static int handle_alloc(BlockDriverState *bs, uint64_t guest_offset,
    uint64_t *host_offset, uint64_t *bytes, QCowL2Meta **m)
{
    BDRVQcowState *s = bs->opaque;
    int l2_index;
    uint64_t *l2_table;
    uint64_t entry;
    unsigned int nb_clusters;
    int ret;

    uint64_t alloc_cluster_offset;

    trace_qcow2_handle_alloc(qemu_coroutine_self(), guest_offset, *host_offset,
                             *bytes);
    assert(*bytes > 0);

    /*
     * Calculate the number of clusters to look for. We stop at L2 table
     * boundaries to keep things simple.
     */
    nb_clusters =
        size_to_clusters(s, offset_into_cluster(s, guest_offset) + *bytes);

    l2_index = offset_to_l2_index(s, guest_offset);
    nb_clusters = MIN(nb_clusters, s->l2_size - l2_index);

    /* Find L2 entry for the first involved cluster */
    ret = get_cluster_table(bs, guest_offset, &l2_table, &l2_index);
    if (ret < 0) {
        return ret;
    }

    entry = be64_to_cpu(l2_table[l2_index]);

    /* For the moment, overwrite compressed clusters one by one */
    if (entry & QCOW_OFLAG_COMPRESSED) {
        nb_clusters = 1;
    } else {
        nb_clusters = count_cow_clusters(s, nb_clusters, l2_table, l2_index);
    }

    /* This function is only called when there were no non-COW clusters, so if
     * we can't find any unallocated or COW clusters either, something is
     * wrong with our code. */
    assert(nb_clusters > 0);

    ret = qcow2_cache_put(bs, s->l2_table_cache, (void**) &l2_table);
    if (ret < 0) {
        return ret;
    }

    /* Allocate, if necessary at a given offset in the image file */
    alloc_cluster_offset = start_of_cluster(s, *host_offset);
    ret = do_alloc_cluster_offset(bs, guest_offset, &alloc_cluster_offset,
                                  &nb_clusters);
    if (ret < 0) {
        goto fail;
    }

    /* Can't extend contiguous allocation */
    if (nb_clusters == 0) {
        *bytes = 0;
        return 0;
    }

    /*
     * Save info needed for meta data update.
     *
     * requested_sectors: Number of sectors from the start of the first
     * newly allocated cluster to the end of the (possibly shortened
     * before) write request.
     *
     * avail_sectors: Number of sectors from the start of the first
     * newly allocated to the end of the last newly allocated cluster.
     *
     * nb_sectors: The number of sectors from the start of the first
     * newly allocated cluster to the end of the area that the write
     * request actually writes to (excluding COW at the end)
     */
    int requested_sectors =
        (*bytes + offset_into_cluster(s, guest_offset))
        >> BDRV_SECTOR_BITS;
    int avail_sectors = nb_clusters
                        << (s->cluster_bits - BDRV_SECTOR_BITS);
    int alloc_n_start = offset_into_cluster(s, guest_offset)
                        >> BDRV_SECTOR_BITS;
    int nb_sectors = MIN(requested_sectors, avail_sectors);
    QCowL2Meta *old_m = *m;

    *m = g_malloc0(sizeof(**m));

    **m = (QCowL2Meta) {
        .next           = old_m,

        .alloc_offset   = alloc_cluster_offset,
        .offset         = start_of_cluster(s, guest_offset),
        .nb_clusters    = nb_clusters,
        .nb_available   = nb_sectors,

        .cow_start = {
            .offset     = 0,
            .nb_sectors = alloc_n_start,
        },
        .cow_end = {
            .offset     = nb_sectors * BDRV_SECTOR_SIZE,
            .nb_sectors = avail_sectors - nb_sectors,
        },
    };
    qemu_co_queue_init(&(*m)->dependent_requests);
    QLIST_INSERT_HEAD(&s->cluster_allocs, *m, next_in_flight);

    *host_offset = alloc_cluster_offset + offset_into_cluster(s, guest_offset);
    *bytes = MIN(*bytes, (nb_sectors * BDRV_SECTOR_SIZE)
                         - offset_into_cluster(s, guest_offset));
    assert(*bytes != 0);

    return 1;

fail:
    if (*m && (*m)->nb_clusters > 0) {
        QLIST_REMOVE(*m, next_in_flight);
    }
    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.