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? > > > + if (skb_orig_data != skb->data) { > > + skb->data = skb_orig_data; > > + skb->len = skb_orig_len; > > + } > > + if (unlikely(!vh)) > > + return 0; > > + > > + return ntohs(vh->h_vlan_TCI); > > +} > > + > > Only since I had to respond above: this is non-obvious enough to > deserve a function comment. Something like the following? > > /* For SOCK_DGRAM, data starts at the network protocol, after any VLAN > * headers. sll_protocol must point to the network protocol. The > * (outer) VLAN TCI is still accessible as auxdata. > */ > > > +static __be16 vlan_get_protocol_dgram(struct sk_buff *skb) > > +{ > > + __be16 proto = skb->protocol; > > + > > + if (unlikely(eth_type_vlan(proto))) { > > + u8 *skb_orig_data = skb->data; > > + int skb_orig_len = skb->len; > > + > > + skb_push(skb, skb->data - skb_mac_header(skb)); > > + proto = __vlan_get_protocol(skb, proto, NULL); > > + if (skb_orig_data != skb->data) { > > + skb->data = skb_orig_data; > > + skb->len = skb_orig_len; > > + } > > + } > > + > > + return proto; > > +} > > + > > static void prb_del_retire_blk_timer(struct tpacket_kbdq_core *pkc) > > { > > del_timer_sync(&pkc->retire_blk_timer); > > @@ -1007,10 +1044,16 @@ static void prb_clear_rxhash(struct tpacket_kbdq_core *pkc, > > static void prb_fill_vlan_info(struct tpacket_kbdq_core *pkc, > > struct tpacket3_hdr *ppd) > > { > > + struct packet_sock *po = container_of(pkc, struct packet_sock, rx_ring.prb_bdqc); > > + > > if (skb_vlan_tag_present(pkc->skb)) { > > ppd->hv1.tp_vlan_tci = skb_vlan_tag_get(pkc->skb); > > ppd->hv1.tp_vlan_tpid = ntohs(pkc->skb->vlan_proto); > > ppd->tp_status = TP_STATUS_VLAN_VALID | TP_STATUS_VLAN_TPID_VALID; > > + } else if (unlikely(po->sk.sk_type == SOCK_DGRAM && eth_type_vlan(pkc->skb->protocol))) { > > + ppd->hv1.tp_vlan_tci = vlan_get_tci(pkc->skb); > > + ppd->hv1.tp_vlan_tpid = ntohs(pkc->skb->protocol); > > + ppd->tp_status = TP_STATUS_VLAN_VALID | TP_STATUS_VLAN_TPID_VALID; > > } else { > > ppd->hv1.tp_vlan_tci = 0; > > ppd->hv1.tp_vlan_tpid = 0; > > @@ -2428,6 +2471,10 @@ static int tpacket_rcv(struct sk_buff *skb, struct net_device *dev, > > h.h2->tp_vlan_tci = skb_vlan_tag_get(skb); > > h.h2->tp_vlan_tpid = ntohs(skb->vlan_proto); > > status |= TP_STATUS_VLAN_VALID | TP_STATUS_VLAN_TPID_VALID; > > + } else if (unlikely(sk->sk_type == SOCK_DGRAM && eth_type_vlan(skb->protocol))) { > > + h.h2->tp_vlan_tci = vlan_get_tci(skb); > > + h.h2->tp_vlan_tpid = ntohs(skb->protocol); > > + status |= TP_STATUS_VLAN_VALID | TP_STATUS_VLAN_TPID_VALID; > > } else { > > h.h2->tp_vlan_tci = 0; > > h.h2->tp_vlan_tpid = 0; > > @@ -2457,7 +2504,8 @@ static int tpacket_rcv(struct sk_buff *skb, struct net_device *dev, > > sll->sll_halen = dev_parse_header(skb, sll->sll_addr); > > sll->sll_family = AF_PACKET; > > sll->sll_hatype = dev->type; > > - sll->sll_protocol = skb->protocol; > > + sll->sll_protocol = (sk->sk_type == SOCK_DGRAM) ? > > + vlan_get_protocol_dgram(skb) : skb->protocol; > > sll->sll_pkttype = skb->pkt_type; > > if (unlikely(packet_sock_flag(po, PACKET_SOCK_ORIGDEV))) > > sll->sll_ifindex = orig_dev->ifindex; > > @@ -3482,7 +3530,8 @@ static int packet_recvmsg(struct socket *sock, struct msghdr *msg, size_t len, > > /* Original length was stored in sockaddr_ll fields */ > > origlen = PACKET_SKB_CB(skb)->sa.origlen; > > sll->sll_family = AF_PACKET; > > - sll->sll_protocol = skb->protocol; > > + sll->sll_protocol = (sock->type == SOCK_DGRAM) ? > > + vlan_get_protocol_dgram(skb) : skb->protocol; > > } > > > > sock_recv_cmsgs(msg, sk, skb); > > @@ -3539,6 +3588,10 @@ static int packet_recvmsg(struct socket *sock, struct msghdr *msg, size_t len, > > aux.tp_vlan_tci = skb_vlan_tag_get(skb); > > aux.tp_vlan_tpid = ntohs(skb->vlan_proto); > > aux.tp_status |= TP_STATUS_VLAN_VALID | TP_STATUS_VLAN_TPID_VALID; > > + } else if (unlikely(sock->type == SOCK_DGRAM && eth_type_vlan(skb->protocol))) { > > + aux.tp_vlan_tci = vlan_get_tci(skb); > > + aux.tp_vlan_tpid = ntohs(skb->protocol); > > + aux.tp_status |= TP_STATUS_VLAN_VALID | TP_STATUS_VLAN_TPID_VALID; > > } else { > > aux.tp_vlan_tci = 0; > > aux.tp_vlan_tpid = 0; > > -- > > 2.43.0 > > > >
