Re: [PATCH v2 6/8] tap: change signature of function tap_push_l2h()

On Thu, 12 Jun 2025 00:21:50 -0400 Jon Maloy wrote:

In the following commits it must be possible for the callers of funtion tap_push_l2h() to specify which source mac address should be added to the ethernet header sent over the tap interface. As a preparation, we now add a new argument to that function, still without actually using it.

Signed-off-by: Jon Maloy --- tap.c | 18 +++++++++++------- tap.h | 3 ++- tcp.c | 4 ++-- 3 files changed, 15 insertions(+), 10 deletions(-)

diff --git a/tap.c b/tap.c index 6db5d88..634c012 100644 --- a/tap.c +++ b/tap.c @@ -171,17 +171,21 @@ const struct in6_addr *tap_ip6_daddr(const struct ctx *c, * tap_push_l2h() - Build an L2 header for an inbound packet * @c: Execution context * @buf: Buffer address at which to generate header + * @src_mac: MAC address to be used as source for message

Maybe ", NULL means default"?

* @proto: Ethernet protocol number for L3 * * Return: pointer at which to write the packet's payload */ -void *tap_push_l2h(const struct ctx *c, void *buf, uint16_t proto) +void *tap_push_l2h(const struct ctx *c, void *buf, + const void *src_mac, uint16_t proto) { struct ethhdr *eh = (struct ethhdr *)buf;

- /* TODO: ARP table lookup */ memcpy(eh->h_dest, c->guest_mac, ETH_ALEN); - memcpy(eh->h_source, c->our_tap_mac, ETH_ALEN); + if (src_mac) + memcpy(eh->h_source, src_mac, ETH_ALEN); + else + memcpy(eh->h_source, c->our_tap_mac, ETH_ALEN); eh->h_proto = ntohs(proto); return eh + 1; } @@ -261,7 +265,7 @@ void tap_udp4_send(const struct ctx *c, struct in_addr src, in_port_t sport, { size_t l4len = dlen + sizeof(struct udphdr); char buf[USHRT_MAX]; - struct iphdr *ip4h = tap_push_l2h(c, buf, ETH_P_IP); + struct iphdr *ip4h = tap_push_l2h(c, buf, NULL, ETH_P_IP); struct udphdr *uh = tap_push_ip4h(ip4h, src, dst, l4len, IPPROTO_UDP); char *data = tap_push_uh4(uh, src, sport, dst, dport, in, dlen);

@@ -281,7 +285,7 @@ void tap_icmp4_send(const struct ctx *c, struct in_addr src, struct in_addr dst, const void *in, size_t l4len) { char buf[USHRT_MAX]; - struct iphdr *ip4h = tap_push_l2h(c, buf, ETH_P_IP); + struct iphdr *ip4h = tap_push_l2h(c, buf, NULL, ETH_P_IP); struct icmphdr *icmp4h = tap_push_ip4h(ip4h, src, dst, l4len, IPPROTO_ICMP);

@@ -367,7 +371,7 @@ void tap_udp6_send(const struct ctx *c, { size_t l4len = dlen + sizeof(struct udphdr); char buf[USHRT_MAX]; - struct ipv6hdr *ip6h = tap_push_l2h(c, buf, ETH_P_IPV6); + struct ipv6hdr *ip6h = tap_push_l2h(c, buf, NULL, ETH_P_IPV6); struct udphdr *uh = tap_push_ip6h(ip6h, src, dst, l4len, IPPROTO_UDP, flow); char *data = tap_push_uh6(uh, src, sport, dst, dport, in, dlen); @@ -389,7 +393,7 @@ void tap_icmp6_send(const struct ctx *c, const void *in, size_t l4len) { char buf[USHRT_MAX]; - struct ipv6hdr *ip6h = tap_push_l2h(c, buf, ETH_P_IPV6); + struct ipv6hdr *ip6h = tap_push_l2h(c, buf, NULL, ETH_P_IPV6); struct icmp6hdr *icmp6h = tap_push_ip6h(ip6h, src, dst, l4len, IPPROTO_ICMPV6, 0);

diff --git a/tap.h b/tap.h index 6fe3d15..e640d95 100644 --- a/tap.h +++ b/tap.h @@ -70,7 +70,8 @@ static inline void tap_hdr_update(struct tap_hdr *thdr, size_t l2len) }

unsigned long tap_l2_max_len(const struct ctx *c); -void *tap_push_l2h(const struct ctx *c, void *buf, uint16_t proto); +void *tap_push_l2h(const struct ctx *c, void *buf, + const void *src_mac, uint16_t proto); void *tap_push_ip4h(struct iphdr *ip4h, struct in_addr src, struct in_addr dst, size_t l4len, uint8_t proto); void *tap_push_uh4(struct udphdr *uh, struct in_addr src, in_port_t sport, diff --git a/tcp.c b/tcp.c index 777f345..1a32424 100644 --- a/tcp.c +++ b/tcp.c @@ -1898,7 +1898,7 @@ static void tcp_rst_no_conn(const struct ctx *c, int af, return;

if (af == AF_INET) { - struct iphdr *ip4h = tap_push_l2h(c, buf, ETH_P_IP); + struct iphdr *ip4h = tap_push_l2h(c, buf, NULL, ETH_P_IP); const struct in_addr *rst_src = daddr; const struct in_addr *rst_dst = saddr;

@@ -1908,7 +1908,7 @@ static void tcp_rst_no_conn(const struct ctx *c, int af, *rst_src, *rst_dst);

} else { - struct ipv6hdr *ip6h = tap_push_l2h(c, buf, ETH_P_IPV6); + struct ipv6hdr *ip6h = tap_push_l2h(c, buf, NULL, ETH_P_IPV6); const struct in6_addr *rst_src = daddr; const struct in6_addr *rst_dst = saddr;

-- Stefano