On Wed, Oct 09, 2019 at 11:00:07AM -0500, Jesse Hathaway wrote:
> We have been experiencing a route lookup race condition on our internet facing
> Linux routers. I have been able to reproduce the issue, but would love more
> help in isolating the cause.
>
> Looking up a route found in the main table returns `*` rather than the
> directly
> connected interface about once for every 10-20 million requests. From my
> reading of the iproute2 source code an asterisk is indicative of the kernel
> returning and interface index of 0 rather than the correct directly connected
> interface.
>
> This is reproducible with the following bash snippet on 5.4-rc2:
>
> $ cat route-race
> #!/bin/bash
>
> # Generate 50 million individual route gets to feed as batch input to `ip`
> function ip-cmds() {
> route_get='route get 192.168.11.142 from 192.168.180.10 iif vlan180'
> for ((i = 0; i < 50000000; i++)); do
> printf '%s\n' "${route_get}"
> done
>
> }
>
> ip-cmds | ip -d -o -batch - | grep -E 'dev \*' | uniq -c
>
> Example output:
>
> $ ./route-race
> 6 unicast 192.168.11.142 from 192.168.180.10 dev * table main
> \ cache iif vlan180
>
> These routers have multiple routing tables and are ingesting full BGP routing
> tables from multiple ISPs:
>
> $ ip route show table all | wc -l
> 3105543
>
> $ ip route show table main | wc -l
> 54
>
> Please let me know what other information I can provide, thanks in advance,
I think it's working as expected. Here is my theory:
If CPU0 is executing both the route get request and forwarding packets
through the directly connected interface, then the following can happen:
<CPU0, t0> - In process context, per-CPU dst entry cached in the nexthop
is found. Not yet dumped to user space
<Any CPU, t1> - Routes are added / removed, therefore invalidating the
cache by bumping 'net->ipv4.rt_genid'
<CPU0, t2> - In softirq, packet is forwarded through the nexthop. The
cached dst entry is found to be invalid. Therefore, it is replaced by a
newer dst entry. dst_dev_put() is called on old entry which assigns the
blackhole netdev to 'dst->dev'. This netdev has an ifindex of 0 because
it is not registered.
<CPU0, t3> - After softirq finished executing, your route get request
from t0 is resumed and the old dst entry is dumped to user space with
ifindex of 0.
I tested this on my system using your script to generate the route get
requests. I pinned it to the same CPU forwarding packets through the
nexthop. To constantly invalidate the cache I created another script
that simply adds and removes IP addresses from an interface.
If I stop the packet forwarding or the script that invalidates the
cache, then I don't see any '*' answers to my route get requests.
BTW, the blackhole netdev was added in 5.3. I assume (didn't test) that
with older kernel versions you'll see 'lo' instead of '*'.
