Re: [PATCH v5] af_packet: Handle outgoing VLAN packets without hardware offloading

[Willem de Bruijn]

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>
> ---
>  net/packet/af_packet.c | 64 ++++++++++++++++++++++++++++++++++++++++--
>  1 file changed, 62 insertions(+), 2 deletions(-)
> 
> diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
> index ea3ebc160e25..53d51ac87ac6 100644
> --- a/net/packet/af_packet.c
> +++ b/net/packet/af_packet.c
> @@ -538,6 +538,52 @@ 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)
> +{
> +	unsigned int vlan_depth = skb->mac_len;
> +	struct vlan_hdr vhdr, *vh;
> +	u8 *skb_head = skb->data;
> +	int skb_len = skb->len;
> +
> +	if (vlan_depth) {
> +		if (WARN_ON(vlan_depth < VLAN_HLEN))
> +			return 0;
> +		vlan_depth -= VLAN_HLEN;
> +	} else {
> +		vlan_depth = ETH_HLEN;
> +	}
> +
> +	skb_push(skb, skb->data - skb_mac_header(skb));
> +	vh = skb_header_pointer(skb, vlan_depth, sizeof(vhdr), &vhdr);
> +	if (skb_head != skb->data) {
> +		skb->data = skb_head;
> +		skb->len = skb_len;
> +	}
> +	if (unlikely(!vh))
> +		return 0;
> +
> +	return ntohs(vh->h_vlan_TCI);

As stated in the conversation: no need for the vlan_depth code.

skb->data is either at the link layer header or immediately beyond it
(i.e., at the outer vlan tag).

> +}
> +
> +static __be16 vlan_get_protocol_dgram(struct sk_buff *skb)
> +{
> +	__be16 proto = skb->protocol;
> +
> +	if (unlikely(eth_type_vlan(proto))) {
> +		u8 *skb_head = skb->data;

Since skb->head is a different thing from skb->data, please call this
orig_data or so.
> +		int skb_len = skb->len;
> +
> +		skb_push(skb, skb->data - skb_mac_header(skb));
> +		proto = __vlan_get_protocol(skb, proto, NULL);
> +		if (skb_head != skb->data) {
> +			skb->data = skb_head;
> +			skb->len = skb_len;
> +		}
> +	}

I don't see a way around this temporary skb->data mangling, so +1
unless someone else suggests a cleaner way.

On second thought, one option:

This adds some parsing overhead in the datapath. SOCK_RAW does not
need it, as it can see the whole VLAN tag. Perhaps limit the new
branches to SOCK_DGRAM cases? Then the above can also be simplified.

> +
> +	return proto;
> +}
> +
>  static void prb_del_retire_blk_timer(struct tpacket_kbdq_core *pkc)
>  {
>  	del_timer_sync(&pkc->retire_blk_timer);
> @@ -1011,6 +1057,10 @@ 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 (unlikely(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 +2478,10 @@ 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(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 +2511,8 @@ 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 +3537,8 @@ 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 +3595,10 @@ 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(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
>