netdev - Re: [PATCH v6] af_packet: Handle outgoing VLAN packets without hardware offloading

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Date: Mon, 10 Jun 2024 19:18:43 -0400
From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
To: Chengen Du <chengen.du@...onical.com>, 
 Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc: davem@...emloft.net, 
 edumazet@...gle.com, 
 kuba@...nel.org, 
 pabeni@...hat.com, 
 kaber@...sh.net, 
 netdev@...r.kernel.org, 
 linux-kernel@...r.kernel.org, 
 stable@...r.kernel.org
Subject: Re: [PATCH v6] af_packet: Handle outgoing VLAN packets without
 hardware offloading

Chengen Du wrote:
> Hi Willem,
> 
> I'm sorry, but I would like to confirm the issue further.
> 
> On Mon, Jun 10, 2024 at 4:19 AM Willem de Bruijn
> <willemdebruijn.kernel@...il.com> wrote:
> >
> > Chengen Du wrote:
> > > The issue initially stems from libpcap. The ethertype will be overwritten
> > > as the VLAN TPID if the network interface lacks hardware VLAN offloading.
> > > In the outbound packet path, if hardware VLAN offloading is unavailable,
> > > the VLAN tag is inserted into the payload but then cleared from the sk_buff
> > > struct. Consequently, this can lead to a false negative when checking for
> > > the presence of a VLAN tag, causing the packet sniffing outcome to lack
> > > VLAN tag information (i.e., TCI-TPID). As a result, the packet capturing
> > > tool may be unable to parse packets as expected.
> > >
> > > The TCI-TPID is missing because the prb_fill_vlan_info() function does not
> > > modify the tp_vlan_tci/tp_vlan_tpid values, as the information is in the
> > > payload and not in the sk_buff struct. The skb_vlan_tag_present() function
> > > only checks vlan_all in the sk_buff struct. In cooked mode, the L2 header
> > > is stripped, preventing the packet capturing tool from determining the
> > > correct TCI-TPID value. Additionally, the protocol in SLL is incorrect,
> > > which means the packet capturing tool cannot parse the L3 header correctly.
> > >
> > > Link: https://github.com/the-tcpdump-group/libpcap/issues/1105
> > > Link: https://lore.kernel.org/netdev/20240520070348.26725-1-chengen.du@canonical.com/T/#u
> > > Fixes: 393e52e33c6c ("packet: deliver VLAN TCI to userspace")
> > > Cc: stable@...r.kernel.org
> > > Signed-off-by: Chengen Du <chengen.du@...onical.com>
> >
> > Overall, solid.
> >
> > > ---
> > >  net/packet/af_packet.c | 57 ++++++++++++++++++++++++++++++++++++++++--
> > >  1 file changed, 55 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
> > > index ea3ebc160e25..8cffbe1f912d 100644
> > > --- a/net/packet/af_packet.c
> > > +++ b/net/packet/af_packet.c
> > > @@ -538,6 +538,43 @@ static void *packet_current_frame(struct packet_sock *po,
> > >       return packet_lookup_frame(po, rb, rb->head, status);
> > >  }
> > >
> > > +static u16 vlan_get_tci(struct sk_buff *skb)
> > > +{
> > > +     struct vlan_hdr vhdr, *vh;
> > > +     u8 *skb_orig_data = skb->data;
> > > +     int skb_orig_len = skb->len;
> > > +
> > > +     skb_push(skb, skb->data - skb_mac_header(skb));
> > > +     vh = skb_header_pointer(skb, ETH_HLEN, sizeof(vhdr), &vhdr);
> >
> > Don't harcode Ethernet.
> >
> > According to documentation VLANs are used with other link layers.
> >
> > More importantly, in practice PF_PACKET allows inserting this
> > skb->protocol on any device.
> >
> > We don't use link layer specific constants anywhere in the packet
> > socket code for this reason. But instead dev->hard_header_len.
> >
> > One caveat there is variable length link layer headers, where
> > dev->min_header_len != dev->hard_header_len. Will just have to fail
> > on those.
> 
> Thank you for pointing out this error. I would like to confirm if I
> need to use dev->hard_header_len to get the correct header length and
> return zero if dev->min_header_len != dev->hard_header_len to handle
> variable-length link layer headers. Is there something I
> misunderstand, or are there other aspects I need to consider further?

That's right.

The min_header_len != hard_header_len check is annoying and may seem
pedantic. But it's the only way to trust that the next header starts
at hard_header_len.