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Date: Sat, 16 Apr 2022 09:33:20 +0200 From: Jakub Kicinski <kuba@...nel.org> To: Peilin Ye <yepeilin.cs@...il.com> 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, 15 Apr 2022 23:56:33 -0700 Peilin Ye wrote: > On Fri, Apr 15, 2022 at 07:11:33PM +0200, Jakub Kicinski wrote: > > > 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. Maybe pr_warn_once(), no need for a stacktrace. > > 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? atomic_t would also work (if it has enough bits). Whatever is simplest TBH. It's just about correctness, I don't think seq is widely used. Thanks!
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