Archipelago » qemu

/*

 * Block driver for the QCOW version 2 format

 *

 * Copyright (c) 2004-2006 Fabrice Bellard

 *

 * Permission is hereby granted, free of charge, to any person obtaining a copy

 * of this software and associated documentation files (the "Software"), to deal

 * in the Software without restriction, including without limitation the rights

 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell

 * copies of the Software, and to permit persons to whom the Software is

 * furnished to do so, subject to the following conditions:

 *

 * The above copyright notice and this permission notice shall be included in

 * all copies or substantial portions of the Software.

 *

 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR

 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,

 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL

 * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER

 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,

 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN

 * THE SOFTWARE.

 */

#include "qemu-common.h"

#include "block_int.h"

#include "module.h"

#include <zlib.h>

#include "aes.h"

#include "block/qcow2.h"

/*

  Differences with QCOW:

  - Support for multiple incremental snapshots.

  - Memory management by reference counts.

  - Clusters which have a reference count of one have the bit

    QCOW_OFLAG_COPIED to optimize write performance.

  - Size of compressed clusters is stored in sectors to reduce bit usage

    in the cluster offsets.

  - Support for storing additional data (such as the VM state) in the

    snapshots.

  - If a backing store is used, the cluster size is not constrained

    (could be backported to QCOW).

  - L2 tables have always a size of one cluster.

*/

typedef struct {

    uint32_t magic;

    uint32_t len;

} QCowExtension;

#define  QCOW_EXT_MAGIC_END 0

#define  QCOW_EXT_MAGIC_BACKING_FORMAT 0xE2792ACA

static int qcow_probe(const uint8_t *buf, int buf_size, const char *filename)

{

    const QCowHeader *cow_header = (const void *)buf;

    if (buf_size >= sizeof(QCowHeader) &&

        be32_to_cpu(cow_header->magic) == QCOW_MAGIC &&

        be32_to_cpu(cow_header->version) == QCOW_VERSION)

        return 100;

    else

        return 0;

}

/* 

 * read qcow2 extension and fill bs

 * start reading from start_offset

 * finish reading upon magic of value 0 or when end_offset reached

 * unknown magic is skipped (future extension this version knows nothing about)

 * return 0 upon success, non-0 otherwise

 */

static int qcow_read_extensions(BlockDriverState *bs, uint64_t start_offset,

                                uint64_t end_offset)

{

    BDRVQcowState *s = bs->opaque;

    QCowExtension ext;

    uint64_t offset;

#ifdef DEBUG_EXT

    printf("qcow_read_extensions: start=%ld end=%ld\n", start_offset, end_offset);

#endif

    offset = start_offset;

    while (offset < end_offset) {

#ifdef DEBUG_EXT

        /* Sanity check */

        if (offset > s->cluster_size)

            printf("qcow_handle_extension: suspicious offset %lu\n", offset);

        printf("attemting to read extended header in offset %lu\n", offset);

#endif

        if (bdrv_pread(s->hd, offset, &ext, sizeof(ext)) != sizeof(ext)) {

            fprintf(stderr, "qcow_handle_extension: ERROR: pread fail from offset %llu\n",

                    (unsigned long long)offset);

            return 1;

        }

        be32_to_cpus(&ext.magic);

        be32_to_cpus(&ext.len);

        offset += sizeof(ext);

#ifdef DEBUG_EXT

        printf("ext.magic = 0x%x\n", ext.magic);

#endif

        switch (ext.magic) {

        case QCOW_EXT_MAGIC_END:

            return 0;

        case QCOW_EXT_MAGIC_BACKING_FORMAT:

            if (ext.len >= sizeof(bs->backing_format)) {

                fprintf(stderr, "ERROR: ext_backing_format: len=%u too large"

                        " (>=%zu)\n",

                        ext.len, sizeof(bs->backing_format));

                return 2;

            }

            if (bdrv_pread(s->hd, offset , bs->backing_format,

                           ext.len) != ext.len)

                return 3;

            bs->backing_format[ext.len] = '\0';

#ifdef DEBUG_EXT

            printf("Qcow2: Got format extension %s\n", bs->backing_format);

#endif

            offset = ((offset + ext.len + 7) & ~7);

            break;

        default:

            /* unknown magic -- just skip it */

            offset = ((offset + ext.len + 7) & ~7);

            break;

        }

    }

    return 0;

}

static int qcow_open(BlockDriverState *bs, const char *filename, int flags)

{

    BDRVQcowState *s = bs->opaque;

    int len, i, shift, ret;

    QCowHeader header;

    uint64_t ext_end;

    ret = bdrv_file_open(&s->hd, filename, flags);

    if (ret < 0)

        return ret;

    if (bdrv_pread(s->hd, 0, &header, sizeof(header)) != sizeof(header))

        goto fail;

    be32_to_cpus(&header.magic);

    be32_to_cpus(&header.version);

    be64_to_cpus(&header.backing_file_offset);

    be32_to_cpus(&header.backing_file_size);

    be64_to_cpus(&header.size);

    be32_to_cpus(&header.cluster_bits);

    be32_to_cpus(&header.crypt_method);

    be64_to_cpus(&header.l1_table_offset);

    be32_to_cpus(&header.l1_size);

    be64_to_cpus(&header.refcount_table_offset);

    be32_to_cpus(&header.refcount_table_clusters);

    be64_to_cpus(&header.snapshots_offset);

    be32_to_cpus(&header.nb_snapshots);

    if (header.magic != QCOW_MAGIC || header.version != QCOW_VERSION)

        goto fail;

    if (header.cluster_bits < MIN_CLUSTER_BITS ||

        header.cluster_bits > MAX_CLUSTER_BITS)

        goto fail;

    if (header.crypt_method > QCOW_CRYPT_AES)

        goto fail;

    s->crypt_method_header = header.crypt_method;

    if (s->crypt_method_header)

        bs->encrypted = 1;

    s->cluster_bits = header.cluster_bits;

    s->cluster_size = 1 << s->cluster_bits;

    s->cluster_sectors = 1 << (s->cluster_bits - 9);

    s->l2_bits = s->cluster_bits - 3; /* L2 is always one cluster */

    s->l2_size = 1 << s->l2_bits;

    bs->total_sectors = header.size / 512;

    s->csize_shift = (62 - (s->cluster_bits - 8));

    s->csize_mask = (1 << (s->cluster_bits - 8)) - 1;

    s->cluster_offset_mask = (1LL << s->csize_shift) - 1;

    s->refcount_table_offset = header.refcount_table_offset;

    s->refcount_table_size =

        header.refcount_table_clusters << (s->cluster_bits - 3);

    s->snapshots_offset = header.snapshots_offset;

    s->nb_snapshots = header.nb_snapshots;

    /* read the level 1 table */

    s->l1_size = header.l1_size;

    shift = s->cluster_bits + s->l2_bits;

    s->l1_vm_state_index = (header.size + (1LL << shift) - 1) >> shift;

    /* the L1 table must contain at least enough entries to put

       header.size bytes */

    if (s->l1_size < s->l1_vm_state_index)

        goto fail;

    s->l1_table_offset = header.l1_table_offset;

    if (s->l1_size > 0) {

        s->l1_table = qemu_mallocz(

            align_offset(s->l1_size * sizeof(uint64_t), 512));

        if (bdrv_pread(s->hd, s->l1_table_offset, s->l1_table, s->l1_size * sizeof(uint64_t)) !=

            s->l1_size * sizeof(uint64_t))

            goto fail;

        for(i = 0;i < s->l1_size; i++) {

            be64_to_cpus(&s->l1_table[i]);

        }

    }

    /* alloc L2 cache */

    s->l2_cache = qemu_malloc(s->l2_size * L2_CACHE_SIZE * sizeof(uint64_t));

    s->cluster_cache = qemu_malloc(s->cluster_size);

    /* one more sector for decompressed data alignment */

    s->cluster_data = qemu_malloc(QCOW_MAX_CRYPT_CLUSTERS * s->cluster_size

                                  + 512);

    s->cluster_cache_offset = -1;

    if (qcow2_refcount_init(bs) < 0)

        goto fail;

    QLIST_INIT(&s->cluster_allocs);

    /* read qcow2 extensions */

    if (header.backing_file_offset)

        ext_end = header.backing_file_offset;

    else

        ext_end = s->cluster_size;

    if (qcow_read_extensions(bs, sizeof(header), ext_end))

        goto fail;

    /* read the backing file name */

    if (header.backing_file_offset != 0) {

        len = header.backing_file_size;

        if (len > 1023)

            len = 1023;

        if (bdrv_pread(s->hd, header.backing_file_offset, bs->backing_file, len) != len)

            goto fail;

        bs->backing_file[len] = '\0';

    }

    if (qcow2_read_snapshots(bs) < 0)

        goto fail;

#ifdef DEBUG_ALLOC

    qcow2_check_refcounts(bs);

#endif

    return 0;

 fail:

    qcow2_free_snapshots(bs);

    qcow2_refcount_close(bs);

    qemu_free(s->l1_table);

    qemu_free(s->l2_cache);

    qemu_free(s->cluster_cache);

    qemu_free(s->cluster_data);

    bdrv_delete(s->hd);

    return -1;

}

static int qcow_set_key(BlockDriverState *bs, const char *key)

{

    BDRVQcowState *s = bs->opaque;

    uint8_t keybuf[16];

    int len, i;

    memset(keybuf, 0, 16);

    len = strlen(key);

    if (len > 16)

        len = 16;

    /* XXX: we could compress the chars to 7 bits to increase

       entropy */

    for(i = 0;i < len;i++) {

        keybuf[i] = key[i];

    }

    s->crypt_method = s->crypt_method_header;

    if (AES_set_encrypt_key(keybuf, 128, &s->aes_encrypt_key) != 0)

        return -1;

    if (AES_set_decrypt_key(keybuf, 128, &s->aes_decrypt_key) != 0)

        return -1;

#if 0

    /* test */

    {

        uint8_t in[16];

        uint8_t out[16];

        uint8_t tmp[16];

        for(i=0;i<16;i++)

            in[i] = i;

        AES_encrypt(in, tmp, &s->aes_encrypt_key);

        AES_decrypt(tmp, out, &s->aes_decrypt_key);

        for(i = 0; i < 16; i++)

            printf(" %02x", tmp[i]);

        printf("\n");

        for(i = 0; i < 16; i++)

            printf(" %02x", out[i]);

        printf("\n");

    }

#endif

    return 0;

}

static int qcow_is_allocated(BlockDriverState *bs, int64_t sector_num,

                             int nb_sectors, int *pnum)

{

    uint64_t cluster_offset;

    *pnum = nb_sectors;

    cluster_offset = qcow2_get_cluster_offset(bs, sector_num << 9, pnum);

    return (cluster_offset != 0);

}

/* handle reading after the end of the backing file */

int qcow2_backing_read1(BlockDriverState *bs,

                  int64_t sector_num, uint8_t *buf, int nb_sectors)

{

    int n1;

    if ((sector_num + nb_sectors) <= bs->total_sectors)

        return nb_sectors;

    if (sector_num >= bs->total_sectors)

        n1 = 0;

    else

        n1 = bs->total_sectors - sector_num;

    memset(buf + n1 * 512, 0, 512 * (nb_sectors - n1));

    return n1;

}

typedef struct QCowAIOCB {

    BlockDriverAIOCB common;

    int64_t sector_num;

    QEMUIOVector *qiov;

    uint8_t *buf;

    void *orig_buf;

    int nb_sectors;

    int n;

    uint64_t cluster_offset;

    uint8_t *cluster_data;

    BlockDriverAIOCB *hd_aiocb;

    struct iovec hd_iov;

    QEMUIOVector hd_qiov;

    QEMUBH *bh;

    QCowL2Meta l2meta;

    QLIST_ENTRY(QCowAIOCB) next_depend;

} QCowAIOCB;

static void qcow_aio_cancel(BlockDriverAIOCB *blockacb)

{

    QCowAIOCB *acb = (QCowAIOCB *)blockacb;

    if (acb->hd_aiocb)

        bdrv_aio_cancel(acb->hd_aiocb);

    qemu_aio_release(acb);

}

static AIOPool qcow_aio_pool = {

    .aiocb_size         = sizeof(QCowAIOCB),

    .cancel             = qcow_aio_cancel,

};

static void qcow_aio_read_cb(void *opaque, int ret);

static void qcow_aio_read_bh(void *opaque)

{

    QCowAIOCB *acb = opaque;

    qemu_bh_delete(acb->bh);

    acb->bh = NULL;

    qcow_aio_read_cb(opaque, 0);

}

static int qcow_schedule_bh(QEMUBHFunc *cb, QCowAIOCB *acb)

{

    if (acb->bh)

        return -EIO;

    acb->bh = qemu_bh_new(cb, acb);

    if (!acb->bh)

        return -EIO;

    qemu_bh_schedule(acb->bh);

    return 0;

}

static void qcow_aio_read_cb(void *opaque, int ret)

{

    QCowAIOCB *acb = opaque;

    BlockDriverState *bs = acb->common.bs;

    BDRVQcowState *s = bs->opaque;

    int index_in_cluster, n1;

    acb->hd_aiocb = NULL;

    if (ret < 0)

        goto done;

    /* post process the read buffer */

    if (!acb->cluster_offset) {

        /* nothing to do */

    } else if (acb->cluster_offset & QCOW_OFLAG_COMPRESSED) {

        /* nothing to do */

    } else {

        if (s->crypt_method) {

            qcow2_encrypt_sectors(s, acb->sector_num, acb->buf, acb->buf,

                            acb->n, 0,

                            &s->aes_decrypt_key);

        }

    }

    acb->nb_sectors -= acb->n;

    acb->sector_num += acb->n;

    acb->buf += acb->n * 512;

    if (acb->nb_sectors == 0) {

        /* request completed */

        ret = 0;

        goto done;

    }

    /* prepare next AIO request */

    acb->n = acb->nb_sectors;

    acb->cluster_offset =

        qcow2_get_cluster_offset(bs, acb->sector_num << 9, &acb->n);

    index_in_cluster = acb->sector_num & (s->cluster_sectors - 1);

    if (!acb->cluster_offset) {

        if (bs->backing_hd) {

            /* read from the base image */

            n1 = qcow2_backing_read1(bs->backing_hd, acb->sector_num,

                               acb->buf, acb->n);

            if (n1 > 0) {

                acb->hd_iov.iov_base = (void *)acb->buf;

                acb->hd_iov.iov_len = acb->n * 512;

                qemu_iovec_init_external(&acb->hd_qiov, &acb->hd_iov, 1);

                acb->hd_aiocb = bdrv_aio_readv(bs->backing_hd, acb->sector_num,

                                    &acb->hd_qiov, acb->n,

                                    qcow_aio_read_cb, acb);

                if (acb->hd_aiocb == NULL)

                    goto done;

            } else {

                ret = qcow_schedule_bh(qcow_aio_read_bh, acb);

                if (ret < 0)

                    goto done;

            }

        } else {

            /* Note: in this case, no need to wait */

            memset(acb->buf, 0, 512 * acb->n);

            ret = qcow_schedule_bh(qcow_aio_read_bh, acb);

            if (ret < 0)

                goto done;

        }

    } else if (acb->cluster_offset & QCOW_OFLAG_COMPRESSED) {

        /* add AIO support for compressed blocks ? */

        if (qcow2_decompress_cluster(s, acb->cluster_offset) < 0)

            goto done;

        memcpy(acb->buf,

               s->cluster_cache + index_in_cluster * 512, 512 * acb->n);

        ret = qcow_schedule_bh(qcow_aio_read_bh, acb);

        if (ret < 0)

            goto done;

    } else {

        if ((acb->cluster_offset & 511) != 0) {

            ret = -EIO;

            goto done;

        }

        acb->hd_iov.iov_base = (void *)acb->buf;

        acb->hd_iov.iov_len = acb->n * 512;

        qemu_iovec_init_external(&acb->hd_qiov, &acb->hd_iov, 1);

        acb->hd_aiocb = bdrv_aio_readv(s->hd,

                            (acb->cluster_offset >> 9) + index_in_cluster,

                            &acb->hd_qiov, acb->n, qcow_aio_read_cb, acb);

        if (acb->hd_aiocb == NULL)

            goto done;

    }

    return;

done:

    if (acb->qiov->niov > 1) {

        qemu_iovec_from_buffer(acb->qiov, acb->orig_buf, acb->qiov->size);

        qemu_vfree(acb->orig_buf);

    }

    acb->common.cb(acb->common.opaque, ret);

    qemu_aio_release(acb);

}

static QCowAIOCB *qcow_aio_setup(BlockDriverState *bs,

        int64_t sector_num, QEMUIOVector *qiov, int nb_sectors,

        BlockDriverCompletionFunc *cb, void *opaque, int is_write)

{

    QCowAIOCB *acb;

    acb = qemu_aio_get(&qcow_aio_pool, bs, cb, opaque);

    if (!acb)

        return NULL;

    acb->hd_aiocb = NULL;

    acb->sector_num = sector_num;

    acb->qiov = qiov;

    if (qiov->niov > 1) {

        acb->buf = acb->orig_buf = qemu_blockalign(bs, qiov->size);

        if (is_write)

            qemu_iovec_to_buffer(qiov, acb->buf);

    } else {

        acb->buf = (uint8_t *)qiov->iov->iov_base;

    }

    acb->nb_sectors = nb_sectors;

    acb->n = 0;

    acb->cluster_offset = 0;

    acb->l2meta.nb_clusters = 0;

    QLIST_INIT(&acb->l2meta.dependent_requests);

    return acb;

}

static BlockDriverAIOCB *qcow_aio_readv(BlockDriverState *bs,

        int64_t sector_num, QEMUIOVector *qiov, int nb_sectors,

        BlockDriverCompletionFunc *cb, void *opaque)

{

    QCowAIOCB *acb;

    acb = qcow_aio_setup(bs, sector_num, qiov, nb_sectors, cb, opaque, 0);

    if (!acb)

        return NULL;

    qcow_aio_read_cb(acb, 0);

    return &acb->common;

}

static void qcow_aio_write_cb(void *opaque, int ret);

static void run_dependent_requests(QCowL2Meta *m)

{

    QCowAIOCB *req;

    QCowAIOCB *next;

    /* Take the request off the list of running requests */

    if (m->nb_clusters != 0) {

        QLIST_REMOVE(m, next_in_flight);

    }

    /*

     * Restart all dependent requests.

     * Can't use QLIST_FOREACH here - the next link might not be the same

     * any more after the callback  (request could depend on a different

     * request now)

     */

    for (req = m->dependent_requests.lh_first; req != NULL; req = next) {

        next = req->next_depend.le_next;

        qcow_aio_write_cb(req, 0);

    }

    /* Empty the list for the next part of the request */

    QLIST_INIT(&m->dependent_requests);

}

static void qcow_aio_write_cb(void *opaque, int ret)

{

    QCowAIOCB *acb = opaque;

    BlockDriverState *bs = acb->common.bs;

    BDRVQcowState *s = bs->opaque;

    int index_in_cluster;

    const uint8_t *src_buf;

    int n_end;

    acb->hd_aiocb = NULL;

    if (ret >= 0) {

        ret = qcow2_alloc_cluster_link_l2(bs, &acb->l2meta);

    }

    run_dependent_requests(&acb->l2meta);

    if (ret < 0)

        goto done;

    acb->nb_sectors -= acb->n;

    acb->sector_num += acb->n;

    acb->buf += acb->n * 512;

    if (acb->nb_sectors == 0) {

        /* request completed */

        ret = 0;

        goto done;

    }

    index_in_cluster = acb->sector_num & (s->cluster_sectors - 1);

    n_end = index_in_cluster + acb->nb_sectors;

    if (s->crypt_method &&

        n_end > QCOW_MAX_CRYPT_CLUSTERS * s->cluster_sectors)

        n_end = QCOW_MAX_CRYPT_CLUSTERS * s->cluster_sectors;

    ret = qcow2_alloc_cluster_offset(bs, acb->sector_num << 9,

        index_in_cluster, n_end, &acb->n, &acb->l2meta);

    if (ret < 0) {

        goto done;

    }

    acb->cluster_offset = acb->l2meta.cluster_offset;

    /* Need to wait for another request? If so, we are done for now. */

    if (acb->l2meta.nb_clusters == 0 && acb->l2meta.depends_on != NULL) {

        QLIST_INSERT_HEAD(&acb->l2meta.depends_on->dependent_requests,

            acb, next_depend);

        return;

    }

    assert((acb->cluster_offset & 511) == 0);

    if (s->crypt_method) {

        if (!acb->cluster_data) {

            acb->cluster_data = qemu_mallocz(QCOW_MAX_CRYPT_CLUSTERS *

                                             s->cluster_size);

        }

        qcow2_encrypt_sectors(s, acb->sector_num, acb->cluster_data, acb->buf,

                        acb->n, 1, &s->aes_encrypt_key);

        src_buf = acb->cluster_data;

    } else {

        src_buf = acb->buf;

    }

    acb->hd_iov.iov_base = (void *)src_buf;

    acb->hd_iov.iov_len = acb->n * 512;

    qemu_iovec_init_external(&acb->hd_qiov, &acb->hd_iov, 1);

    acb->hd_aiocb = bdrv_aio_writev(s->hd,

                                    (acb->cluster_offset >> 9) + index_in_cluster,

                                    &acb->hd_qiov, acb->n,

                                    qcow_aio_write_cb, acb);

    if (acb->hd_aiocb == NULL)

        goto done;

    return;

done:

    if (acb->qiov->niov > 1)

        qemu_vfree(acb->orig_buf);

    acb->common.cb(acb->common.opaque, ret);

    qemu_aio_release(acb);

}

static BlockDriverAIOCB *qcow_aio_writev(BlockDriverState *bs,

        int64_t sector_num, QEMUIOVector *qiov, int nb_sectors,

        BlockDriverCompletionFunc *cb, void *opaque)

{

    BDRVQcowState *s = bs->opaque;

    QCowAIOCB *acb;

    s->cluster_cache_offset = -1; /* disable compressed cache */

    acb = qcow_aio_setup(bs, sector_num, qiov, nb_sectors, cb, opaque, 1);

    if (!acb)

        return NULL;

    qcow_aio_write_cb(acb, 0);

    return &acb->common;

}

static void qcow_close(BlockDriverState *bs)

{

    BDRVQcowState *s = bs->opaque;

    qemu_free(s->l1_table);

    qemu_free(s->l2_cache);

    qemu_free(s->cluster_cache);

    qemu_free(s->cluster_data);

    qcow2_refcount_close(bs);

    bdrv_delete(s->hd);

}

/*

 * Updates the variable length parts of the qcow2 header, i.e. the backing file

 * name and all extensions. qcow2 was not designed to allow such changes, so if

 * we run out of space (we can only use the first cluster) this function may

 * fail.

 *

 * Returns 0 on success, -errno in error cases.

 */

static int qcow2_update_ext_header(BlockDriverState *bs,

    const char *backing_file, const char *backing_fmt)

{

    size_t backing_file_len = 0;

    size_t backing_fmt_len = 0;

    BDRVQcowState *s = bs->opaque;

    QCowExtension ext_backing_fmt = {0, 0};

    int ret;

    /* Backing file format doesn't make sense without a backing file */

    if (backing_fmt && !backing_file) {

        return -EINVAL;

    }

    /* Prepare the backing file format extension if needed */

    if (backing_fmt) {

        ext_backing_fmt.len = cpu_to_be32(strlen(backing_fmt));

        ext_backing_fmt.magic = cpu_to_be32(QCOW_EXT_MAGIC_BACKING_FORMAT);

        backing_fmt_len = ((sizeof(ext_backing_fmt)

            + strlen(backing_fmt) + 7) & ~7);

    }

    /* Check if we can fit the new header into the first cluster */

    if (backing_file) {

        backing_file_len = strlen(backing_file);

    }

    size_t header_size = sizeof(QCowHeader) + backing_file_len

        + backing_fmt_len;

    if (header_size > s->cluster_size) {

        return -ENOSPC;

    }

    /* Rewrite backing file name and qcow2 extensions */

    size_t ext_size = header_size - sizeof(QCowHeader);

    uint8_t buf[ext_size];

    size_t offset = 0;

    size_t backing_file_offset = 0;

    if (backing_file) {

        if (backing_fmt) {

            int padding = backing_fmt_len -

                (sizeof(ext_backing_fmt) + strlen(backing_fmt));

            memcpy(buf + offset, &ext_backing_fmt, sizeof(ext_backing_fmt));

            offset += sizeof(ext_backing_fmt);

            memcpy(buf + offset, backing_fmt, strlen(backing_fmt));

            offset += strlen(backing_fmt);

            memset(buf + offset, 0, padding);

            offset += padding;

        }

        memcpy(buf + offset, backing_file, backing_file_len);

        backing_file_offset = sizeof(QCowHeader) + offset;

    }

    ret = bdrv_pwrite(s->hd, sizeof(QCowHeader), buf, ext_size);

    if (ret < 0) {

        goto fail;

    }

    /* Update header fields */

    uint64_t be_backing_file_offset = cpu_to_be64(backing_file_offset);

    uint32_t be_backing_file_size = cpu_to_be32(backing_file_len);

    ret = bdrv_pwrite(s->hd, offsetof(QCowHeader, backing_file_offset),

        &be_backing_file_offset, sizeof(uint64_t));

    if (ret < 0) {

        goto fail;

    }

    ret = bdrv_pwrite(s->hd, offsetof(QCowHeader, backing_file_size),

        &be_backing_file_size, sizeof(uint32_t));

    if (ret < 0) {

        goto fail;

    }

    ret = 0;

fail:

    return ret;

}

static int qcow2_change_backing_file(BlockDriverState *bs,

    const char *backing_file, const char *backing_fmt)

{

    return qcow2_update_ext_header(bs, backing_file, backing_fmt);

}

static int get_bits_from_size(size_t size)

{

    int res = 0;

    if (size == 0) {

        return -1;

    }

    while (size != 1) {

        /* Not a power of two */

        if (size & 1) {

            return -1;

        }

        size >>= 1;

        res++;

    }

    return res;

}

static int preallocate(BlockDriverState *bs)

{

    BDRVQcowState *s = bs->opaque;

    uint64_t nb_sectors;

    uint64_t offset;

    int num;

    int ret;

    QCowL2Meta meta;

    nb_sectors = bdrv_getlength(bs) >> 9;

    offset = 0;

    QLIST_INIT(&meta.dependent_requests);

    meta.cluster_offset = 0;

    while (nb_sectors) {

        num = MIN(nb_sectors, INT_MAX >> 9);

        ret = qcow2_alloc_cluster_offset(bs, offset, 0, num, &num, &meta);

        if (ret < 0) {

            return -1;

        }

        if (qcow2_alloc_cluster_link_l2(bs, &meta) < 0) {

            qcow2_free_any_clusters(bs, meta.cluster_offset, meta.nb_clusters);

            return -1;

        }

        /* There are no dependent requests, but we need to remove our request

         * from the list of in-flight requests */

        run_dependent_requests(&meta);

        /* TODO Preallocate data if requested */

        nb_sectors -= num;

        offset += num << 9;

    }

    /*

     * It is expected that the image file is large enough to actually contain

     * all of the allocated clusters (otherwise we get failing reads after

     * EOF). Extend the image to the last allocated sector.

     */

    if (meta.cluster_offset != 0) {

        uint8_t buf[512];

        memset(buf, 0, 512);

        bdrv_write(s->hd, (meta.cluster_offset >> 9) + num - 1, buf, 1);

    }

    return 0;

}

static int qcow_create2(const char *filename, int64_t total_size,

                        const char *backing_file, const char *backing_format,

                        int flags, size_t cluster_size, int prealloc)

{

    int fd, header_size, backing_filename_len, l1_size, i, shift, l2_bits;

    int ref_clusters, backing_format_len = 0;

    int rounded_ext_bf_len = 0;

    QCowHeader header;

    uint64_t tmp, offset;

    QCowCreateState s1, *s = &s1;

    QCowExtension ext_bf = {0, 0};

    int ret;

    memset(s, 0, sizeof(*s));

    fd = open(filename, O_WRONLY | O_CREAT | O_TRUNC | O_BINARY, 0644);

    if (fd < 0)

        return -1;

    memset(&header, 0, sizeof(header));

    header.magic = cpu_to_be32(QCOW_MAGIC);

    header.version = cpu_to_be32(QCOW_VERSION);

    header.size = cpu_to_be64(total_size * 512);

    header_size = sizeof(header);

    backing_filename_len = 0;

    if (backing_file) {

        if (backing_format) {

            ext_bf.magic = QCOW_EXT_MAGIC_BACKING_FORMAT;

            backing_format_len = strlen(backing_format);

            ext_bf.len = backing_format_len;

            rounded_ext_bf_len = (sizeof(ext_bf) + ext_bf.len + 7) & ~7;

            header_size += rounded_ext_bf_len;

        }

        header.backing_file_offset = cpu_to_be64(header_size);

        backing_filename_len = strlen(backing_file);

        header.backing_file_size = cpu_to_be32(backing_filename_len);

        header_size += backing_filename_len;

    }

    /* Cluster size */

    s->cluster_bits = get_bits_from_size(cluster_size);

    if (s->cluster_bits < MIN_CLUSTER_BITS ||

        s->cluster_bits > MAX_CLUSTER_BITS)

    {

        fprintf(stderr, "Cluster size must be a power of two between "

            "%d and %dk\n",

            1 << MIN_CLUSTER_BITS,

            1 << (MAX_CLUSTER_BITS - 10));

        return -EINVAL;

    }

    s->cluster_size = 1 << s->cluster_bits;

    header.cluster_bits = cpu_to_be32(s->cluster_bits);

    header_size = (header_size + 7) & ~7;

    if (flags & BLOCK_FLAG_ENCRYPT) {

        header.crypt_method = cpu_to_be32(QCOW_CRYPT_AES);

    } else {

        header.crypt_method = cpu_to_be32(QCOW_CRYPT_NONE);

    }

    l2_bits = s->cluster_bits - 3;

    shift = s->cluster_bits + l2_bits;

    l1_size = (((total_size * 512) + (1LL << shift) - 1) >> shift);

    offset = align_offset(header_size, s->cluster_size);

    s->l1_table_offset = offset;

    header.l1_table_offset = cpu_to_be64(s->l1_table_offset);

    header.l1_size = cpu_to_be32(l1_size);

    offset += align_offset(l1_size * sizeof(uint64_t), s->cluster_size);

    s->refcount_table = qemu_mallocz(s->cluster_size);