int qcow2_update_snapshot_refcount(BlockDriverState *bs,
    int64_t l1_table_offset, int l1_size, int addend)
{
    BDRVQcowState *s = bs->opaque;
    uint64_t *l1_table, *l2_table, l2_offset, offset, l1_size2, l1_allocated;
    int64_t old_offset, old_l2_offset;
    int i, j, l1_modified = 0, nb_csectors, refcount;
    int ret;

    l2_table = NULL;
    l1_table = NULL;
    l1_size2 = l1_size * sizeof(uint64_t);

    s->cache_discards = true;

    /* WARNING: qcow2_snapshot_goto relies on this function not using the
     * l1_table_offset when it is the current s->l1_table_offset! Be careful
     * when changing this! */
    if (l1_table_offset != s->l1_table_offset) {
        l1_table = g_malloc0(align_offset(l1_size2, 512));
        l1_allocated = 1;

        ret = bdrv_pread(bs->file, l1_table_offset, l1_table, l1_size2);
        if (ret < 0) {
            goto fail;
        }

        for(i = 0;i < l1_size; i++)
            be64_to_cpus(&l1_table[i]);
    } else {
        assert(l1_size == s->l1_size);
        l1_table = s->l1_table;
        l1_allocated = 0;
    }

    for(i = 0; i < l1_size; i++) {
        l2_offset = l1_table[i];
        if (l2_offset) {
            old_l2_offset = l2_offset;
            l2_offset &= L1E_OFFSET_MASK;

            ret = qcow2_cache_get(bs, s->l2_table_cache, l2_offset,
                (void**) &l2_table);
            if (ret < 0) {
                goto fail;
            }

            for(j = 0; j < s->l2_size; j++) {
                uint64_t cluster_index;

                offset = be64_to_cpu(l2_table[j]);
                old_offset = offset;
                offset &= ~QCOW_OFLAG_COPIED;

                switch (qcow2_get_cluster_type(offset)) {
                    case QCOW2_CLUSTER_COMPRESSED:
                        nb_csectors = ((offset >> s->csize_shift) &
                                       s->csize_mask) + 1;
                        if (addend != 0) {
                            ret = update_refcount(bs,
                                (offset & s->cluster_offset_mask) & ~511,
                                nb_csectors * 512, addend,
                                QCOW2_DISCARD_SNAPSHOT);
                            if (ret < 0) {
                                goto fail;
                            }
                        }
                        /* compressed clusters are never modified */
                        refcount = 2;
                        break;

                    case QCOW2_CLUSTER_NORMAL:
                    case QCOW2_CLUSTER_ZERO:
                        cluster_index = (offset & L2E_OFFSET_MASK) >> s->cluster_bits;
                        if (!cluster_index) {
                            /* unallocated */
                            refcount = 0;
                            break;
                        }
                        if (addend != 0) {
                            refcount = qcow2_update_cluster_refcount(bs,
                                    cluster_index, addend,
                                    QCOW2_DISCARD_SNAPSHOT);
                        } else {
                            refcount = get_refcount(bs, cluster_index);
                        }

                        if (refcount < 0) {
                            ret = refcount;
                            goto fail;
                        }
                        break;

                    case QCOW2_CLUSTER_UNALLOCATED:
                        refcount = 0;
                        break;

                    default:
                        abort();
                }

                if (refcount == 1) {
                    offset |= QCOW_OFLAG_COPIED;
                }
                if (offset != old_offset) {
                    if (addend > 0) {
                        qcow2_cache_set_dependency(bs, s->l2_table_cache,
                            s->refcount_block_cache);
                    }
                    l2_table[j] = cpu_to_be64(offset);
                    qcow2_cache_entry_mark_dirty(s->l2_table_cache, l2_table);
                }
            }

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


            if (addend != 0) {
                refcount = qcow2_update_cluster_refcount(bs, l2_offset >>
                        s->cluster_bits, addend, QCOW2_DISCARD_SNAPSHOT);
            } else {
                refcount = get_refcount(bs, l2_offset >> s->cluster_bits);
            }
            if (refcount < 0) {
                ret = refcount;
                goto fail;
            } else if (refcount == 1) {
                l2_offset |= QCOW_OFLAG_COPIED;
            }
            if (l2_offset != old_l2_offset) {
                l1_table[i] = l2_offset;
                l1_modified = 1;
            }
        }
    }

    ret = bdrv_flush(bs);
fail:
    if (l2_table) {
        qcow2_cache_put(bs, s->l2_table_cache, (void**) &l2_table);
    }

    s->cache_discards = false;
    qcow2_process_discards(bs, ret);

    /* Update L1 only if it isn't deleted anyway (addend = -1) */
    if (ret == 0 && addend >= 0 && l1_modified) {
        for (i = 0; i < l1_size; i++) {
            cpu_to_be64s(&l1_table[i]);
        }

        ret = bdrv_pwrite_sync(bs->file, l1_table_offset, l1_table, l1_size2);

        for (i = 0; i < l1_size; i++) {
            be64_to_cpus(&l1_table[i]);
        }
    }
    if (l1_allocated)
        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.