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Date: Tue, 4 Jun 2024 14:14:37 +0800
From: Chengen Du <chengen.du@...onical.com>
To: 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 v5] af_packet: Handle outgoing VLAN packets without
hardware offloading
Hi Willem,
Thank you for your comments on patch v4.
I have made some modifications and would like to discuss some of your
suggestions in detail.
On Tue, Jun 4, 2024 at 1:50 PM Chengen Du <chengen.du@...onical.com> 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>
> ---
> 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 you mentioned, we need the outermost VLAN tag to fit the protocol
we referenced in tp_vlan_tpid.
In if_vlan.h, __vlan_get_protocol() only provides the protocol and may
affect other usage scenarios if we modify it to also provide TCI.
There is a similar function called __vlan_get_tag(), which also aims
to extract TCI from the L2 header, but it doesn't check the header
size as __vlan_get_protocol() does.
To prevent affecting other usage scenarios, I introduced a new
function to achieve our purpose.
Additionally, the starting point of skb->data differs in SOCK_RAW and
SOCK_DGRAM cases.
I accounted for this by using skb_push to ensure that skb->data starts
from the L2 header.
Would you recommend modifying __vlan_get_tag() to add the size check
logic instead?
If so, we would also need to consider how to integrate the size check
logic into both __vlan_get_protocol() and __vlan_get_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;
> + 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;
> + }
> + }
> +
> + 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
>
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