[PATCH iproute2 2/2] ip tunnel: warn when changing IPv6 tunnel without tunnel name

[Andrea Claudi <aclaudi@redhat.com>]

Tunnel change fails if a tunnel name is not specified while using
'ip -6 tunnel change'. However, no warning message is printed and
no error code is returned.

$ ip -6 tunnel add ip6tnl1 mode ip6gre local fd::1 remote fd::2 tos inherit ttl 127 encaplimit none dev dummy0
$ ip -6 tunnel change dev dummy0 local 2001:1234::1 remote 2001:1234::2
$ ip -6 tunnel show ip6tnl1
ip6tnl1: gre/ipv6 remote fd::2 local fd::1 dev dummy0 encaplimit none hoplimit 127 tclass inherit flowlabel 0x00000 (flowinfo 0x00000000)

This commit checks if tunnel interface name is equal to an empty
string: in this case, it prints a warning message to the user.
It intentionally avoids to return an error to not break existing
script setup.

This is the output after this commit:
$ ip -6 tunnel add ip6tnl1 mode ip6gre local fd::1 remote fd::2 tos inherit ttl 127 encaplimit none dev dummy0
$ ip -6 tunnel change dev dummy0 local 2001:1234::1 remote 2001:1234::2
Tunnel interface name not specified
$ ip -6 tunnel show ip6tnl1
ip6tnl1: gre/ipv6 remote fd::2 local fd::1 dev dummy0 encaplimit none hoplimit 127 tclass inherit flowlabel 0x00000 (flowinfo 0x00000000)

Reviewed-by: Matteo Croce <mcroce@redhat.com>
Signed-off-by: Andrea Claudi <aclaudi@redhat.com>
---
 ip/ip6tunnel.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/ip/ip6tunnel.c b/ip/ip6tunnel.c
index 999408ed801b1..e3da11eb4518e 100644
--- a/ip/ip6tunnel.c
+++ b/ip/ip6tunnel.c
@@ -386,6 +386,9 @@ static int do_add(int cmd, int argc, char **argv)
 	if (parse_args(argc, argv, cmd, &p) < 0)
 		return -1;
 
+	if (!*p.name)
+		fprintf(stderr, "Tunnel interface name not specified\n");
+
 	if (p.proto == IPPROTO_GRE)
 		basedev = "ip6gre0";
 	else if (p.i_flags & VTI_ISVTI)
-- 
2.20.1