Subject: Re: [PATCH net 2/2] ip6_gre: Fix skb_under_panic in __gre6_xmit()

From: Peilin Ye <[email-masked]>
Date: Fri, 15 Apr 2022 23:56:33 -0700

On Fri, Apr 15, 2022 at 07:11:33PM +0200, Jakub Kicinski wrote:
> On Thu, 14 Apr 2022 13:08:54 -0700 Peilin Ye wrote:
> > > We should also reject using SEQ with collect_md, but that's a separate
> > > issue.  
> > 
> > Could you explain this a bit more?  It seems that commit 77a5196a804e
> > ("gre: add sequence number for collect md mode.") added this
> > intentionally.
> 
> Interesting. Maybe a better way of dealing with the problem would be
> rejecting SEQ if it's not set on the device itself.

According to ip-link(8), the 'external' option is mutually exclusive
with the '[o]seq' option.  In other words, a collect_md mode IP6GRETAP
device should always have the TUNNEL_SEQ flag off in its
'tunnel->parms.o_flags'.

(However, I just tried:

  $ ip link add dev ip6gretap11 type ip6gretap oseq external
					       ^^^^ ^^^^^^^^
 ...and my 'ip' executed it with no error.  I will take a closer look at
 iproute2 later; maybe it's undefined behavior...)

How about:

1. If 'external', then 'oseq' means "always turn off NETIF_F_LLTX, so
it's okay to set TUNNEL_SEQ in e.g. eBPF";

2. Otherwise, if 'external' but NOT 'oseq', then whenever we see a
TUNNEL_SEQ in skb's tunnel info, we do something like WARN_ONCE() then
return -EINVAL.

?

> When the device is set up without the SEQ bit enabled it disables Tx
> locking (look for LLTX). This means that multiple CPUs can try to do
> the tunnel->o_seqno++ in parallel. Not catastrophic but racy for sure.

Thanks for the explanation!  At first glance, I was wondering why don't
we make 'o_seqno' atomic until I found commit b790e01aee74 ("ip_gre:
lockless xmit").  I quote:

"""
Even using an atomic_t o_seq, we would increase chance for packets being
out of order at receiver.
"""

I don't fully understand this out-of-order yet, but it seems that making
'o_seqno' atomic is not an option?

Thanks,
Peilin Ye