static int alloc_refcount_block(BlockDriverState *bs,
    int64_t cluster_index, uint16_t **refcount_block)
{
    BDRVQcowState *s = bs->opaque;
    unsigned int refcount_table_index;
    int ret;

    BLKDBG_EVENT(bs->file, BLKDBG_REFBLOCK_ALLOC);

    /* Find the refcount block for the given cluster */
    refcount_table_index = cluster_index >> (s->cluster_bits - REFCOUNT_SHIFT);

    if (refcount_table_index < s->refcount_table_size) {

        uint64_t refcount_block_offset =
            s->refcount_table[refcount_table_index] & REFT_OFFSET_MASK;

        /* If it's already there, we're done */
        if (refcount_block_offset) {
             return load_refcount_block(bs, refcount_block_offset,
                 (void**) refcount_block);
        }
    }

    /*
     * If we came here, we need to allocate something. Something is at least
     * a cluster for the new refcount block. It may also include a new refcount
     * table if the old refcount table is too small.
     *
     * Note that allocating clusters here needs some special care:
     *
     * - We can't use the normal qcow2_alloc_clusters(), it would try to
     *   increase the refcount and very likely we would end up with an endless
     *   recursion. Instead we must place the refcount blocks in a way that
     *   they can describe them themselves.
     *
     * - We need to consider that at this point we are inside update_refcounts
     *   and potentially doing an initial refcount increase. This means that
     *   some clusters have already been allocated by the caller, but their
     *   refcount isn't accurate yet. If we allocate clusters for metadata, we
     *   need to return -EAGAIN to signal the caller that it needs to restart
     *   the search for free clusters.
     *
     * - alloc_clusters_noref and qcow2_free_clusters may load a different
     *   refcount block into the cache
     */

    *refcount_block = NULL;

    /* We write to the refcount table, so we might depend on L2 tables */
    ret = qcow2_cache_flush(bs, s->l2_table_cache);
    if (ret < 0) {
        return ret;
    }

    /* Allocate the refcount block itself and mark it as used */
    int64_t new_block = alloc_clusters_noref(bs, s->cluster_size);
    if (new_block < 0) {
        return new_block;
    }

#ifdef DEBUG_ALLOC2
    fprintf(stderr, "qcow2: Allocate refcount block %d for %" PRIx64
        " at %" PRIx64 "\n",
        refcount_table_index, cluster_index << s->cluster_bits, new_block);
#endif

    if (in_same_refcount_block(s, new_block, cluster_index << s->cluster_bits)) {
        /* Zero the new refcount block before updating it */
        ret = qcow2_cache_get_empty(bs, s->refcount_block_cache, new_block,
            (void**) refcount_block);
        if (ret < 0) {
            goto fail_block;
        }

        memset(*refcount_block, 0, s->cluster_size);

        /* The block describes itself, need to update the cache */
        int block_index = (new_block >> s->cluster_bits) &
            ((1 << (s->cluster_bits - REFCOUNT_SHIFT)) - 1);
        (*refcount_block)[block_index] = cpu_to_be16(1);
    } else {
        /* Described somewhere else. This can recurse at most twice before we
         * arrive at a block that describes itself. */
        ret = update_refcount(bs, new_block, s->cluster_size, 1,
                              QCOW2_DISCARD_NEVER);
        if (ret < 0) {
            goto fail_block;
        }

        ret = qcow2_cache_flush(bs, s->refcount_block_cache);
        if (ret < 0) {
            goto fail_block;
        }

        /* Initialize the new refcount block only after updating its refcount,
         * update_refcount uses the refcount cache itself */
        ret = qcow2_cache_get_empty(bs, s->refcount_block_cache, new_block,
            (void**) refcount_block);
        if (ret < 0) {
            goto fail_block;
        }

        memset(*refcount_block, 0, s->cluster_size);
    }

    /* Now the new refcount block needs to be written to disk */
    BLKDBG_EVENT(bs->file, BLKDBG_REFBLOCK_ALLOC_WRITE);
    qcow2_cache_entry_mark_dirty(s->refcount_block_cache, *refcount_block);
    ret = qcow2_cache_flush(bs, s->refcount_block_cache);
    if (ret < 0) {
        goto fail_block;
    }

    /* If the refcount table is big enough, just hook the block up there */
    if (refcount_table_index < s->refcount_table_size) {
        uint64_t data64 = cpu_to_be64(new_block);
        BLKDBG_EVENT(bs->file, BLKDBG_REFBLOCK_ALLOC_HOOKUP);
        ret = bdrv_pwrite_sync(bs->file,
            s->refcount_table_offset + refcount_table_index * sizeof(uint64_t),
            &data64, sizeof(data64));
        if (ret < 0) {
            goto fail_block;
        }

        s->refcount_table[refcount_table_index] = new_block;

        /* The new refcount block may be where the caller intended to put its
         * data, so let it restart the search. */
        return -EAGAIN;
    }

    ret = qcow2_cache_put(bs, s->refcount_block_cache, (void**) refcount_block);
    if (ret < 0) {
        goto fail_block;
    }

    /*
     * If we come here, we need to grow the refcount table. Again, a new
     * refcount table needs some space and we can't simply allocate to avoid
     * endless recursion.
     *
     * Therefore let's grab new refcount blocks at the end of the image, which
     * will describe themselves and the new refcount table. This way we can
     * reference them only in the new table and do the switch to the new
     * refcount table at once without producing an inconsistent state in
     * between.
     */
    BLKDBG_EVENT(bs->file, BLKDBG_REFTABLE_GROW);

    /* Calculate the number of refcount blocks needed so far */
    uint64_t refcount_block_clusters = 1 << (s->cluster_bits - REFCOUNT_SHIFT);
    uint64_t blocks_used = DIV_ROUND_UP(cluster_index, refcount_block_clusters);

    /* And now we need at least one block more for the new metadata */
    uint64_t table_size = next_refcount_table_size(s, blocks_used + 1);
    uint64_t last_table_size;
    uint64_t blocks_clusters;
    do {
        uint64_t table_clusters =
            size_to_clusters(s, table_size * sizeof(uint64_t));
        blocks_clusters = 1 +
            ((table_clusters + refcount_block_clusters - 1)
            / refcount_block_clusters);
        uint64_t meta_clusters = table_clusters + blocks_clusters;

        last_table_size = table_size;
        table_size = next_refcount_table_size(s, blocks_used +
            ((meta_clusters + refcount_block_clusters - 1)
            / refcount_block_clusters));

    } while (last_table_size != table_size);

#ifdef DEBUG_ALLOC2
    fprintf(stderr, "qcow2: Grow refcount table %" PRId32 " => %" PRId64 "\n",
        s->refcount_table_size, table_size);
#endif

    /* Create the new refcount table and blocks */
    uint64_t meta_offset = (blocks_used * refcount_block_clusters) *
        s->cluster_size;
    uint64_t table_offset = meta_offset + blocks_clusters * s->cluster_size;
    uint16_t *new_blocks = g_malloc0(blocks_clusters * s->cluster_size);
    uint64_t *new_table = g_malloc0(table_size * sizeof(uint64_t));

    /* Fill the new refcount table */
    memcpy(new_table, s->refcount_table,
        s->refcount_table_size * sizeof(uint64_t));
    new_table[refcount_table_index] = new_block;

    int i;
    for (i = 0; i < blocks_clusters; i++) {
        new_table[blocks_used + i] = meta_offset + (i * s->cluster_size);
    }

    /* Fill the refcount blocks */
    uint64_t table_clusters = size_to_clusters(s, table_size * sizeof(uint64_t));
    int block = 0;
    for (i = 0; i < table_clusters + blocks_clusters; i++) {
        new_blocks[block++] = cpu_to_be16(1);
    }

    /* Write refcount blocks to disk */
    BLKDBG_EVENT(bs->file, BLKDBG_REFBLOCK_ALLOC_WRITE_BLOCKS);
    ret = bdrv_pwrite_sync(bs->file, meta_offset, new_blocks,
        blocks_clusters * s->cluster_size);
    g_free(new_blocks);
    if (ret < 0) {
        goto fail_table;
    }

    /* Write refcount table to disk */
    for(i = 0; i < table_size; i++) {
        cpu_to_be64s(&new_table[i]);
    }

    BLKDBG_EVENT(bs->file, BLKDBG_REFBLOCK_ALLOC_WRITE_TABLE);
    ret = bdrv_pwrite_sync(bs->file, table_offset, new_table,
        table_size * sizeof(uint64_t));
    if (ret < 0) {
        goto fail_table;
    }

    for(i = 0; i < table_size; i++) {
        be64_to_cpus(&new_table[i]);
    }

    /* Hook up the new refcount table in the qcow2 header */
    uint8_t data[12];
    cpu_to_be64w((uint64_t*)data, table_offset);
    cpu_to_be32w((uint32_t*)(data + 8), table_clusters);
    BLKDBG_EVENT(bs->file, BLKDBG_REFBLOCK_ALLOC_SWITCH_TABLE);
    ret = bdrv_pwrite_sync(bs->file, offsetof(QCowHeader, refcount_table_offset),
        data, sizeof(data));
    if (ret < 0) {
        goto fail_table;
    }

    /* And switch it in memory */
    uint64_t old_table_offset = s->refcount_table_offset;
    uint64_t old_table_size = s->refcount_table_size;

    g_free(s->refcount_table);
    s->refcount_table = new_table;
    s->refcount_table_size = table_size;
    s->refcount_table_offset = table_offset;

    /* Free old table. */
    qcow2_free_clusters(bs, old_table_offset, old_table_size * sizeof(uint64_t),
                        QCOW2_DISCARD_OTHER);

    ret = load_refcount_block(bs, new_block, (void**) refcount_block);
    if (ret < 0) {
        return ret;
    }

    /* If we were trying to do the initial refcount update for some cluster
     * allocation, we might have used the same clusters to store newly
     * allocated metadata. Make the caller search some new space. */
    return -EAGAIN;

fail_table:
    g_free(new_table);
fail_block:
    if (*refcount_block != NULL) {
        qcow2_cache_put(bs, s->refcount_block_cache, (void**) refcount_block);
    }
    return ret;
}

The product performs a calculation that can produce an integer overflow or wraparound when the logic assumes that the resulting value will always be larger than the original value. This occurs when an integer value is incremented to a value that is too large to store in the associated representation. When this occurs, the value may become a very small or negative number.