Subject: Re: [PATCH RFC] ipv6: fix route selection if kernel is not compiled with CONFIG_IPV6_ROUTER_PREF

From: Nicolas Dichtel <[email-masked]>
Date: Wed, 10 Jul 2013 09:54:58 +0200

Le 09/07/2013 23:57, Hannes Frederic Sowa a écrit :
> Hello Nicolas!
>
> I am currently trying to fix the nexthop selection for kernels compiled
> without support for CONFIG_IPV6_ROUTER_PREF. I attached my current patch
> below. While testing I got the following kernel panic in ecmp code:
>
> [   80.144667] ------------[ cut here ]------------
> [   80.145172] kernel BUG at net/ipv6/ip6_fib.c:733!
> [   80.145172] invalid opcode: 0000 [#1] SMP
> [   80.145172] Modules linked in: 8021q nf_conntrack_netbios_ns nf_conntrack_broadcast ipt_MASQUERADE ip6table_mangle ip6t_REJECT nf_conntrack_ipv6 nf_defrag_ipv6 iptable_nat nf_nat_ipv4 nf_nat iptable_mangle nf_conntrack_ipv4 nf_defrag_ipv4 xt_conntrack nf_conntrack ebtable_filter ebtables ip6table_filter ip6_tables snd_hda_intel snd_hda_codec snd_hwdep snd_seq snd_seq_device snd_pcm snd_page_alloc snd_timer virtio_balloon snd soundcore i2c_piix4 i2c_core virtio_net virtio_blk
> [   80.145172] CPU: 1 PID: 786 Comm: ping6 Not tainted 3.10.0+ #118
> [   80.145172] Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011
> [   80.145172] task: ffff880117fa0000 ti: ffff880118770000 task.ti: ffff880118770000
> [   80.145172] RIP: 0010:[<ffffffff815f3b5d>]  [<ffffffff815f3b5d>] fib6_add+0x75d/0x830
> [   80.145172] RSP: 0018:ffff880118771798  EFLAGS: 00010202
> [   80.145172] RAX: 0000000000000000 RBX: 0000000000000000 RCX: ffff88011350e480
> [   80.145172] RDX: ffff88011350e238 RSI: 0000000000000004 RDI: ffff88011350f738
> [   80.145172] RBP: ffff880118771848 R08: ffff880117903280 R09: 0000000000000001
> [   80.145172] R10: 0000000000000000 R11: 0000000000000000 R12: ffff88011350f680
> [   80.145172] R13: ffff880117903280 R14: ffff880118771890 R15: ffff88011350ef90
> [   80.145172] FS:  00007f02b5127740(0000) GS:ffff88011fd00000(0000) knlGS:0000000000000000
> [   80.145172] CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
> [   80.145172] CR2: 00007f981322a000 CR3: 00000001181b1000 CR4: 00000000000006e0
> [   80.145172] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> [   80.145172] DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
> [   80.145172] Stack:
> [   80.145172]  0000000000000001 ffff880100000000 ffff880100000000 ffff880117903280
> [   80.145172]  0000000000000000 ffff880119a4cf00 0000000000000400 00000000000007fa
> [   80.145172]  0000000000000000 0000000000000000 0000000000000000 ffff88011350f680
> [   80.145172] Call Trace:
> [   80.145172]  [<ffffffff815eeceb>] ? rt6_bind_peer+0x4b/0x90
> [   80.145172]  [<ffffffff815ed985>] __ip6_ins_rt+0x45/0x70
> [   80.145172]  [<ffffffff815eee35>] ip6_ins_rt+0x35/0x40
> [   80.145172]  [<ffffffff815ef1e4>] ip6_pol_route.isra.44+0x3a4/0x4b0
> [   80.145172]  [<ffffffff815ef34a>] ip6_pol_route_output+0x2a/0x30
> [   80.145172]  [<ffffffff81616077>] fib6_rule_action+0xd7/0x210
> [   80.145172]  [<ffffffff815ef320>] ? ip6_pol_route_input+0x30/0x30
> [   80.145172]  [<ffffffff81553026>] fib_rules_lookup+0xc6/0x140
> [   80.145172]  [<ffffffff81616374>] fib6_rule_lookup+0x44/0x80
> [   80.145172]  [<ffffffff815ef320>] ? ip6_pol_route_input+0x30/0x30
> [   80.145172]  [<ffffffff815edea3>] ip6_route_output+0x73/0xb0
> [   80.145172]  [<ffffffff815dfdf3>] ip6_dst_lookup_tail+0x2c3/0x2e0
> [   80.145172]  [<ffffffff813007b1>] ? list_del+0x11/0x40
> [   80.145172]  [<ffffffff81082a4c>] ? remove_wait_queue+0x3c/0x50
> [   80.145172]  [<ffffffff815dfe4d>] ip6_dst_lookup_flow+0x3d/0xa0
> [   80.145172]  [<ffffffff815fda77>] rawv6_sendmsg+0x267/0xc20
> [   80.145172]  [<ffffffff815a8a83>] inet_sendmsg+0x63/0xb0
> [   80.145172]  [<ffffffff8128eb93>] ? selinux_socket_sendmsg+0x23/0x30
> [   80.145172]  [<ffffffff815218d6>] sock_sendmsg+0xa6/0xd0
> [   80.145172]  [<ffffffff81524a68>] SYSC_sendto+0x128/0x180
> [   80.145172]  [<ffffffff8109825c>] ? update_curr+0xec/0x170
> [   80.145172]  [<ffffffff81041d09>] ? kvm_clock_get_cycles+0x9/0x10
> [   80.145172]  [<ffffffff810afd1e>] ? __getnstimeofday+0x3e/0xd0
> [   80.145172]  [<ffffffff8152509e>] SyS_sendto+0xe/0x10
> [   80.145172]  [<ffffffff8164efd9>] system_call_fastpath+0x16/0x1b
> [   80.145172] Code: fe ff ff 41 f6 45 2a 06 0f 85 ca fe ff ff 49 8b 7e 08 4c 89 ee e8 94 ef ff ff e9 b9 fe ff ff 48 8b 82 28 05 00 00 e9 01 ff ff ff <0f> 0b 49 8b 54 24 30 0d 00 00 40 00 89 83 14 01 00 00 48 89 53
> [   80.145172] RIP  [<ffffffff815f3b5d>] fib6_add+0x75d/0x830
> [   80.145172]  RSP <ffff880118771798>
> [   80.387413] ---[ end trace 02f20b7a8b81ed95 ]---
> [   80.390154] Kernel panic - not syncing: Fatal exception in interrupt
>
> The relevant code is:
>
>      725                 /* For each sibling in the list, increment the counter of
>      726                  * siblings. BUG() if counters does not match, list of siblings
>      727                  * is broken!
>      728                  */
>      729                 rt6i_nsiblings = 0;
>      730                 list_for_each_entry_safe(sibling, temp_sibling,
>      731                                          &rt->rt6i_siblings, rt6i_siblings) {
>      732                         sibling->rt6i_nsiblings++;
>      733                         BUG_ON(sibling->rt6i_nsiblings != rt->rt6i_nsiblings);
>      734                         rt6i_nsiblings++;
>      735                 }
>      736                 BUG_ON(rt6i_nsiblings != rt->rt6i_nsiblings);
>      737         }
>
> Currently, I don't know if I my patch is to blame or something in the ecmp logic.
>
> Another thing I just noticed is:
>
>     1240         /* Remove this entry from other siblings */
>     1241         if (rt->rt6i_nsiblings) {
>     1242                 struct rt6_info *sibling, *next_sibling;
>     1243
>     1244                 list_for_each_entry_safe(sibling, next_sibling,
>     1245                                          &rt->rt6i_siblings, rt6i_siblings)
>     1246                         sibling->rt6i_nsiblings--;
>     1247                 rt->rt6i_nsiblings = 0;
>     1248                 list_del_init(&rt->rt6i_siblings);
>     1249         }
>
> Are we sure we decrement all sibling's rt6i_nsiblings? Shouldn't we
> start iterating from fn->leaf? But this does not seem to cause it,
> because my trace does not report any calls to fib6_del_route.
Note sure to follow you, but all siblings are listed in rt6i_siblings, so it 
must be enough.

>
> You could try reproduce it by having an interface autoconfigured with
> a default router with NUD_VALID neighbour. I then added an unused vlan
> interface (vid 100 in my case) and added the following ip addresses:
>
> ip -6 a a 2001:ffff::1/64 dev eth0.100
> ip -6 r a 2000::/3 nexthop via 2001:ffff::30 nexthop via 2001:ffff::31 nexthop via 2001:ffff::32 nexthop via 2001:ffff::33
>
> (all nexthops should not be reachable)
>
> After starting a ping6 2000::1 the box should panic soon, after the
> first nexthop entry times out.
>
> Perhaps you could give me a hint?
I will run some tests with your patch. Will see.

I assume you didn't reproduce this without your patch.

Regards,
Nicolas

>
> Thanks a lot,
>
>    Hannes
>
>
> [PATCH RFC] ipv6: fix route selection if kernel not compiled with CONFIG_IPV6_ROUTER_PREF
>
> This is a follow-up patch to 3630d40067a21d4dfbadc6002bb469ce26ac5d52
> ("ipv6: rt6_check_neigh should successfully verify neigh if no NUD
> information are available").
>
> Since the removal of rt->n in rt6_info we can end up with a dst ==
> NULL in rt6_check_neigh. In case the kernel is not compiled with
> CONFIG_IPV6_ROUTER_PREF we should also select a route with unkown
> NUD state but we must not avoid doing round robin selection on routes
> with the same target. So introduce and pass down a boolean ``do_rr'' to
> indicate when we should update rt->rr_ptr. As soon as no route is valid
> we do backtracking and do a lookup on a higher level in the fib trie.
>
> To hold correct state on the NUD selection we need to create a neighbour
> entry as soon as we tried to validate a nexthop.
>
> I changed the return value of rt6_check_neigh to:
>     1 in case of the dst entry validated
>    -1 in case of we had no dst_entry and we need to do rr now
>    -2 in case a we had a dst_entry and it did not validate
>
> In case of CONFIG_IPV6_ROUTER_PREF, rt6_probe does not allocate an
> neighbour entry (!CONFIG_IPV6_ROUTER_PREF: rt6_probe is a nop). Because
> of this, we have to create a neighbour entry on nexthop validation to
> track earlier validation errors. We recheck NUD state here to shortcurcuit
> NUD_NOARP neighbours.
>
> This seems to be the least complex fix for stable and net. I'll introduce
> a new route lookup flag 'idempotent' as soon as next opens to not let
> ip route get trigger active NUD validation if CONFIG_IPV6_ROUTER_PREF
> is enabled. Currently we trigger active NUD validation if compiled with
> CONFIG_IPV6_ROUTER_PREF.
>
> It also seems advantageous to make CONFIG_IPV6_ROUTER_PREF a runtime
> switch and just select the default operation on compile-time.
>
> v2:
> a) improved rt6_check_neigh logic and documented return values
>
> Reported-by: Pierre Emeriaud <[email-masked]>
> Cc: YOSHIFUJI Hideaki <[email-masked]>
> ---
>   net/ipv6/route.c | 62 +++++++++++++++++++++++++++++++++++---------------------
>   1 file changed, 39 insertions(+), 23 deletions(-)
>
> diff --git a/net/ipv6/route.c b/net/ipv6/route.c
> index bd5fd70..c5d9e68 100644
> --- a/net/ipv6/route.c
> +++ b/net/ipv6/route.c
> @@ -531,28 +531,34 @@ static inline int rt6_check_dev(struct rt6_info *rt, int oif)
>   	return 0;
>   }
>
> -static inline bool rt6_check_neigh(struct rt6_info *rt)
> +/* This function checks if a neighbour is reachable for routing
> + * purposes. It returns -2 in case the neighbour should not get
> + * selected as a viable router, -1 in case it should get selected with
> + * lowest score and afterwards trying roundrobin. 1 indicates a
> + * successfull verification.
> + */
> +static inline int rt6_check_neigh(struct rt6_info *rt)
>   {
>   	struct neighbour *neigh;
> -	bool ret = false;
> +	int ret = -2;
>
>   	if (rt->rt6i_flags & RTF_NONEXTHOP ||
>   	    !(rt->rt6i_flags & RTF_GATEWAY))
> -		return true;
> +		return 1;
>
>   	rcu_read_lock_bh();
>   	neigh = __ipv6_neigh_lookup_noref(rt->dst.dev, &rt->rt6i_gateway);
>   	if (neigh) {
>   		read_lock(&neigh->lock);
>   		if (neigh->nud_state & NUD_VALID)
> -			ret = true;
> +			ret = 1;
>   #ifdef CONFIG_IPV6_ROUTER_PREF
>   		else if (!(neigh->nud_state & NUD_FAILED))
> -			ret = true;
> +			ret = 1;
>   #endif
>   		read_unlock(&neigh->lock);
> -	} else if (IS_ENABLED(CONFIG_IPV6_ROUTER_PREF)) {
> -		ret = true;
> +	} else {
> +		ret = IS_ENABLED(CONFIG_IPV6_ROUTER_PREF) ? 1 : -1;
>   	}
>   	rcu_read_unlock_bh();
>
> @@ -566,43 +572,52 @@ static int rt6_score_route(struct rt6_info *rt, int oif,
>
>   	m = rt6_check_dev(rt, oif);
>   	if (!m && (strict & RT6_LOOKUP_F_IFACE))
> -		return -1;
> +		return -2;
>   #ifdef CONFIG_IPV6_ROUTER_PREF
>   	m |= IPV6_DECODE_PREF(IPV6_EXTRACT_PREF(rt->rt6i_flags)) << 2;
>   #endif
> -	if (!rt6_check_neigh(rt) && (strict & RT6_LOOKUP_F_REACHABLE))
> -		return -1;
> +	if (strict & RT6_LOOKUP_F_REACHABLE) {
> +		int n = rt6_check_neigh(rt);
> +		if (n < 0)
> +			return n;
> +	}
>   	return m;
>   }
>
>   static struct rt6_info *find_match(struct rt6_info *rt, int oif, int strict,
> -				   int *mpri, struct rt6_info *match)
> +				   int *mpri, struct rt6_info *match,
> +				   bool *do_rr)
>   {
>   	int m;
> +	bool match_do_rr = false;
>
>   	if (rt6_check_expired(rt))
>   		goto out;
>
>   	m = rt6_score_route(rt, oif, strict);
> -	if (m < 0)
> +	if (m == -1 && !IS_ENABLED(CONFIG_IPV6_ROUTER_PREF)) {
> +		match_do_rr = true;
> +		m = 0; /* lowest valid score */
> +	} else if (m < 0) {
>   		goto out;
> +	}
> +
> +	if (strict & RT6_LOOKUP_F_REACHABLE)
> +		rt6_probe(rt);
>
>   	if (m > *mpri) {
> -		if (strict & RT6_LOOKUP_F_REACHABLE)
> -			rt6_probe(match);
> +		*do_rr = match_do_rr;
>   		*mpri = m;
>   		match = rt;
> -	} else if (strict & RT6_LOOKUP_F_REACHABLE) {
> -		rt6_probe(rt);
>   	}
> -
>   out:
>   	return match;
>   }
>
>   static struct rt6_info *find_rr_leaf(struct fib6_node *fn,
>   				     struct rt6_info *rr_head,
> -				     u32 metric, int oif, int strict)
> +				     u32 metric, int oif, int strict,
> +				     bool *do_rr)
>   {
>   	struct rt6_info *rt, *match;
>   	int mpri = -1;
> @@ -610,10 +625,10 @@ static struct rt6_info *find_rr_leaf(struct fib6_node *fn,
>   	match = NULL;
>   	for (rt = rr_head; rt && rt->rt6i_metric == metric;
>   	     rt = rt->dst.rt6_next)
> -		match = find_match(rt, oif, strict, &mpri, match);
> +		match = find_match(rt, oif, strict, &mpri, match, do_rr);
>   	for (rt = fn->leaf; rt && rt != rr_head && rt->rt6i_metric == metric;
>   	     rt = rt->dst.rt6_next)
> -		match = find_match(rt, oif, strict, &mpri, match);
> +		match = find_match(rt, oif, strict, &mpri, match, do_rr);
>
>   	return match;
>   }
> @@ -622,15 +637,16 @@ static struct rt6_info *rt6_select(struct fib6_node *fn, int oif, int strict)
>   {
>   	struct rt6_info *match, *rt0;
>   	struct net *net;
> +	bool do_rr = false;
>
>   	rt0 = fn->rr_ptr;
>   	if (!rt0)
>   		fn->rr_ptr = rt0 = fn->leaf;
>
> -	match = find_rr_leaf(fn, rt0, rt0->rt6i_metric, oif, strict);
> +	match = find_rr_leaf(fn, rt0, rt0->rt6i_metric, oif, strict,
> +			     &do_rr);
>
> -	if (!match &&
> -	    (strict & RT6_LOOKUP_F_REACHABLE)) {
> +	if (do_rr) {
>   		struct rt6_info *next = rt0->dst.rt6_next;
>
>   		/* no entries matched; do round-robin */
>