Re: [PATCH v11 7/9] tcp: forward external source MAC address through tap interface

On Sat, 27 Sep 2025 15:25:20 -0400 Jon Maloy wrote:

We forward the incoming mac address through the tap interface when receiving incoming packets from network local hosts.

This is a part of the solution to bug https://bugs.passt.top/show_bug.cgi?id=120

Signed-off-by: Jon Maloy Reviewed-by: David Gibson

--- v3: - Adapted to the signature change in nl_mac_get() function, so that we now consider only the template interface when checking the ARP/NDP table. v4: - Adapted to previous name changes in this series v5: - Added lookup in ARP/NDP cache and/or table on incoming messages in case flow->tap_omac wasn't initialized at flow creation, i.e., the flow was initiated from the guest. v6: - Using MAC_IS_ZERO() instead of mac_undefined() - I ignored David's suggestion to move the test for undefined flow->tap_omac to tcp_{buf,vu}_data_from_sock(), at least for now. It looks like a sub-optimization to first have to look up the flow instance in these calls, and then do the test for MAC_IS_ZERO(), even though we might in theory end up with fewer such calls (how often?). --- passt.c | 7 +++---- passt.h | 3 +-- pasta.c | 2 +- tap.c | 2 +- tcp.c | 13 +++++++++++-- tcp.h | 2 +- tcp_buf.c | 37 +++++++++++++++++-------------------- tcp_internal.h | 4 ++-- tcp_vu.c | 5 ++--- 9 files changed, 39 insertions(+), 36 deletions(-)

diff --git a/passt.c b/passt.c index fdd275a..a2ce2d9 100644 --- a/passt.c +++ b/passt.c @@ -159,11 +159,10 @@ static void timer_init(struct ctx *c, const struct timespec *now) /** * proto_update_l2_buf() - Update scatter-gather L2 buffers in protocol handlers * @eth_d: Ethernet destination address, NULL if unchanged - * @eth_s: Ethernet source address, NULL if unchanged */ -void proto_update_l2_buf(const unsigned char *eth_d, const unsigned char *eth_s) +void proto_update_l2_buf(const unsigned char *eth_d) { - tcp_update_l2_buf(eth_d, eth_s); + tcp_update_l2_buf(eth_d); udp_update_l2_buf(eth_d); }

@@ -314,7 +313,7 @@ int main(int argc, char **argv) if ((!c.no_udp && udp_init(&c)) || (!c.no_tcp && tcp_init(&c))) _exit(EXIT_FAILURE);

- proto_update_l2_buf(c.guest_mac, c.our_tap_mac); + proto_update_l2_buf(c.guest_mac);

if (c.ifi4 && !c.no_dhcp) dhcp_init(); diff --git a/passt.h b/passt.h index 0075eb4..ff0236c 100644 --- a/passt.h +++ b/passt.h @@ -326,7 +326,6 @@ struct ctx { bool migrate_exit; };

-void proto_update_l2_buf(const unsigned char *eth_d, - const unsigned char *eth_s); +void proto_update_l2_buf(const unsigned char *eth_d);

#endif /* PASST_H */ diff --git a/pasta.c b/pasta.c index 687406b..a42cfd8 100644 --- a/pasta.c +++ b/pasta.c @@ -411,7 +411,7 @@ void pasta_ns_conf(struct ctx *c) } }

- proto_update_l2_buf(c->guest_mac, NULL); + proto_update_l2_buf(c->guest_mac); }

/** diff --git a/tap.c b/tap.c index 399eeaa..250a0f6 100644 --- a/tap.c +++ b/tap.c @@ -1101,7 +1101,7 @@ void tap_add_packet(struct ctx *c, struct iov_tail *data, memcpy(c->guest_mac, eh->h_source, ETH_ALEN); debug("New guest MAC address observed: %s", eth_ntop(c->guest_mac, bufmac, sizeof(bufmac))); - proto_update_l2_buf(c->guest_mac, NULL); + proto_update_l2_buf(c->guest_mac); }

switch (ntohs(eh->h_proto)) { diff --git a/tcp.c b/tcp.c index 48b1ef2..af05d35 100644 --- a/tcp.c +++ b/tcp.c @@ -919,6 +919,7 @@ static void tcp_fill_header(struct tcphdr *th,

/** * tcp_fill_headers() - Fill 802.3, IP, TCP headers + * @c: Execution context * @conn: Connection pointer * @taph: tap backend specific header * @ip4h: Pointer to IPv4 header, or NULL @@ -929,14 +930,15 @@ static void tcp_fill_header(struct tcphdr *th, * @seq: Sequence number for this segment * @no_tcp_csum: Do not set TCP checksum */ -void tcp_fill_headers(const struct tcp_tap_conn *conn, - struct tap_hdr *taph, +void tcp_fill_headers(const struct ctx *c, struct tcp_tap_conn *conn, + struct tap_hdr *taph, struct ethhdr *eh,

You now pass a 802.3 header here, but the function documentation isn't updated accordingly.

struct iphdr *ip4h, struct ipv6hdr *ip6h, struct tcphdr *th, struct iov_tail *payload, const uint16_t *ip4_check, uint32_t seq, bool no_tcp_csum)

-- Stefano