int64_t qcow2_alloc_bytes(BlockDriverState *bs, int size)
{
    BDRVQcowState *s = bs->opaque;
    int64_t offset, cluster_offset;
    int free_in_cluster;

    BLKDBG_EVENT(bs->file, BLKDBG_CLUSTER_ALLOC_BYTES);
    assert(size > 0 && size <= s->cluster_size);
    if (s->free_byte_offset == 0) {
        offset = qcow2_alloc_clusters(bs, s->cluster_size);
        if (offset < 0) {
            return offset;
        }
        s->free_byte_offset = offset;
    }
 redo:
    free_in_cluster = s->cluster_size -
        offset_into_cluster(s, s->free_byte_offset);
    if (size <= free_in_cluster) {
        /* enough space in current cluster */
        offset = s->free_byte_offset;
        s->free_byte_offset += size;
        free_in_cluster -= size;
        if (free_in_cluster == 0)
            s->free_byte_offset = 0;
        if (offset_into_cluster(s, offset) != 0)
            qcow2_update_cluster_refcount(bs, offset >> s->cluster_bits, 1,
                                          QCOW2_DISCARD_NEVER);
    } else {
        offset = qcow2_alloc_clusters(bs, s->cluster_size);
        if (offset < 0) {
            return offset;
        }
        cluster_offset = start_of_cluster(s, s->free_byte_offset);
        if ((cluster_offset + s->cluster_size) == offset) {
            /* we are lucky: contiguous data */
            offset = s->free_byte_offset;
            qcow2_update_cluster_refcount(bs, offset >> s->cluster_bits, 1,
                                          QCOW2_DISCARD_NEVER);
            s->free_byte_offset += size;
        } else {
            s->free_byte_offset = offset;
            goto redo;
        }
    }

    /* The cluster refcount was incremented, either by qcow2_alloc_clusters()
     * or explicitly by qcow2_update_cluster_refcount().  Refcount blocks must
     * be flushed before the caller's L2 table updates.
     */
    qcow2_cache_set_dependency(bs, s->l2_table_cache, s->refcount_block_cache);
    return offset;
}

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.