2023-07-19 14:15:01

by Benjamin Poirier

Subject: [PATCH net-next v2 3/4] nexthop: Do not return invalid nexthop object during multipath selection

With legacy nexthops, when net.ipv4.fib_multipath_use_neigh is set,
fib_select_multipath() will never set res->nhc to a nexthop that is not
good (as per fib_good_nh()). OTOH, with nexthop objects,
nexthop_select_path_hthr() may return a nexthop that failed the
nexthop_is_good_nh() test even if there was one that passed. Refactor
nexthop_select_path_hthr() to follow a selection logic more similar to
fib_select_multipath().

The issue can be demonstrated with the following sequence of commands. The
first block shows that things work as expected with legacy nexthops. The
last sequence of `ip rou get` in the second block shows the problem case -
some routes still use the .2 nexthop.

sysctl net.ipv4.fib_multipath_use_neigh=1
ip link add dummy1 up type dummy
ip rou add 198.51.100.0/24 nexthop via 192.0.2.1 dev dummy1 onlink nexthop via 192.0.2.2 dev dummy1 onlink
for i in {10..19}; do ip -o rou get 198.51.100.$i; done
ip neigh add 192.0.2.1 dev dummy1 nud failed
echo ".1 failed:" # results should not use .1
for i in {10..19}; do ip -o rou get 198.51.100.$i; done
ip neigh del 192.0.2.1 dev dummy1
ip neigh add 192.0.2.2 dev dummy1 nud failed
echo ".2 failed:" # results should not use .2
for i in {10..19}; do ip -o rou get 198.51.100.$i; done
ip link del dummy1

ip link add dummy1 up type dummy
ip nexthop add id 1 via 192.0.2.1 dev dummy1 onlink
ip nexthop add id 2 via 192.0.2.2 dev dummy1 onlink
ip nexthop add id 1001 group 1/2
ip rou add 198.51.100.0/24 nhid 1001
for i in {10..19}; do ip -o rou get 198.51.100.$i; done
ip neigh add 192.0.2.1 dev dummy1 nud failed
echo ".1 failed:" # results should not use .1
for i in {10..19}; do ip -o rou get 198.51.100.$i; done
ip neigh del 192.0.2.1 dev dummy1
ip neigh add 192.0.2.2 dev dummy1 nud failed
echo ".2 failed:" # results should not use .2
for i in {10..19}; do ip -o rou get 198.51.100.$i; done
ip link del dummy1

Signed-off-by: Ido Schimmel <[email protected]>
Signed-off-by: Benjamin Poirier <[email protected]>
---
net/ipv4/nexthop.c | 14 ++++++++------
1 file changed, 8 insertions(+), 6 deletions(-)

diff --git a/net/ipv4/nexthop.c b/net/ipv4/nexthop.c
index c12acbf39659..93f14d39fef6 100644
--- a/net/ipv4/nexthop.c
+++ b/net/ipv4/nexthop.c
@@ -1194,20 +1194,22 @@ static struct nexthop *nexthop_select_path_hthr(struct nh_group *nhg, int hash)
for (i = 0; i < nhg->num_nh; ++i) {
struct nh_grp_entry *nhge = &nhg->nh_entries[i];

- if (hash > atomic_read(&nhge->hthr.upper_bound))
- continue;
-
/* nexthops always check if it is good and does
* not rely on a sysctl for this behavior
*/
- if (nexthop_is_good_nh(nhge->nh))
- return nhge->nh;
+ if (!nexthop_is_good_nh(nhge->nh))
+ continue;

if (!rc)
rc = nhge->nh;
+
+ if (hash > atomic_read(&nhge->hthr.upper_bound))
+ continue;
+
+ return nhge->nh;
}

- return rc;
+ return rc ? : nhg->nh_entries[0].nh;
}

static struct nexthop *nexthop_select_path_res(struct nh_group *nhg, int hash)

--
2.40.1

2023-07-20 16:03:58

by David Ahern

Subject: Re: [PATCH net-next v2 3/4] nexthop: Do not return invalid nexthop object during multipath selection

On 7/19/23 7:57 AM, Benjamin Poirier wrote:
> With legacy nexthops, when net.ipv4.fib_multipath_use_neigh is set,
> fib_select_multipath() will never set res->nhc to a nexthop that is not
> good (as per fib_good_nh()). OTOH, with nexthop objects,
> nexthop_select_path_hthr() may return a nexthop that failed the
> nexthop_is_good_nh() test even if there was one that passed. Refactor
> nexthop_select_path_hthr() to follow a selection logic more similar to
> fib_select_multipath().
>
> The issue can be demonstrated with the following sequence of commands. The
> first block shows that things work as expected with legacy nexthops. The
> last sequence of `ip rou get` in the second block shows the problem case -
> some routes still use the .2 nexthop.
>
> sysctl net.ipv4.fib_multipath_use_neigh=1
> ip link add dummy1 up type dummy
> ip rou add 198.51.100.0/24 nexthop via 192.0.2.1 dev dummy1 onlink nexthop via 192.0.2.2 dev dummy1 onlink
> for i in {10..19}; do ip -o rou get 198.51.100.$i; done
> ip neigh add 192.0.2.1 dev dummy1 nud failed
> echo ".1 failed:" # results should not use .1
> for i in {10..19}; do ip -o rou get 198.51.100.$i; done
> ip neigh del 192.0.2.1 dev dummy1
> ip neigh add 192.0.2.2 dev dummy1 nud failed
> echo ".2 failed:" # results should not use .2
> for i in {10..19}; do ip -o rou get 198.51.100.$i; done
> ip link del dummy1
>
> ip link add dummy1 up type dummy
> ip nexthop add id 1 via 192.0.2.1 dev dummy1 onlink
> ip nexthop add id 2 via 192.0.2.2 dev dummy1 onlink
> ip nexthop add id 1001 group 1/2
> ip rou add 198.51.100.0/24 nhid 1001
> for i in {10..19}; do ip -o rou get 198.51.100.$i; done
> ip neigh add 192.0.2.1 dev dummy1 nud failed
> echo ".1 failed:" # results should not use .1
> for i in {10..19}; do ip -o rou get 198.51.100.$i; done
> ip neigh del 192.0.2.1 dev dummy1
> ip neigh add 192.0.2.2 dev dummy1 nud failed
> echo ".2 failed:" # results should not use .2
> for i in {10..19}; do ip -o rou get 198.51.100.$i; done
> ip link del dummy1
>
> Signed-off-by: Ido Schimmel <[email protected]>
> Signed-off-by: Benjamin Poirier <[email protected]>
> ---
> net/ipv4/nexthop.c | 14 ++++++++------
> 1 file changed, 8 insertions(+), 6 deletions(-)
>

Reviewed-by: David Ahern <[email protected]>