linux-kernel - Re: [PATCH net 2/2] net/af_packet: fix tx skb network header on SOCK

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Message-ID: <fa5895ae62e0f9c1eb8f662295ca920d1da7e88f.camel@redhat.com>
Date:   Thu, 12 Jan 2023 13:48:51 +0100
From:   Paolo Abeni <pabeni@...hat.com>
To:     Hervé Boisse <admin@...geek.ovh>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>
Cc:     netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH net 2/2] net/af_packet: fix tx skb network header on
 SOCK_RAW sockets over VLAN device

Hello,

On Tue, 2023-01-10 at 20:17 +0100, Hervé Boisse wrote:
> When an application sends a packet on a SOCK_RAW socket over a VLAN device,
> there is a possibility that the skb network header is incorrectly set.
> 
> The issue happens when the device used to send the packet is a VLAN device
> whose underlying device has no VLAN tx hardware offloading support.
> In that case, the VLAN driver reports a LL header size increased by 4 bytes
> to take into account the tag that will be added in software.
> 
> However, the socket user has no clue about that and still provides a normal
> LL header without tag.
> This results in the network header of the skb being shifted 4 bytes too far
> in the packet. This shift makes tc classifiers fail as they point to
> incorrect data.

I'm unsure I read correctly the use case: teh user-space application is
providing an L2 header and is expecting the Linux stack to add a vlan
tag? Or the linux application is sending packets on top of a vlan
device and desire no tag on the egress packet? or something else?

(Note: I think that in the fisrt 2 cases above the fix belong to the
user-space application).

Thanks!

Paolo