Consider ethernet header size when returning MTU.

PiperOrigin-RevId: 834925762

Author: milantracy

pkg/tcpip/link/ethernet/ethernet.go

@@ -54,7 +54,11 @@ func (e *Endpoint) LinkAddress() tcpip.LinkAddress {
 
 // MTU implements stack.LinkEndpoint.
 func (e *Endpoint) MTU() uint32 {
-	return e.Endpoint.MTU()
+	// It prevents upper-layers from sending larger than expected packets.
+	if mtu := e.Endpoint.MTU(); mtu > header.EthernetMinimumSize {
+		return mtu - header.EthernetMinimumSize
+	}
+	return 0
 }
 
 // DeliverNetworkPacket implements stack.NetworkDispatcher.

pkg/tcpip/link/ethernet/ethernet_test.go

@@ -143,13 +143,21 @@ func TestMTU(t *testing.T) {
 			maxFrameSize: 0,
 			expectedMTU:  0,
 		},
+		{
+			maxFrameSize: header.EthernetMinimumSize - 1,
+			expectedMTU:  0,
+		},
 		{
 			maxFrameSize: header.EthernetMinimumSize,
-			expectedMTU:  header.EthernetMinimumSize,
+			expectedMTU:  0,
+		},
+		{
+			maxFrameSize: header.EthernetMinimumSize + 1,
+			expectedMTU:  1,
 		},
 		{
 			maxFrameSize: maxFrameSize,
-			expectedMTU:  maxFrameSize,
+			expectedMTU:  maxFrameSize - header.EthernetMinimumSize,
 		},
 	}
 

test/rtnetlink/linux/setlink_test.sh

@@ -20,7 +20,7 @@ source "$(dirname "$0")/rtnetlink_test.sh"
 # Create a new veth pair in the current namespace and change the MTU.
 ip link add name test_veth01 type veth peer name test_veth02
 ip link set test_veth01 mtu 3000
-ip link show test_veth01 | grep -E "mtu 3000"
+ip link show test_veth01 | grep -E "mtu (2986|3000)"
 ip link del name test_veth01
 # Check that test_veth02 has been destroyed.
 if ! wait_for ! ip link show test_veth02; then

test/syscalls/linux/socket_netlink_route.cc

@@ -16,6 +16,7 @@
 #include <fcntl.h>
 #include <ifaddrs.h>
 #include <linux/fib_rules.h>
+#include <linux/if_ether.h>
 #include <linux/netlink.h>
 #include <linux/rtnetlink.h>
 #include <linux/veth.h>
@@ -320,7 +321,7 @@ TEST_P(NetlinkSetLinkTest, ChangeMTU) {
   EXPECT_NO_ERRNO(NetlinkRequestAckOrError(fd, kSeq, &req, sizeof(req)));
 
   // Update the local loopback_link's MTU to the requested value.
-  loopback_link.mtu = req.mtu;
+  loopback_link.mtu = req.mtu - ETH_HLEN;
   // Verify the new MTU.
   struct searchrequest {
     struct nlmsghdr hdr;