculate some
        // parameters.
        // Note: blocksize and bytes_per_inode are never recalculated.
 retry:
        // N.B. a block group can have no more than blocks_per_group blocks
        ngroups = div_roundup(nblocks - first_block, blocks_per_group);

        group_desc_blocks = div_roundup(ngroups, blocksize / sizeof(*gd));
        // TODO: reserved blocks must be marked as such in the bitmaps,
        // or resulting filesystem is corrupt
        if (ENABLE_FEATURE_MKFS_EXT2_RESERVED_GDT) {
                /*
                 * From e2fsprogs: Calculate the number of GDT blocks to reserve for online
                 * filesystem growth.
                 * The absolute maximum number of GDT blocks we can reserve is determined by
                 * the number of block pointers that can fit into a single block.
                 * We set it at 1024x the current filesystem size, or
                 * the upper block count limit (2^32), whichever is lower.
                 */
                uint32_t reserved_group_desc_blocks = 0xFFFFFFFF; // maximum block number
                if (nblocks < reserved_group_desc_blocks / 1024)
                        reserved_group_desc_blocks = nblocks * 1024;
                reserved_group_desc_blocks = div_roundup(reserved_group_desc_blocks - first_block, blocks_per_group);
                reserved_group_desc_blocks = div_roundup(reserved_group_desc_blocks, blocksize / sizeof(*gd)) - group_desc_blocks;
                if (reserved_group_desc_blocks > blocksize / sizeof(uint32_t))
                        reserved_group_desc_blocks = blocksize / sizeof(uint32_t);
                //TODO: STORE_LE(sb->s_reserved_gdt_blocks, reserved_group_desc_blocks);
                group_desc_blocks += reserved_group_desc_blocks;
        }

        {
                // N.B. e2fsprogs does as follows!
                uint32_t overhead, remainder;
                // ninodes is the max number of inodes in this filesystem
                uint32_t ninodes = ((uint64_t) nblocks_full * blocksize) / bytes_per_inode;
                if (ninodes < EXT2_GOOD_OLD_FIRST_INO+1)
                        ninodes = EXT2_GOOD_OLD_FIRST_INO+1;
                inodes_per_group = div_roundup(ninodes, ngroups);
                // minimum number because the first EXT2_GOOD_OLD_FIRST_INO-1 are reserved
                if (inodes_per_group < 16)
                        inodes_per_group = 16;
                // a block group can't have more inodes than blocks
                if (inodes_per_group > blocks_per_group)
                        inodes_per_group = blocks_per_group;
                // adjust inodes per group so they completely fill the inode table blocks in the descriptor
                inodes_per_group = (div_roundup(inodes_per_group * inodesize, blocksize) * blocksize) / inodesize;
                // make sure the number of inodes per group is a multiple of 8
                inodes_per_group &= ~7;
                inode_table_blocks = div_roundup(inodes_per_group * inodesize, blocksize);

                // to be useful, lost+found should occupy at least 2 blocks (but not exceeding 16*1024 bytes),
                // and at most EXT2_NDIR_BLOCKS. So reserve these blocks right now
                /* Or e2fsprogs comment verbatim (what does it mean?):
                 * Ensure that lost+found is at least 2 blocks, so we always
                 * test large empty blocks for big-block filesystems. */
                lost_and_found_blocks = MIN(EXT2_NDIR_BLOCKS, 16 >> (blocksize_log2 - EXT2_MIN_BLOCK_LOG_SIZE));

                // the last group needs more attention: isn't it too small for possible overhead?
                overhead = (has_super(ngroups - 1) ? (1/*sb*/ + group_desc_blocks) : 0) + 1/*bbmp*/ + 1/*ibmp*/ + inode_table_blocks;
                remainder = (nblocks - first_block) % blocks_per_group;
                ////can't happen, nblocks >= 60 guarantees this
                ////if ((1 == ngroups)
                //// && remainder
                //// && (remainder < overhead + 1/* "/" */ + lost_and_found_blocks)
                ////) {
                ////    bb_error_msg_and_die("way small device");
                ////}

                // Standard mke2fs uses 50. Looks like a bug in our calculation
                // of "remainder" or "overhead" - we don't match standard mke2fs
                // when we transition from one group to two groups
                // (a bit after 8M image size), but it works for two->three groups
                // transition (at 16M).
                if (remainder && (remainder < overhead + 50)) {
//bb_info_msg("CHOP[%u]", remainder);
                        nblocks -= remainder;
                        goto retry;
                }
        }

        if (nblocks_full - nblocks)
                printf("warning: %u blocks unused\n\n", nblocks_full - nblocks);
        printf(
                "Filesystem label=%s\n"
                "OS type: Linux\n"
                "Block size=%u (log=%u)\n"
                "Fragment size=%u (log=%u)\n"
                "%u inodes, %u blocks\n"
                "%u blocks (%u%%) reserved for the super user\n"
                "First data block=%u\n"
                "Maximum filesystem blocks=%u\n"
                "%u block groups\n"
                "%u blocks per group, %u fragments per group\n"
                "%u inodes per group"
                , label
                , blocksize, blocksize_log2 - EXT2_MIN_BLOCK_LOG_SIZE
                , blocksize, blocksize_log2 - EXT2_MIN_BLOCK_LOG_SIZE
                , inodes_per_group * ngroups, nblocks
                , nreserved, reserved_percent
                , first_block
                , group_desc_blocks * (blocksize / (unsigned)sizeof(*gd)) * blocks_per_group
                , ngroups
                , blocks_per_group, blocks_per_group
                , inodes_per_group
        );
        {
                const char *fmt = "\nSuperblock backups stored on blocks:\n"
                        "\t%u";
                pos = first_block;
                for (i = 1; i < ngroups; i++) {
                        pos += blocks_per_group;
                        if (has_super(i)) {
                                printf(fmt, (unsigned)pos);
                                fmt = ", %u";
                        }
                }
        }
        bb_putchar('\n');

        if (option_mask32 & OPT_n) {
                if (ENABLE_FEATURE_CLEAN_UP)
                        close(fd);
                return EXIT_SUCCESS;
        }

        // TODO: 3/5 refuse if mounted
        // TODO: 4/5 compat options
        // TODO: 1/5 sanity checks
        // TODO: 0/5 more verbose error messages
        // TODO: 4/5 bigendianness: recheck, wait for ARM reporters
        // TODO: 2/5 reserved GDT: how to mark but not allocate?
        // TODO: 3/5 dir_index?

        // fill the superblock
        sb = xzalloc(1024);
        STORE_LE(sb->s_rev_level, EXT2_DYNAMIC_REV); // revision 1 filesystem
        STORE_LE(sb->s_magic, EXT2_SUPER_MAGIC);
        STORE_LE(sb->s_inode_size, inodesize);
        // set "Required extra isize" and "Desired extra isize" fields to 28
        if (inodesize != sizeof(*inode))
                STORE_LE(sb->s_reserved[21], 0x001C001C);
        STORE_LE(sb->s_first_ino, EXT2_GOOD_OLD_FIRST_INO);
        STORE_LE(sb->s_log_block_size, blocksize_log2 - EXT2_MIN_BLOCK_LOG_SIZE);
        STORE_LE(sb->s_log_frag_size, blocksize_log2 - EXT2_MIN_BLOCK_LOG_SIZE);
        // first 1024 bytes of the device are for boot record. If block size is 1024 bytes, then
        // the first block is 1, otherwise 0
        STORE_LE(sb->s_first_data_block, first_block);
        // block and inode bitmaps occupy no more than one block, so maximum number of blocks is
        STORE_LE(sb->s_blocks_per_group, blocks_per_group);
        STORE_LE(sb->s_frags_per_group, blocks_per_group);
        // blocks
        STORE_LE(sb->s_blocks_count, nblocks);
        // reserve blocks for superuser
        STORE_LE(sb->s_r_blocks_count, nreserved);
        // ninodes
        STORE_LE(sb->s_inodes_per_group, inodes_per_group);
        STORE_LE(sb->s_inodes_count, inodes_per_group * ngroups);
        STORE_LE(sb->s_free_inodes_count, inodes_per_group * ngroups - EXT2_GOOD_OLD_FIRST_INO);
        // timestamps
        timestamp = time(NULL);
        STORE_LE(sb->s_mkfs_time, timestamp);
        STORE_LE(sb->s_wtime, timestamp);
        STORE_LE(sb->s_lastcheck, timestamp);
        // misc. Values are chosen to match mke2fs 1.41.9
        STORE_LE(sb->s_state, 1); // TODO: what's 1?
        STORE_LE(sb->s_creator_os, EXT2_OS_LINUX);
        STORE_LE(sb->s_checkinterval, 24*60*60 * 180); // 180 days
        STORE_LE(sb->s_errors, EXT2_ERRORS_DEFAULT);
        // mke2fs 1.41.9 also sets EXT3_FEATURE_COMPAT_RESIZE_INODE
        // and if >= 0.5GB, EXT3_FEATURE_RO_COMPAT_LARGE_FILE.
        // we use values which match "mke2fs -O ^resize_inode":
        // in this case 1.41.9 never sets EXT3_FEATURE_RO_COMPAT_LARGE_FILE.
        STORE_LE(sb->s_feature_compat, EXT2_FEATURE_COMPAT_SUPP
                | (EXT2_FEATURE_COMPAT_RESIZE_INO * ENABLE_FEATURE_MKFS_EXT2_RESERVED_GDT)
                | (EXT2_FEATURE_COMPAT_DIR_INDEX * ENABLE_FEATURE_MKFS_EXT2_DIR_INDEX)
        );
        STORE_LE(sb->s_feature_incompat, EXT2_FEATURE_INCOMPAT_FILETYPE);
        STORE_LE(sb->s_feature_ro_compat, EXT2_FEATURE_RO_COMPAT_SPARSE_SUPER);
        STORE_LE(sb->s_flags, EXT2_FLAGS_UNSIGNED_HASH * ENABLE_FEATURE_MKFS_EXT2_DIR_INDEX);
        generate_uuid(sb->s_uuid);
        if (ENABLE_FEATURE_MKFS_EXT2_DIR_INDEX) {
                STORE_LE(sb->s_def_hash_version, EXT2_HASH_HALF_MD4);
                generate_uuid((uint8_t *)sb->s_hash_seed);
        }
        /*
         * From e2fsprogs: add "jitter" to the superblock's check interval so that we
         * don't check all the filesystems at the same time.  We use a
         * kludgy hack of using the UUID to derive a random jitter value.
         */
        STORE_LE(sb->s_max_mnt_count,
                EXT2_DFL_MAX_MNT_COUNT
                + (sb->s_uuid[ARRAY_SIZE(sb->s_uuid)-1] % EXT2_DFL_MAX_MNT_COUNT));

        // write the label
        safe_strncpy((char *)sb->s_volume_name, label, sizeof(sb->s_volume_name));

        // calculate filesystem skeleton structures
        gd = xzalloc(group_desc_blocks * blocksize);
        buf = xmalloc(blocksize);
        sb->s_free_blocks_count = 0;
        for (i = 0, pos = first_block, n = nblocks - first_block;
                i < ngroups;
                i++, pos += blocks_per_group, n -= blocks_per_group
        ) {
                uint32_t overhead = pos + (has_super(i) ? (1/*sb*/ + group_desc_blocks) : 0);
                uint32_t free_blocks;
                // fill group descriptors
                STORE_LE(gd[i].bg_block_bitmap, overhead + 0);
                STORE_LE(gd[i].bg_inode_bitmap, overhead + 1);
                STORE_LE(gd[i].bg_inode_table, overhead + 2);
                overhead = overhead - pos + 1/*bbmp*/ + 1/*ibmp*/ + inode_table_blocks;
                gd[i].bg_free_inodes_count = inodes_per_group;
                //STORE_LE(gd[i].bg_used_dirs_count, 0);
                // N.B. both "/" and "/lost+found" are within the first block group
                // "/" occupies 1 block, "/lost+found" occupies lost_and_found_blocks...
                if (0 == i) {
                        // ... thus increased overhead for the first block group ...
                        overhead += 1 + lost_and_found_blocks;
                        // ... and 2 used directories
                        STORE_LE(gd[i].bg_used_dirs_count, 2);
                        // well known reserved inodes belong to the first block too
                        gd[i].bg_free_inodes_count -= EXT2_GOOD_OLD_FIRST_INO;
                }

                // cache free block count of the group
                free_blocks = (n < blocks_per_group ? n : blocks_per_group) - overhead;

                // mark preallocated blocks as allocated
//bb_info_msg("ALLOC: [%u][%u][%u]", blocksize, overhead, blocks_per_group - (free_blocks + overhead));
                allocate(buf, blocksize,
                        // reserve "overhead" blocks
                        overhead,
                        // mark unused trailing blocks
                        blocks_per_group - (free_blocks + overhead)
                );
                // dump block bitmap
                PUT((uint64_t)(FETCH_LE32(gd[i].bg_block_bitmap)) * blocksize, buf, blocksize);
                STORE_LE(gd[i].bg_free_blocks_count, free_blocks);

                // mark preallocated inodes as allocated
                allocate(buf, blocksize,
                        // mark reserved inodes
                        inodes_per_group - gd[i].bg_free_inodes_count,
                        // mark unused trailing inodes
                        blocks_per_group - inodes_per_group
                );
                // dump inode bitmap
                //PUT((uint64_t)(FETCH_LE32(gd[i].bg_block_bitmap)) * blocksize, buf, blocksize);
                //but it's right after block bitmap, so we can just:
                xwrite(fd, buf, blocksize);
                STORE_LE(gd[i].bg_free_inodes_count, gd[i].bg_free_inodes_count);

                // count overall free blocks
                sb->s_free_blocks_count += free_blocks;
        }
        STORE_LE(sb->s_free_blocks_count, sb->s_free_blocks_count);

        // dump filesystem skeleton structures
//      printf("Writing superblocks and filesystem accounting information: ");
        for (i = 0, pos = first_block; i < ngroups; i++, pos += blocks_per_group) {
                // dump superblock and group descriptors and their backups
                if (has_super(i)) {
                        // N.B. 1024 byte blocks are special
                        PUT(((uint64_t)pos * blocksize) + ((0 == i && 1024 != blocksize) ? 1024 : 0),
                                        sb, 1024);
                        PUT(((uint64_t)pos * blocksize) + blocksize,
                                        gd, group_desc_blocks * blocksize);
                }
        }

        // zero boot sectors
        memset(buf, 0, blocksize);
        PUT(0, buf, 1024); // N.B. 1024 <= blocksize, so buf[0..1023] contains zeros
        // zero inode tables
        for (i = 0; i < ngroups; ++i)
                for (n = 0; n < inode_table_blocks; ++n)
                        PUT((uint64_t)(FETCH_LE32(gd[i].bg_inode_table) + n) * blocksize,
                                buf, blocksize);

        // prepare directory inode
        inode = (struct ext2_inode *)buf;
        STORE_LE(inode->i_mode, S_IFDIR | S_IRWXU | S_IRGRP | S_IROTH | S_IXGRP | S_IXOTH);
        STORE_LE(inode->i_mtime, timestamp);
        STORE_LE(inode->i_atime, timestamp);
        STORE_LE(inode->i_ctime, timestamp);
        STORE_LE(inode->i_size, blocksize);
        // inode->i_blocks stores the number of 512 byte data blocks
        // (512, because it goes directly to struct stat without scaling)
        STORE_LE(inode->i_blocks, blocksize / 512);

        // dump root dir inode
        STORE_LE(inode->i_links_count, 3); // "/.", "/..", "/lost+found/.." point to this inode
        STORE_LE(inode->i_block[0], FETCH_LE32(gd[0].bg_inode_table) + inode_table_blocks);
        PUT(((uint64_t)FETCH_LE32(gd[0].bg_inode_table) * blocksize) + (EXT2_ROOT_INO-1) * inodesize,
                                buf, inodesize);

        // dump lost+found dir inode
        STORE_LE(inode->i_links_count, 2); // both "/lost+found" and "/lost+found/." point to this inode
        STORE_LE(inode->i_size, lost_and_found_blocks * blocksize);
        STORE_LE(inode->i_blocks, (lost_and_found_blocks * blocksize) / 512);
        n = FETCH_LE32(inode->i_block[0]) + 1;
        for (i = 0; i < lost_and_found_blocks; ++i)
                STORE_LE(inode->i_block[i], i + n); // use next block
//bb_info_msg("LAST BLOCK USED[%u]", i + n);
        PUT(((uint64_t)FETCH_LE32(gd[0].bg_inode_table) * blocksize) + (EXT2_GOOD_OLD_FIRST_INO-1) * inodesize,
                                buf, inodesize);

        // dump directories
        memset(buf, 0, blocksize);
        dir = (struct ext2_dir *)buf;

        // dump 2nd+ blocks of "/lost+found"
        STORE_LE(dir->rec_len1, blocksize); // e2fsck 1.41.4 compat (1.41.9 does not need this)
        for (i = 1; i < lost_and_found_blocks; ++i)
                PUT((uint64_t)(FETCH_LE32(gd[0].bg_inode_table) + inode_table_blocks + 1+i) * blocksize,
                                buf, blocksize);

        // dump 1st block of "/lost+found"
        STORE_LE(dir->inode1, EXT2_GOOD_OLD_FIRST_INO);
        STORE_LE(dir->rec_len1, 12);
        STORE_LE(dir->name_len1, 1);
        STORE_LE(dir->file_type1, EXT2_FT_DIR);
        dir->name1[0] = '.';
        STORE_LE(dir->inode2, EXT2_ROOT_INO);
        STORE_LE(dir->rec_len2, blocksize - 12);
        STORE_LE(dir->name_len2, 2);
        STORE_LE(dir->file_type2, EXT2_FT_DIR);
        dir->name2[0] = '.'; dir->name2[1] = '.';
        PUT((uint64_t)(FETCH_LE32(gd[0].bg_inode_table) + inode_table_blocks + 1) * blocksize, buf, blocksize);

        // dump root dir block
        STORE_LE(dir->inode1, EXT2_ROOT_INO);
        STORE_LE(dir->rec_len2, 12);
        STORE_LE(dir->inode3, EXT2_GOOD_OLD_FIRST_INO);
        STORE_LE(dir->rec_len3, blocksize - 12 - 12);
        STORE_LE(dir->name_len3, 10);
        STORE_LE(dir->file_type3, EXT2_FT_DIR);
        strcpy(dir->name3, "lost+found");
        PUT((uint64_t)(FETCH_LE32(gd[0].bg_inode_table) + inode_table_blocks + 0) * blocksize, buf, blocksize);

        // cleanup
        if (ENABLE_FEATURE_CLEAN_UP) {
                free(buf);
                free(gd);
                free(sb);
        }

        xclose(fd);
        return EXIT_SUCCESS;
}