[PATCH v2] af_packet: Handle outgoing VLAN packets without hardware offloading

[Chengen Du]

The issue initially stems from libpcap [1]. 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.

[1] https://github.com/the-tcpdump-group/libpcap/issues/1105

Fixes: f6fb8f100b80 ("af-packet: TPACKET_V3 flexible buffer implementation.")
Signed-off-by: Chengen Du <chengen.du@xxxxxxxxxxxxx>
---
 net/packet/af_packet.c | 18 ++++++++++++++++--
 1 file changed, 16 insertions(+), 2 deletions(-)

diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
index ea3ebc160e25..82b36e90d73b 100644
--- a/net/packet/af_packet.c
+++ b/net/packet/af_packet.c
@@ -1011,6 +1011,10 @@ static void prb_fill_vlan_info(struct tpacket_kbdq_core *pkc,
 		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 (eth_type_vlan(pkc->skb->protocol)) {
+		ppd->hv1.tp_vlan_tci = ntohs(vlan_eth_hdr(pkc->skb)->h_vlan_TCI);
+		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 +2432,10 @@ 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 (eth_type_vlan(skb->protocol)) {
+			h.h2->tp_vlan_tci = ntohs(vlan_eth_hdr(skb)->h_vlan_TCI);
+			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 +2465,8 @@ 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 = (skb->protocol == htons(ETH_P_8021Q)) ?
+		vlan_eth_hdr(skb)->h_vlan_encapsulated_proto : 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 +3491,8 @@ 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 = (skb->protocol == htons(ETH_P_8021Q)) ?
+			vlan_eth_hdr(skb)->h_vlan_encapsulated_proto : skb->protocol;
 	}
 
 	sock_recv_cmsgs(msg, sk, skb);
@@ -3539,6 +3549,10 @@ 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 (eth_type_vlan(skb->protocol)) {
+			aux.tp_vlan_tci = ntohs(vlan_eth_hdr(skb)->h_vlan_TCI);
+			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;
-- 
2.40.1