int qcow2_expand_zero_clusters(BlockDriverState *bs)
{
    BDRVQcowState *s = bs->opaque;
    uint64_t *l1_table = NULL;
    uint64_t nb_clusters;
    uint8_t *expanded_clusters;
    int ret;
    int i, j;

    nb_clusters = size_to_clusters(s, bs->file->total_sectors *
                                   BDRV_SECTOR_SIZE);
    expanded_clusters = g_malloc0((nb_clusters + 7) / 8);

    ret = expand_zero_clusters_in_l1(bs, s->l1_table, s->l1_size,
                                     &expanded_clusters, &nb_clusters);
    if (ret < 0) {
        goto fail;
    }

    /* Inactive L1 tables may point to active L2 tables - therefore it is
     * necessary to flush the L2 table cache before trying to access the L2
     * tables pointed to by inactive L1 entries (else we might try to expand
     * zero clusters that have already been expanded); furthermore, it is also
     * necessary to empty the L2 table cache, since it may contain tables which
     * are now going to be modified directly on disk, bypassing the cache.
     * qcow2_cache_empty() does both for us. */
    ret = qcow2_cache_empty(bs, s->l2_table_cache);
    if (ret < 0) {
        goto fail;
    }

    for (i = 0; i < s->nb_snapshots; i++) {
        int l1_sectors = (s->snapshots[i].l1_size * sizeof(uint64_t) +
                BDRV_SECTOR_SIZE - 1) / BDRV_SECTOR_SIZE;

        l1_table = g_realloc(l1_table, l1_sectors * BDRV_SECTOR_SIZE);

        ret = bdrv_read(bs->file, s->snapshots[i].l1_table_offset /
                BDRV_SECTOR_SIZE, (void *)l1_table, l1_sectors);
        if (ret < 0) {
            goto fail;
        }

        for (j = 0; j < s->snapshots[i].l1_size; j++) {
            be64_to_cpus(&l1_table[j]);
        }

        ret = expand_zero_clusters_in_l1(bs, l1_table, s->snapshots[i].l1_size,
                                         &expanded_clusters, &nb_clusters);
        if (ret < 0) {
            goto fail;
        }
    }

    ret = 0;

fail:
    g_free(expanded_clusters);
    g_free(l1_table);
    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.