static int cloop_open(BlockDriverState *bs, QDict *options, int flags,
                      Error **errp)
{
    BDRVCloopState *s = bs->opaque;
    uint32_t offsets_size, max_compressed_block_size = 1, i;
    int ret;

    bs->read_only = 1;

    /* read header */
    ret = bdrv_pread(bs->file, 128, &s->block_size, 4);
    if (ret < 0) {
        return ret;
    }
    s->block_size = be32_to_cpu(s->block_size);
    if (s->block_size % 512) {
        error_setg(errp, "block_size %u must be a multiple of 512",
                   s->block_size);
        return -EINVAL;
    }
    if (s->block_size == 0) {
        error_setg(errp, "block_size cannot be zero");
        return -EINVAL;
    }

    /* cloop's create_compressed_fs.c warns about block sizes beyond 256 KB but
     * we can accept more.  Prevent ridiculous values like 4 GB - 1 since we
     * need a buffer this big.
     */
    if (s->block_size > MAX_BLOCK_SIZE) {
        error_setg(errp, "block_size %u must be %u MB or less",
                   s->block_size,
                   MAX_BLOCK_SIZE / (1024 * 1024));
        return -EINVAL;
    }

    ret = bdrv_pread(bs->file, 128 + 4, &s->n_blocks, 4);
    if (ret < 0) {
        return ret;
    }
    s->n_blocks = be32_to_cpu(s->n_blocks);

    /* read offsets */
    if (s->n_blocks > UINT32_MAX / sizeof(uint64_t)) {
        /* Prevent integer overflow */
        error_setg(errp, "n_blocks %u must be %zu or less",
                   s->n_blocks,
                   UINT32_MAX / sizeof(uint64_t));
        return -EINVAL;
    }
    offsets_size = s->n_blocks * sizeof(uint64_t);
    if (offsets_size > 512 * 1024 * 1024) {
        /* Prevent ridiculous offsets_size which causes memory allocation to
         * fail or overflows bdrv_pread() size.  In practice the 512 MB
         * offsets[] limit supports 16 TB images at 256 KB block size.
         */
        error_setg(errp, "image requires too many offsets, "
                   "try increasing block size");
        return -EINVAL;
    }
    s->offsets = g_malloc(offsets_size);

    ret = bdrv_pread(bs->file, 128 + 4 + 4, s->offsets, offsets_size);
    if (ret < 0) {
        goto fail;
    }

    for(i=0;i<s->n_blocks;i++) {
        uint64_t size;

        s->offsets[i] = be64_to_cpu(s->offsets[i]);
        if (i == 0) {
            continue;
        }

        if (s->offsets[i] < s->offsets[i - 1]) {
            error_setg(errp, "offsets not monotonically increasing at "
                       "index %u, image file is corrupt", i);
            ret = -EINVAL;
            goto fail;
        }

        size = s->offsets[i] - s->offsets[i - 1];

        /* Compressed blocks should be smaller than the uncompressed block size
         * but maybe compression performed poorly so the compressed block is
         * actually bigger.  Clamp down on unrealistic values to prevent
         * ridiculous s->compressed_block allocation.
         */
        if (size > 2 * MAX_BLOCK_SIZE) {
            error_setg(errp, "invalid compressed block size at index %u, "
                       "image file is corrupt", i);
            ret = -EINVAL;
            goto fail;
        }

        if (size > max_compressed_block_size) {
            max_compressed_block_size = size;
        }
    }

    /* initialize zlib engine */
    s->compressed_block = g_malloc(max_compressed_block_size + 1);
    s->uncompressed_block = g_malloc(s->block_size);
    if (inflateInit(&s->zstream) != Z_OK) {
        ret = -EINVAL;
        goto fail;
    }
    s->current_block = s->n_blocks;

    s->sectors_per_block = s->block_size/512;
    bs->total_sectors = s->n_blocks * s->sectors_per_block;
    qemu_co_mutex_init(&s->lock);
    return 0;

fail:
    g_free(s->offsets);
    g_free(s->compressed_block);
    g_free(s->uncompressed_block);
    return ret;
}