link: add support for IFLA_GRO_MAX_SIZE

Add support for the new IFLA_GRO_MAX_SIZE attribute which is needed to
enable BIG TCP[1] properly. Define the attribute in the local link
attributes (nl/link_linux.go) because it isn't yet present in Go's
x/sys/unix package. Also add a test for it.

[1] https://patchwork.kernel.org/project/netdevbpf/cover/20220513183408.686447-1-eric.dumazet@gmail.com/

Signed-off-by: Nikolay Aleksandrov <razor@blackwall.org>

link.go

@@ -47,6 +47,7 @@ type LinkAttrs struct {
 	NumRxQueues  int
 	GSOMaxSize   uint32
 	GSOMaxSegs   uint32
+	GROMaxSize   uint32
 	Vfs          []VfInfo // virtual functions available on link
 	Group        uint32
 	Slave        LinkSlave

link_linux.go

@@ -1401,6 +1401,11 @@ func (h *Handle) linkModify(link Link, flags int) error {
 		req.AddData(gsoAttr)
 	}
 
+	if base.GROMaxSize > 0 {
+		groAttr := nl.NewRtAttr(nl.IFLA_GRO_MAX_SIZE, nl.Uint32Attr(base.GROMaxSize))
+		req.AddData(groAttr)
+	}
+
 	if base.Group > 0 {
 		groupAttr := nl.NewRtAttr(unix.IFLA_GROUP, nl.Uint32Attr(base.Group))
 		req.AddData(groupAttr)
@@ -1941,6 +1946,8 @@ func LinkDeserialize(hdr *unix.NlMsghdr, m []byte) (Link, error) {
 			base.GSOMaxSize = native.Uint32(attr.Value[0:4])
 		case unix.IFLA_GSO_MAX_SEGS:
 			base.GSOMaxSegs = native.Uint32(attr.Value[0:4])
+		case nl.IFLA_GRO_MAX_SIZE:
+			base.GROMaxSize = native.Uint32(attr.Value[0:4])
 		case unix.IFLA_VFINFO_LIST:
 			data, err := nl.ParseRouteAttr(attr.Value)
 			if err != nil {

link_test.go

@@ -985,6 +985,32 @@ func TestLinkAddDelDummyWithGSO(t *testing.T) {
 	}
 }
 
+func TestLinkAddDelDummyWithGRO(t *testing.T) {
+	const (
+		groMaxSize = 1 << 14
+	)
+	minKernelRequired(t, 5, 19)
+	tearDown := setUpNetlinkTest(t)
+	defer tearDown()
+
+	dummy := &Dummy{LinkAttrs: LinkAttrs{Name: "foo", GROMaxSize: groMaxSize}}
+	if err := LinkAdd(dummy); err != nil {
+		t.Fatal(err)
+	}
+	link, err := LinkByName("foo")
+	if err != nil {
+		t.Fatal(err)
+	}
+	dummy, ok := link.(*Dummy)
+	if !ok {
+		t.Fatalf("unexpected link type: %T", link)
+	}
+
+	if dummy.GROMaxSize != groMaxSize {
+		t.Fatalf("GROMaxSize is %d, should be %d", dummy.GROMaxSize, groMaxSize)
+	}
+}
+
 func TestLinkAddDummyWithTxQLen(t *testing.T) {
 	tearDown := setUpNetlinkTest(t)
 	defer tearDown()

nl/link_linux.go

@@ -718,3 +718,8 @@ const (
 	IFLA_BAREUDP_MULTIPROTO_MODE
 	IFLA_BAREUDP_MAX = IFLA_BAREUDP_MULTIPROTO_MODE
 )
+
+const (
+	IFLA_UNSPEC       = iota
+	IFLA_GRO_MAX_SIZE = 0x3a
+)