On  Mon, Feb 22, 2021 at 14:31:38 -0800, Jakub Kicinski wrote:
> On Mon, 22 Feb 2021 17:40:16 +0100 Matthias Schiffer wrote:
> > >> This will not be sufficient for my usecase: To stay compatible with older
> > >> versions of fastd, I can't set the T flag in the first packet of the
> > >> handshake, as it won't be known whether the peer has a new enough fastd
> > >> version to understand packets that have this bit set. Luckily, the second
> > >> handshake byte is always 0 in fastd's protocol, so these packets fail the
> > >> tunnel version check and are passed to userspace regardless.
> > >>
> > >> I'm aware that this usecase is far outside of the original intentions of the
> > >> code and can only be described as a hack, but I still consider this a
> > >> regression in the kernel, as it was working fine in the past, without
> > >> visible warnings.
> > >>  
> > > 
> > > I'm sorry, but for the reasons stated above I disagree about it being
> > > a regression.  
> > 
> > Hmm, is it common for protocol implementations in the kernel to warn about 
> > invalid packets they receive? While L2TP uses connected sockets and thus 
> > usually no unrelated packets end up in the socket, a simple UDP port scan 
> > originating from the configured remote address/port will trigger the "short 
> > packet" warning now (nmap uses a zero-length payload for UDP scans by 
> > default). Log spam caused by a malicous party might also be a concern.
> 
> Indeed, seems like appropriate counters would be a good fit here? 
> The prints are both potentially problematic for security and lossy.

Yes, I agree with this argument.