netdev - Re: [PATCH net 3/3] selftests: net: fib_nexthops: add test for group refcount imbalance bug

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Date:   Sun, 21 Nov 2021 19:53:19 +0200
From:   Ido Schimmel <idosch@...sch.org>
To:     Nikolay Aleksandrov <razor@...ckwall.org>
Cc:     netdev@...r.kernel.org, davem@...emloft.net, kuba@...nel.org,
        dsahern@...il.com, Nikolay Aleksandrov <nikolay@...dia.com>
Subject: Re: [PATCH net 3/3] selftests: net: fib_nexthops: add test for group
 refcount imbalance bug

On Sun, Nov 21, 2021 at 05:24:53PM +0200, Nikolay Aleksandrov wrote:
> From: Nikolay Aleksandrov <nikolay@...dia.com>
> 
> The new selftest runs a sequence which causes circular refcount
> dependency between deleted objects which cannot be released and results
> in a netdevice refcount imbalance.
> 
> Signed-off-by: Nikolay Aleksandrov <nikolay@...dia.com>
> ---
>  tools/testing/selftests/net/fib_nexthops.sh | 56 +++++++++++++++++++++
>  1 file changed, 56 insertions(+)
> 
> diff --git a/tools/testing/selftests/net/fib_nexthops.sh b/tools/testing/selftests/net/fib_nexthops.sh
> index b5a69ad191b0..48d88a36ae27 100755
> --- a/tools/testing/selftests/net/fib_nexthops.sh
> +++ b/tools/testing/selftests/net/fib_nexthops.sh
> @@ -629,6 +629,59 @@ ipv6_fcnal()
>  	log_test $? 0 "Nexthops removed on admin down"
>  }
>  
> +ipv6_grp_refs()
> +{
> +	run_cmd "$IP link set dev veth1 up"
> +	run_cmd "$IP link add veth1.10 link veth1 up type vlan id 10"
> +	run_cmd "$IP link add veth1.20 link veth1 up type vlan id 20"
> +	run_cmd "$IP -6 addr add 2001:db8:91::1/64 dev veth1.10"
> +	run_cmd "$IP -6 addr add 2001:db8:92::1/64 dev veth1.20"
> +	run_cmd "$IP -6 neigh add 2001:db8:91::2 lladdr 00:11:22:33:44:55 dev veth1.10"
> +	run_cmd "$IP -6 neigh add 2001:db8:92::2 lladdr 00:11:22:33:44:55 dev veth1.20"
> +	run_cmd "$IP nexthop add id 100 via 2001:db8:91::2 dev veth1.10"
> +	run_cmd "$IP nexthop add id 101 via 2001:db8:92::2 dev veth1.20"
> +	run_cmd "$IP nexthop add id 102 group 100"
> +	run_cmd "$IP route add 2001:db8:101::1/128 nhid 102"
> +
> +	# create per-cpu dsts through nh 100
> +	run_cmd "ip netns exec me mausezahn -6 veth1.10 -B 2001:db8:101::1 -A 2001:db8:91::1 -c 5 -t tcp "dp=1-1023, flags=syn" >/dev/null 2>&1"

I see that other test cases in this file that are using mausezahn check
that it exists. See ipv4_torture() for example

> +
> +	# remove nh 100 from the group to delete the route potentially leaving
> +	# a stale per-cpu dst

Not sure I understand the comment. Maybe:

"Remove nh 100 from the group. If the bug described in the previous
commit is not fixed, the nexthop continues to cache a per-CPU dst entry
that holds a reference on the IPv6 route."

?

> +	run_cmd "$IP nexthop replace id 102 group 101"
> +	run_cmd "$IP route del 2001:db8:101::1/128"
> +
> +	# add both nexthops to the group so a reference is taken on them
> +	run_cmd "$IP nexthop replace id 102 group 100/101"
> +
> +	# if the bug exists at this point we have an unlinked IPv6 route

I would mention that by "the bug" you are referring to the bug described
in previous commit

> +	# (but not freed due to stale dst) with a reference over the group
> +	# so we delete the group which will again only unlink it due to the
> +	# route reference
> +	run_cmd "$IP nexthop del id 102"
> +
> +	# delete the nexthop with stale dst, since we have an unlinked
> +	# group with a ref to it and an unlinked IPv6 route with ref to the
> +	# group, the nh will only be unlinked and not freed so the stale dst
> +	# remains forever and we get a net device refcount imbalance
> +	run_cmd "$IP nexthop del id 100"
> +
> +	# if the bug exists this command will hang because the net device
> +	# cannot be removed
> +	timeout -s KILL 5 ip netns exec me ip link del veth1.10 >/dev/null 2>&1
> +
> +	# we can't cleanup if the command is hung trying to delete the netdev
> +	if [ $? -eq 137 ]; then
> +		return 1
> +	fi
> +
> +	# cleanup
> +	run_cmd "$IP link del veth1.20"
> +	run_cmd "$IP nexthop flush"
> +
> +	return 0
> +}
> +
>  ipv6_grp_fcnal()
>  {
>  	local rc
> @@ -734,6 +787,9 @@ ipv6_grp_fcnal()
>  
>  	run_cmd "$IP nexthop add id 108 group 31/24"
>  	log_test $? 2 "Nexthop group can not have a blackhole and another nexthop"
> +
> +	ipv6_grp_refs
> +	log_test $? 0 "Nexthop group replace refcounts"
>  }
>  
>  ipv6_res_grp_fcnal()
> -- 
> 2.31.1
>