On 2/22/21 9:32 AM, Kaustubh Pandey wrote: > When netdevice MTU is increased via sysfs, NETDEV_CHANGEMTU is raised. > > addrconf_notify -> rt6_mtu_change -> rt6_mtu_change_route -> > fib6_nh_mtu_change > > As part of handling NETDEV_CHANGEMTU notification we land up on a > condition where if route mtu is less than dev mtu and route mtu equals > ipv6_devconf mtu, route mtu gets updated. > > Due to this v6 traffic end up using wrong MTU then configured earlier. > This commit fixes this by removing comparison with ipv6_devconf > and updating route mtu only when it is greater than incoming dev mtu. > > This can be easily reproduced with below script: > pre-condition: > device up(mtu = 1500) and route mtu for both v4 and v6 is 1500 > > test-script: > ip route change 192.168.0.0/24 dev eth0 src 192.168.0.1 mtu 1400 > ip -6 route change 2001::/64 dev eth0 metric 256 mtu 1400 > echo 1400 > /sys/class/net/eth0/mtu > ip route change 192.168.0.0/24 dev eth0 src 192.168.0.1 mtu 1500 > echo 1500 > /sys/class/net/eth0/mtu > > Signed-off-by: Kaustubh Pandey <kapan...@codeaurora.org> > --- > net/ipv6/route.c | 3 +-- > 1 file changed, 1 insertion(+), 2 deletions(-) > > diff --git a/net/ipv6/route.c b/net/ipv6/route.c > index 1536f49..653b6c7 100644 > --- a/net/ipv6/route.c > +++ b/net/ipv6/route.c > @@ -4813,8 +4813,7 @@ static int fib6_nh_mtu_change(struct fib6_nh *nh, void > *_arg) > struct inet6_dev *idev = __in6_dev_get(arg->dev); > u32 mtu = f6i->fib6_pmtu; > > - if (mtu >= arg->mtu || > - (mtu < arg->mtu && mtu == idev->cnf.mtu6)) > + if (mtu >= arg->mtu) > fib6_metric_set(f6i, RTAX_MTU, arg->mtu); > > spin_lock_bh(&rt6_exception_lock); >
The existing logic mirrors what is done for exceptions, see rt6_mtu_change_route_allowed and commit e9fa1495d738. It seems right to me to drop the mtu == idev->cnf.mtu6 comparison in which case the exceptions should do the same. Added author of e9fa1495d738 in case I am overlooking something. Test case should be added to tools/testing/selftests/net/pmtu.sh, and did you run that script with the proposed change?
