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@xxxxxxxxx> 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@xxxxxxxxxxxxx/T/#u > > > Fixes: 393e52e33c6c ("packet: deliver VLAN TCI to userspace") > > > Cc: stable@xxxxxxxxxxxxxxx > > > Signed-off-by: Chengen Du <chengen.du@xxxxxxxxxxxxx> > > > > 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.
