Message-ID: <666c0feca4abf_17367e294e2@willemb.c.googlers.com.notmuch>
Date: Fri, 14 Jun 2024 05:39:56 -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,
> 
> Thank you for the suggestion.
> I have conducted further tests and found that the results are not as
> we expected.
> 
> I would like to explain my findings based on the following tests:
>     ip link add link ens18 ens18.24 type vlan proto 802.1ad id 24
>     ip link add link ens18.24 ens18.24.25 type vlan proto 802.1Q id 25
>     ifconfig ens18.24 1.0.24.1/24
>     ifconfig ens18.24.25 1.0.25.1/24
>     ping -n 1.0.25.3 > /dev/null 2>&1 &
>     tcpdump -nn -i any -y LINUX_SLL -Q out not tcp and not udp
> 
> I have added more logs and found the following results:
>     af_packet: tpacket_rcv: dev->name [ens18.24.25]
>     af_packet: tpacket_rcv: dev->name [ens18.24]
>     af_packet: vlan_get_tci: dev->name [ens18.24], min_header_len
> [14], hard_header_len [18]
>     af_packet: prb_fill_vlan_info: ppd->hv1.tp_vlan_tci [0],
> ppd->hv1.tp_vlan_tpid [8100]
>     af_packet: prb_fill_vlan_info: currect vlan_tci [19], tp_vlan_tpid [8100]
>     af_packet: tpacket_rcv: dev->name [ens18]
>     af_packet: vlan_get_tci: dev->name [ens18], min_header_len [14],
> hard_header_len [14]
>     af_packet: prb_fill_vlan_info: ppd->hv1.tp_vlan_tci [18],
> ppd->hv1.tp_vlan_tpid [88a8]
>     af_packet: prb_fill_vlan_info: currect vlan_tci [18], tp_vlan_tpid [88a8]
> 
> It seems that the min_header_len has been set even though the device
> is ens18.24.
> I will continue investigating this issue.
> Thank you for your ongoing assistance.

Thanks. Apparently min_header_len cannot be relied on for this. It is
not explicitly set in most drivers, including in the vlan driver.

Let's not make this series conditional on that.

The number of variable length link layer protocols is minimal, and
egregious errors are caught by skb_header_pointer. Just use
dev->hard_header_len as is, without the min_header_len check.

Btw, please remember not to top-post.
https://subspace.kernel.org/etiquette.html#do-not-top-post-when-replying

 
> Best regards,
> Chengen Du
> 
> On Wed, Jun 12, 2024 at 10:07 PM Willem de Bruijn
> <willemdebruijn.kernel@...il.com> wrote:
> >
> > Chengen Du wrote:
> > > Hi Willem,
> > >
> > > On Tue, Jun 11, 2024 at 7:18 AM Willem de Bruijn
> > > <willemdebruijn.kernel@...il.com> wrote:
> > > >
> > > > 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.
> > >
> > > Thank you for your advice.
> > > I have implemented the modification, but I found that the
> > > (min_header_len != hard_header_len) check results in unexpected
> > > behavior in the following test scenario:
> > >     ip link add link ens18 ens18.24 type vlan proto 802.1ad id 24
> > >     ip link add link ens18.24 ens18.24.25 type vlan proto 802.1Q id 25
> > >     ifconfig ens18.24 1.0.24.1/24
> > >     ifconfig ens18.24.25 1.0.25.1/24
> > >     ping -n 1.0.25.3 > /dev/null 2>&1 &
> > >     tcpdump -nn -i any -y LINUX_SLL -Q out not tcp and not udp
> > >
> > > While receiving a packet from ens18.24.25 (802.1Q), the min_header_len
> > > and hard_header_len are 14 and 18, respectively.
> > > This check results in the TCI being 0 instead of 25.
> > > Should we skip this check to display the correct value, or is there
> > > another check that can achieve the same purpose?
> >
> > Interesting. Glad you found this.
> >
> > Makes sense, as VLAN devices have
> >
> >     vlandev->hard_header_len = dev->hard_header_len + VLAN_HLEN;
> >
> > Does
> >
> >     if (min_header_len && min_header_len != hard_header_len)
> >
> > resolve it?
> >
> > Few devices actually set min_header_len. Initially, only Ethernet in
> > ether_setup() and loopback. It was introduced for validation in
> > dev_validate_header, and a min_header_len of 0 just skips some basic
> > validation.
> >
> > As long as VLAN devices do not initialize min_header_len (e.g., by
> > inheriting it from the physical device and incorrectly setting it to
> > ETH_HLEN), then this should be fine.
> >

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