static void inc_refcounts(BlockDriverState *bs,
                          BdrvCheckResult *res,
                          uint16_t *refcount_table,
                          int refcount_table_size,
                          int64_t offset, int64_t size)
{
    BDRVQcowState *s = bs->opaque;
    int64_t start, last, cluster_offset;
    int k;

    if (size <= 0)
        return;

    start = start_of_cluster(s, offset);
    last = start_of_cluster(s, offset + size - 1);
    for(cluster_offset = start; cluster_offset <= last;
        cluster_offset += s->cluster_size) {
        k = cluster_offset >> s->cluster_bits;
        if (k < 0) {
            fprintf(stderr, "ERROR: invalid cluster offset=0x%" PRIx64 "\n",
                cluster_offset);
            res->corruptions++;
        } else if (k >= refcount_table_size) {
            fprintf(stderr, "Warning: cluster offset=0x%" PRIx64 " is after "
                "the end of the image file, can't properly check refcounts.\n",
                cluster_offset);
            res->check_errors++;
        } else {
            if (++refcount_table[k] == 0) {
                fprintf(stderr, "ERROR: overflow cluster offset=0x%" PRIx64
                    "\n", cluster_offset);
                res->corruptions++;
            }
        }
    }
}

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.