rt_uses_gateway was removed?

rt_uses_gateway was removed?

[Julian Anastasov]

	Hello,

	Looks like I'm a bit late with the storm of changes
in the routing.

	By default, after allocation rt_uses_gateway was set to 0.
Later it can be set to 1 if nh_gw is not the final route target,
i.e. it is indirect GW and not a target on LAN (the RT_SCOPE_LINK
check in rt_set_nexthop).

	What remains hidden for the rt_uses_gateway semantic
is this code in rt_set_nexthop:

	if (unlikely(!cached)) {
		/* Routes we intend to cache in nexthop exception or
		 * FIB nexthop have the DST_NOCACHE bit clear.
		 * However, if we are unsuccessful at storing this
		 * route into the cache we really need to set it.
		 */
		if (!rt->rt_gateway)
			rt->rt_gateway = daddr;
		rt_add_uncached_list(rt);
	}

and this code in rt_bind_exception:

	if (!rt->rt_gateway)
		rt->rt_gateway = daddr;

	I.e. even if rt_uses_gateway remains 0, rt_gateway
can contain address, i.e. the target is on LAN and not
behind nh_gw.

	Now I see commit 1550c171935d wrongly changes that to
"If rt_gw_family is set it implies rt_uses_gateway.".
As result, we set rt_gw_family while rt_uses_gateway was 0
for above cases. Think about it in this way: there should be
a reason why we used rt_uses_gateway flag instead a simple
"rt_gateway != 0" check, right?

	Replacing rt->rt_gateway checks with rt_gw_family
checks is right but rt_uses_gateway checks should be put
back because they indicates the route has more hops to
target.

	As the problem is related to some FNHW and non-cached
routes, redirects, etc the easiest way to see the problem is with
'ip route get LOCAL_IP oif eth0' where extra 'via GW' line is
shown.

Regards

--
Julian Anastasov <ja@ssi.bg>

Re: rt_uses_gateway was removed?

[David Ahern]

On 9/15/19 3:08 AM, Julian Anastasov wrote:
> 	Now I see commit 1550c171935d wrongly changes that to
> "If rt_gw_family is set it implies rt_uses_gateway.".
> As result, we set rt_gw_family while rt_uses_gateway was 0
> for above cases. Think about it in this way: there should be
> a reason why we used rt_uses_gateway flag instead a simple
> "rt_gateway != 0" check, right?
> 
> 	Replacing rt->rt_gateway checks with rt_gw_family
> checks is right but rt_uses_gateway checks should be put
> back because they indicates the route has more hops to
> target.
> 
> 	As the problem is related to some FNHW and non-cached
> routes, redirects, etc the easiest way to see the problem is with
> 'ip route get LOCAL_IP oif eth0' where extra 'via GW' line is
> shown.

Hi Julian:

Thanks for the detailed report. Yes, I misunderstood the subtle point of
rt_uses_gateway. I will look at a fix this week.

David