linux-kernel - Re: [RFC PATCH net] ipv4: ip_gre: Fix drops of small packets in ipgre

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Message-ID: <CANn89iKP3VPExdyZt+eLFk3rE5=6yRckTPySfh5MvcEqPNm6aA@mail.gmail.com>
Date: Sun, 22 Sep 2024 12:20:03 +0200
From: Eric Dumazet <edumazet@...gle.com>
To: Anton Danilov <littlesmilingcloud@...il.com>
Cc: netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>, 
	David Ahern <dsahern@...nel.org>, Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, 
	Suman Ghosh <sumang@...vell.com>, Shigeru Yoshida <syoshida@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH net] ipv4: ip_gre: Fix drops of small packets in ipgre_xmit

On Sat, Sep 21, 2024 at 11:55 PM Anton Danilov
<littlesmilingcloud@...il.com> wrote:
>
> Regression Description:
>
> Depending on the GRE tunnel device options, small packets are being
> dropped. This occurs because the pskb_network_may_pull function fails due
> to insufficient space in the network header. For example, if only the key
> option is specified for the tunnel device, packets of sizes up to 27
> (including the IPv4 header itself) will be dropped. This affects both
> locally originated and forwarded packets.
>
> How to reproduce (for local originated packets):
>
>   ip link add dev gre1 type gre ikey 1.9.8.4 okey 1.9.8.4 \
>           local <your-ip> remote <any-ip>
>
>   ip link set mtu 1400 dev gre1
>   ip link set up dev gre1
>   ip address add 192.168.13.1/24 dev gre1
>   ping -s 1374 -c 10 192.168.13.2

This size does not match the changelog ? (packets of sizes up to 27...)

>   tcpdump -vni gre1
>   tcpdump -vni <your-ext-iface> 'ip proto 47'
>   ip -s -s -d link show dev gre1

Please provide a real selftest, because in this particular example,
the path taken by the packets should not reach the
pskb_network_may_pull(skb, pull_len)),
because dev->header_ops should be NULL ?