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Message-ID: <20220416065633.GA10882@bytedance>
Date: Fri, 15 Apr 2022 23:56:33 -0700
From: Peilin Ye <yepeilin.cs@...il.com>
To: Jakub Kicinski <kuba@...nel.org>
Cc: "David S. Miller" <davem@...emloft.net>,
Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
David Ahern <dsahern@...nel.org>,
Peilin Ye <peilin.ye@...edance.com>,
Cong Wang <cong.wang@...edance.com>,
Feng Zhou <zhoufeng.zf@...edance.com>, netdev@...r.kernel.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH net 2/2] ip6_gre: Fix skb_under_panic in __gre6_xmit()
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
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