[PATCH net] route: check sysctl_fib_multipath_use_neigh earlier than hash

[PATCH net] route: check sysctl_fib_multipath_use_neigh earlier than hash

[Xin Long]

Prior to this patch, when one packet is hashed into path [1]
(hash <= nh_upper_bound) and it's neigh is dead, it will try
path [2]. However, if path [2]'s neigh is alive but it's
hash > nh_upper_bound, it will not return this alive path.
This packet will never be sent even if path [2] is alive.

 3.3.3.1/24:
  nexthop via 1.1.1.254 dev eth1 weight 1 <--[1] (dead neigh)
  nexthop via 2.2.2.254 dev eth2 weight 1 <--[2]

With sysctl_fib_multipath_use_neigh set is supposed to find an
available path respecting to the l3/l4 hash. But if there is
no available route with this hash, it should at least return
an alive route even with other hash.

This patch is to fix it by processing fib_multipath_use_neigh
earlier than the hash check, so that it will at least return
an alive route if there is when fib_multipath_use_neigh is
enabled. It's also compatible with before when there are alive
routes with the l3/l4 hash.

Fixes: a6db4494d218 ("net: ipv4: Consider failed nexthops in multipath routes")
Reported-by: Jianlin Shi <jishi@redhat.com>
Signed-off-by: Xin Long <lucien.xin@gmail.com>
---
 net/ipv4/fib_semantics.c | 20 +++++++++++---------
 1 file changed, 11 insertions(+), 9 deletions(-)

diff --git a/net/ipv4/fib_semantics.c b/net/ipv4/fib_semantics.c
index 7d36a95..9d51292 100644
--- a/net/ipv4/fib_semantics.c
+++ b/net/ipv4/fib_semantics.c
@@ -1746,18 +1746,20 @@ void fib_select_multipath(struct fib_result *res, int hash)
 	bool first = false;
 
 	for_nexthops(fi) {
+		if (net->ipv4.sysctl_fib_multipath_use_neigh) {
+			if (!fib_good_nh(nh))
+				continue;
+			if (!first) {
+				res->nh_sel = nhsel;
+				first = true;
+			}
+		}
+
 		if (hash > atomic_read(&nh->nh_upper_bound))
 			continue;
 
-		if (!net->ipv4.sysctl_fib_multipath_use_neigh ||
-		    fib_good_nh(nh)) {
-			res->nh_sel = nhsel;
-			return;
-		}
-		if (!first) {
-			res->nh_sel = nhsel;
-			first = true;
-		}
+		res->nh_sel = nhsel;
+		return;
 	} endfor_nexthops(fi);
 }
 #endif
-- 
2.1.0

Re: [PATCH net] route: check sysctl_fib_multipath_use_neigh earlier than hash

[David Ahern]

On 4/1/18 8:40 AM, Xin Long wrote:
> Prior to this patch, when one packet is hashed into path [1]
> (hash <= nh_upper_bound) and it's neigh is dead, it will try
> path [2]. However, if path [2]'s neigh is alive but it's
> hash > nh_upper_bound, it will not return this alive path.
> This packet will never be sent even if path [2] is alive.
> 
>  3.3.3.1/24:
>   nexthop via 1.1.1.254 dev eth1 weight 1 <--[1] (dead neigh)
>   nexthop via 2.2.2.254 dev eth2 weight 1 <--[2]
> 
> With sysctl_fib_multipath_use_neigh set is supposed to find an
> available path respecting to the l3/l4 hash. But if there is
> no available route with this hash, it should at least return
> an alive route even with other hash.
> 
> This patch is to fix it by processing fib_multipath_use_neigh
> earlier than the hash check, so that it will at least return
> an alive route if there is when fib_multipath_use_neigh is
> enabled. It's also compatible with before when there are alive
> routes with the l3/l4 hash.
> 
> Fixes: a6db4494d218 ("net: ipv4: Consider failed nexthops in multipath routes")
> Reported-by: Jianlin Shi <jishi@redhat.com>
> Signed-off-by: Xin Long <lucien.xin@gmail.com>
> ---
>  net/ipv4/fib_semantics.c | 20 +++++++++++---------
>  1 file changed, 11 insertions(+), 9 deletions(-)
> 

Acked-by: David Ahern <dsa@cumulusnetworks.com>

Re: [PATCH net] route: check sysctl_fib_multipath_use_neigh earlier than hash

[David Miller]

From: Xin Long <lucien.xin@gmail.com>
Date: Sun,  1 Apr 2018 22:40:35 +0800

> Prior to this patch, when one packet is hashed into path [1]
> (hash <= nh_upper_bound) and it's neigh is dead, it will try
> path [2]. However, if path [2]'s neigh is alive but it's
> hash > nh_upper_bound, it will not return this alive path.
> This packet will never be sent even if path [2] is alive.
> 
>  3.3.3.1/24:
>   nexthop via 1.1.1.254 dev eth1 weight 1 <--[1] (dead neigh)
>   nexthop via 2.2.2.254 dev eth2 weight 1 <--[2]
> 
> With sysctl_fib_multipath_use_neigh set is supposed to find an
> available path respecting to the l3/l4 hash. But if there is
> no available route with this hash, it should at least return
> an alive route even with other hash.
> 
> This patch is to fix it by processing fib_multipath_use_neigh
> earlier than the hash check, so that it will at least return
> an alive route if there is when fib_multipath_use_neigh is
> enabled. It's also compatible with before when there are alive
> routes with the l3/l4 hash.
> 
> Fixes: a6db4494d218 ("net: ipv4: Consider failed nexthops in multipath routes")
> Reported-by: Jianlin Shi <jishi@redhat.com>
> Signed-off-by: Xin Long <lucien.xin@gmail.com>

Applied and queued up for -stable, thanks.

Re: [PATCH net] route: check sysctl_fib_multipath_use_neigh earlier than hash

[Sasha Levin]

Hi Xin Long.

[This is an automated email]

This commit has been processed because it contains a "Fixes:" tag.
fixing commit: a6db4494d218 net: ipv4: Consider failed nexthops in multipath routes.

The bot has also determined it's probably a bug fixing patch. (score: 58.0140)

The bot has tested the following trees: v4.15.15, v4.14.32, v4.9.92, 

v4.15.15: Build OK!
v4.14.32: Build OK!
v4.9.92: Build OK!