conf: Limit maximum MTU based on backend frame size

The -m option controls the MTU, that is the maximum transmissible L3
datagram, not including L2 headers.  We currently limit it to ETH_MAX_MTU
which sounds like it makes sense.  But ETH_MAX_MTU is confusing: it's not
consistently used as to whether it means the maximum L3 datagram size or
the maximum L2 frame size.  Even within conf() we explicitly account for
the L2 header size when computing the default --mtu value, but not when
we compute the maximum --mtu value.

Clean this up by reworking the maximum MTU computation to be the minimum of
IP_MAX_MTU (65535) and the maximum sized IP datagram which can fit into
our L2 frames when we account for the L2 header.  The latter can vary
depending on our tap backend, although it doesn't right now.

Link: https://bugs.passt.top/show_bug.cgi?id=66
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

2 files changed, 7 insertions, 7 deletions

diff --git a/conf.c b/conf.c
index b58e2a6..c760f79 100644
--- a/conf.c
+++ b/conf.c
@@ -1434,6 +1434,7 @@ void conf(struct ctx *c, int argc, char **argv)
 	enum fwd_ports_mode fwd_default = FWD_NONE;
 	bool v4_only = false, v6_only = false;
 	unsigned dns4_idx = 0, dns6_idx = 0;
+	unsigned long max_mtu = IP_MAX_MTU;
 	struct fqdn *dnss = c->dns_search;
 	unsigned int ifi4 = 0, ifi6 = 0;
 	const char *logfile = NULL;
@@ -1449,7 +1450,9 @@ void conf(struct ctx *c, int argc, char **argv)
 		fwd_default = FWD_AUTO;
 	}
 
-	c->mtu = ROUND_DOWN(ETH_MAX_MTU - ETH_HLEN, sizeof(uint32_t));
+	if (tap_l2_max_len(c) - ETH_HLEN < max_mtu)
+		max_mtu = tap_l2_max_len(c) - ETH_HLEN;
+	c->mtu = ROUND_DOWN(max_mtu, sizeof(uint32_t));
 	c->tcp.fwd_in.mode = c->tcp.fwd_out.mode = FWD_UNSET;
 	c->udp.fwd_in.mode = c->udp.fwd_out.mode = FWD_UNSET;
 	memcpy(c->our_tap_mac, MAC_OUR_LAA, ETH_ALEN);
@@ -1711,9 +1714,9 @@ void conf(struct ctx *c, int argc, char **argv)
 			if (errno || *e)
 				die("Invalid MTU: %s", optarg);
 
-			if (mtu > ETH_MAX_MTU) {
-				die("MTU %lu too large (max %u)",
-				    mtu, ETH_MAX_MTU);
+			if (mtu > max_mtu) {
+				die("MTU %lu too large (max %lu)",
+				    mtu, max_mtu);
 			}
 
 			c->mtu = mtu;
diff --git a/util.h b/util.h
index 0f70f4d..4d512fa 100644
--- a/util.h
+++ b/util.h
@@ -31,9 +31,6 @@
 #ifndef SECCOMP_RET_KILL_PROCESS
 #define SECCOMP_RET_KILL_PROCESS	SECCOMP_RET_KILL
 #endif
-#ifndef ETH_MAX_MTU
-#define ETH_MAX_MTU			USHRT_MAX
-#endif
 #ifndef IP_MAX_MTU
 #define IP_MAX_MTU			USHRT_MAX
 #endif