ppp0 modem interface causes parseAddr panic

[jswantek]

The parseAddr function (https://github.com/vishvananda/netlink/blob/master/addr_linux.go#L216) assumes that the unix.IFA_ADDRESS case will be hit, setting a valid value to the dst variable. However, if the unix.IFA_ADDRESS case is not hit then dst will remain unset (null) and cause a panic (https://github.com/vishvananda/netlink/blob/master/addr_linux.go#L271).

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x1a31f90]

goroutine 20887 [running]:
github.com/rancher/k3s/vendor/github.com/vishvananda/netlink.parseAddr(0x400648bdec, 0x40, 0x40, 0x0, 0x400867a0a8, 0x4, 0x280, 0x0, 0x0, 0x400648be00, ...)
        /go/src/github.com/rancher/k3s/vendor/github.com/vishvananda/netlink/addr_linux.go:274 +0x170
github.com/rancher/k3s/vendor/github.com/vishvananda/netlink.(*Handle).AddrList(0x6721590, 0x43eff40, 0x400f0859e0, 0x2, 0x0, 0x4, 0x2bc, 0xfe, 0x1)
        /go/src/github.com/rancher/k3s/vendor/github.com/vishvananda/netlink/addr_linux.go:199 +0x180
github.com/rancher/k3s/vendor/github.com/vishvananda/netlink.AddrList(...)
        /go/src/github.com/rancher/k3s/vendor/github.com/vishvananda/netlink/addr_linux.go:174
github.com/rancher/k3s/vendor/github.com/coreos/flannel/pkg/ip.getIfaceAddrs(0x400e1e22c0, 0x0, 0x0, 0x0, 0x0, 0x0)
        /go/src/github.com/rancher/k3s/vendor/github.com/coreos/flannel/pkg/ip/iface.go:36 +0x68
github.com/rancher/k3s/vendor/github.com/coreos/flannel/pkg/ip.GetIfaceIP4Addr(0x400e1e22c0, 0x0, 0x0, 0x400775ee08, 0x1, 0x1)
        /go/src/github.com/rancher/k3s/vendor/github.com/coreos/flannel/pkg/ip/iface.go:40 +0x30
github.com/rancher/k3s/pkg/agent/flannel.LookupExtIface(0x400e1e22c0, 0x75ff34, 0x400cd82f08, 0x75f90c)
        /go/src/github.com/rancher/k3s/pkg/agent/flannel/flannel.go:100 +0xa8
github.com/rancher/k3s/pkg/agent/flannel.flannel(0x444d740, 0x4000e58c00, 0x0, 0x4006a5dd40, 0x34, 0x400b2e43f0, 0x2d, 0x1, 0x0)
        /go/src/github.com/rancher/k3s/pkg/agent/flannel/flannel.go:43 +0x28
github.com/rancher/k3s/pkg/agent/flannel.Run.func1(0x444d740, 0x4000e58c00, 0x4005df4380)
        /go/src/github.com/rancher/k3s/pkg/agent/flannel/setup.go:98 +0x5c
created by github.com/rancher/k3s/pkg/agent/flannel.Run
        /go/src/github.com/rancher/k3s/pkg/agent/flannel/setup.go:97 +0x290

The above stack is a running of flannel (via k3s) that makes use of the netlink library to parse all the host system network routes/addresses. In the event that a ppp0 modem interface exists the above panic occurs.

IP routing information

# route -n
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
0.0.0.0         0.0.0.0         0.0.0.0         U     700    0        0 ppp0
10.31.81.0      0.0.0.0         255.255.255.0   U     101    0        0 lan0
172.17.0.0      0.0.0.0         255.255.0.0     U     0      0        0 docker0

32: ppp0: <POINTOPOINT,MULTICAST,NOARP,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UNKNOWN group default qlen 3
    link/ppp
    inet 10.206.144.193/32 brd 10.206.144.193 scope global noprefixroute ppp0
       valid_lft forever preferred_lft foreve

See the k3s tracked issue for more details: k3s-io/k3s#3512

[jswantek]

Proposed fix: #665

[romainreignier]

I can confirm that #665 fixed the issue.