Hi,

I am not sure if this is an upstream or a SUSE specific bug - I hope it's fine
to report here.

I think the `pmtudisc` option is only supported on `gre` and not on `ip6gre`
interfaces. If this assumption is correct, I think the user should be informed
about this, instead of being returned a "stack smashing detected" exception.

#this is a ip6gre tunnel interface:
$ ip t sh gre5
gre5: any/ip remote 42.2.23.72 local 0.0.0.1 ttl inherit tos inherit/0x02
nopmtudisc ikey 0
$ ip -6 t sh gre5
gre5: gre/ipv6 remote 2a02:1748:f7df:9c81::2 local 2a03:4000:55:d20::1
encaplimit 4 hoplimit 64 tclass 0x00 flowlabel 0x00000 (flowinfo 0x00000000)���������

#attempting to change `nopmtudisc` to `pmtudisc` makes the program very unhappy
$ sudo ip t c gre5 pmtudisc                                                     
*** stack smashing detected ***: terminated                                     
fish: Job 1, 'sudo ip t c gre5 pmtudisc' terminated by signal SIGABRT (Abort)


On a "regular" `gre` interface, it works fine.

#this is a gre tunnel interface:
$ ip t sh gre2
gre2: gre/ip remote 146.59.50.73 local 202.61.255.116 ttl 255 nopmtudisc

#attempting to change `nopmtudisc` to `pmtudisc`...
$ sudo ip t c gre2 pmtudisc

#... succeeds:
$ ip t sh gre2
gre2: gre/ip remote 146.59.50.73 local 202.61.255.116 ttl 255


Let me know if you need any further details or if you prefer me to report it at
kernel.org.
Thank you for reading!

Best,
Georg