Only use callback for GetLinkAddress

GetLinkAddress's callback will be called immediately with a
stack.LinkResolutionResult which will hold the link address
so no need to also return the link address from the function.

Fixes #5151.

PiperOrigin-RevId: 353157857

pkg/tcpip/network/ipv6/ndp_test.go

@@ -162,6 +162,11 @@ func TestStackNDPEndpointInvalidateDefaultRouter(t *testing.T) {
 	}
 }
 
+type linkResolutionResult struct {
+	linkAddr tcpip.LinkAddress
+	ok       bool
+}
+
 // TestNeighorSolicitationWithSourceLinkLayerOption tests that receiving a
 // valid NDP NS message with the Source Link Layer Address option results in a
 // new entry in the link address cache for the sender of the message.
@@ -231,35 +236,28 @@ func TestNeighorSolicitationWithSourceLinkLayerOption(t *testing.T) {
 				Data: hdr.View().ToVectorisedView(),
 			}))
 
-			linkAddr, c, err := s.GetLinkAddress(nicID, lladdr1, lladdr0, ProtocolNumber, nil)
-			if linkAddr != test.expectedLinkAddr {
-				t.Errorf("got link address = %s, want = %s", linkAddr, test.expectedLinkAddr)
+			ch := make(chan stack.LinkResolutionResult, 1)
+			err := s.GetLinkAddress(nicID, lladdr1, lladdr0, ProtocolNumber, func(r stack.LinkResolutionResult) {
+				ch <- r
+			})
+
+			wantInvalid := uint64(0)
+			wantErr := (*tcpip.Error)(nil)
+			wantSucccess := true
+			if len(test.expectedLinkAddr) == 0 {
+				wantInvalid = 1
+				wantErr = tcpip.ErrWouldBlock
+				wantSucccess = false
 			}
 
-			if test.expectedLinkAddr != "" {
-				if err != nil {
-					t.Errorf("s.GetLinkAddress(%d, %s, %s, %d, nil): %s", nicID, lladdr1, lladdr0, ProtocolNumber, err)
-				}
-				if c != nil {
-					t.Errorf("got unexpected channel")
-				}
-
-				// Invalid count should not have increased.
-				if got := invalid.Value(); got != 0 {
-					t.Errorf("got invalid = %d, want = 0", got)
-				}
-			} else {
-				if err != tcpip.ErrWouldBlock {
-					t.Errorf("got s.GetLinkAddress(%d, %s, %s, %d, nil) = (_, _, %v), want = (_, _, %s)", nicID, lladdr1, lladdr0, ProtocolNumber, err, tcpip.ErrWouldBlock)
-				}
-				if c == nil {
-					t.Errorf("expected channel from call to s.GetLinkAddress(%d, %s, %s, %d, nil)", nicID, lladdr1, lladdr0, ProtocolNumber)
-				}
-
-				// Invalid count should have increased.
-				if got := invalid.Value(); got != 1 {
-					t.Errorf("got invalid = %d, want = 1", got)
-				}
+			if err != wantErr {
+				t.Errorf("got s.GetLinkAddress(%d, %s, %s, %d, _) = %s, want = %s", nicID, lladdr1, lladdr0, ProtocolNumber, err, wantErr)
+			}
+			if diff := cmp.Diff(stack.LinkResolutionResult{LinkAddress: test.expectedLinkAddr, Success: wantSucccess}, <-ch); diff != "" {
+				t.Errorf("linkResolutionResult mismatch (-want +got):\n%s", diff)
+			}
+			if got := invalid.Value(); got != wantInvalid {
+				t.Errorf("got invalid = %d, want = %d", got, wantInvalid)
 			}
 		})
 	}
@@ -803,35 +801,28 @@ func TestNeighorAdvertisementWithTargetLinkLayerOption(t *testing.T) {
 				Data: hdr.View().ToVectorisedView(),
 			}))
 
-			linkAddr, c, err := s.GetLinkAddress(nicID, lladdr1, lladdr0, ProtocolNumber, nil)
-			if linkAddr != test.expectedLinkAddr {
-				t.Errorf("got link address = %s, want = %s", linkAddr, test.expectedLinkAddr)
+			ch := make(chan stack.LinkResolutionResult, 1)
+			err := s.GetLinkAddress(nicID, lladdr1, lladdr0, ProtocolNumber, func(r stack.LinkResolutionResult) {
+				ch <- r
+			})
+
+			wantInvalid := uint64(0)
+			wantErr := (*tcpip.Error)(nil)
+			wantSucccess := true
+			if len(test.expectedLinkAddr) == 0 {
+				wantInvalid = 1
+				wantErr = tcpip.ErrWouldBlock
+				wantSucccess = false
 			}
 
-			if test.expectedLinkAddr != "" {
-				if err != nil {
-					t.Errorf("s.GetLinkAddress(%d, %s, %s, %d, nil): %s", nicID, lladdr1, lladdr0, ProtocolNumber, err)
-				}
-				if c != nil {
-					t.Errorf("got unexpected channel")
-				}
-
-				// Invalid count should not have increased.
-				if got := invalid.Value(); got != 0 {
-					t.Errorf("got invalid = %d, want = 0", got)
-				}
-			} else {
-				if err != tcpip.ErrWouldBlock {
-					t.Errorf("got s.GetLinkAddress(%d, %s, %s, %d, nil) = (_, _, %v), want = (_, _, %s)", nicID, lladdr1, lladdr0, ProtocolNumber, err, tcpip.ErrWouldBlock)
-				}
-				if c == nil {
-					t.Errorf("expected channel from call to s.GetLinkAddress(%d, %s, %s, %d, nil)", nicID, lladdr1, lladdr0, ProtocolNumber)
-				}
-
-				// Invalid count should have increased.
-				if got := invalid.Value(); got != 1 {
-					t.Errorf("got invalid = %d, want = 1", got)
-				}
+			if err != wantErr {
+				t.Errorf("got s.GetLinkAddress(%d, %s, %s, %d, _) = %s, want = %s", nicID, lladdr1, lladdr0, ProtocolNumber, err, wantErr)
+			}
+			if diff := cmp.Diff(stack.LinkResolutionResult{LinkAddress: test.expectedLinkAddr, Success: wantSucccess}, <-ch); diff != "" {
+				t.Errorf("linkResolutionResult mismatch (-want +got):\n%s", diff)
+			}
+			if got := invalid.Value(); got != wantInvalid {
+				t.Errorf("got invalid = %d, want = %d", got, wantInvalid)
 			}
 		})
 	}

pkg/tcpip/stack/linkaddrcache.go

@@ -97,12 +97,13 @@ type linkAddrEntry struct {
 	done chan struct{}
 
 	// onResolve is called with the result of address resolution.
-	onResolve []func(tcpip.LinkAddress, bool)
+	onResolve []func(LinkResolutionResult)
 }
 
 func (e *linkAddrEntry) notifyCompletionLocked(linkAddr tcpip.LinkAddress) {
+	res := LinkResolutionResult{LinkAddress: linkAddr, Success: len(linkAddr) != 0}
 	for _, callback := range e.onResolve {
-		callback(linkAddr, len(linkAddr) != 0)
+		callback(res)
 	}
 	e.onResolve = nil
 	if ch := e.done; ch != nil {
@@ -196,7 +197,7 @@ func (c *linkAddrCache) getOrCreateEntryLocked(k tcpip.Address) *linkAddrEntry {
 }
 
 // get reports any known link address for k.
-func (c *linkAddrCache) get(k tcpip.Address, linkRes LinkAddressResolver, localAddr tcpip.Address, nic NetworkInterface, onResolve func(tcpip.LinkAddress, bool)) (tcpip.LinkAddress, <-chan struct{}, *tcpip.Error) {
+func (c *linkAddrCache) get(k tcpip.Address, linkRes LinkAddressResolver, localAddr tcpip.Address, nic NetworkInterface, onResolve func(LinkResolutionResult)) (tcpip.LinkAddress, <-chan struct{}, *tcpip.Error) {
 	c.cache.Lock()
 	defer c.cache.Unlock()
 	entry := c.getOrCreateEntryLocked(k)
@@ -208,7 +209,7 @@ func (c *linkAddrCache) get(k tcpip.Address, linkRes LinkAddressResolver, localA
 		if !time.Now().After(entry.expiration) {
 			// Not expired.
 			if onResolve != nil {
-				onResolve(entry.linkAddr, true)
+				onResolve(LinkResolutionResult{LinkAddress: entry.linkAddr, Success: true})
 			}
 			return entry.linkAddr, nil, nil
 		}

pkg/tcpip/stack/neighbor_cache.go

@@ -126,7 +126,7 @@ func (n *neighborCache) getOrCreateEntry(remoteAddr tcpip.Address, linkRes LinkA
 // packet prompting NUD/link address resolution.
 //
 // TODO(gvisor.dev/issue/5151): Don't return the neighbor entry.
-func (n *neighborCache) entry(remoteAddr, localAddr tcpip.Address, linkRes LinkAddressResolver, onResolve func(tcpip.LinkAddress, bool)) (NeighborEntry, <-chan struct{}, *tcpip.Error) {
+func (n *neighborCache) entry(remoteAddr, localAddr tcpip.Address, linkRes LinkAddressResolver, onResolve func(LinkResolutionResult)) (NeighborEntry, <-chan struct{}, *tcpip.Error) {
 	entry := n.getOrCreateEntry(remoteAddr, linkRes)
 	entry.mu.Lock()
 	defer entry.mu.Unlock()
@@ -142,7 +142,7 @@ func (n *neighborCache) entry(remoteAddr, localAddr tcpip.Address, linkRes LinkA
 		//   a node continues sending packets to that neighbor using the cached
 		//   link-layer address."
 		if onResolve != nil {
-			onResolve(entry.neigh.LinkAddr, true)
+			onResolve(LinkResolutionResult{LinkAddress: entry.neigh.LinkAddr, Success: true})
 		}
 		return entry.neigh, nil, nil
 	case Unknown, Incomplete, Failed:

pkg/tcpip/stack/neighbor_cache_test.go

@@ -1188,12 +1188,9 @@ func TestNeighborCacheKeepFrequentlyUsed(t *testing.T) {
 		if !ok {
 			t.Fatalf("store.entry(%d) not found", i)
 		}
-		_, ch, err := neigh.entry(entry.Addr, "", linkRes, func(linkAddr tcpip.LinkAddress, ok bool) {
-			if !ok {
-				t.Fatal("expected successful address resolution")
-			}
-			if linkAddr != entry.LinkAddr {
-				t.Fatalf("got linkAddr = %s, want = %s", linkAddr, entry.LinkAddr)
+		_, ch, err := neigh.entry(entry.Addr, "", linkRes, func(r LinkResolutionResult) {
+			if diff := cmp.Diff(LinkResolutionResult{LinkAddress: entry.LinkAddr, Success: true}, r); diff != "" {
+				t.Fatalf("got link resolution result mismatch (-want +got):\n%s", diff)
 			}
 		})
 		if err != tcpip.ErrWouldBlock {
@@ -1247,12 +1244,9 @@ func TestNeighborCacheKeepFrequentlyUsed(t *testing.T) {
 			t.Fatalf("store.entry(%d) not found", i)
 		}
 
-		_, ch, err := neigh.entry(entry.Addr, "", linkRes, func(linkAddr tcpip.LinkAddress, ok bool) {
-			if !ok {
-				t.Fatal("expected successful address resolution")
-			}
-			if linkAddr != entry.LinkAddr {
-				t.Fatalf("got linkAddr = %s, want = %s", linkAddr, entry.LinkAddr)
+		_, ch, err := neigh.entry(entry.Addr, "", linkRes, func(r LinkResolutionResult) {
+			if diff := cmp.Diff(LinkResolutionResult{LinkAddress: entry.LinkAddr, Success: true}, r); diff != "" {
+				t.Fatalf("got link resolution result mismatch (-want +got):\n%s", diff)
 			}
 		})
 		if err != tcpip.ErrWouldBlock {
@@ -1423,12 +1417,9 @@ func TestNeighborCacheReplace(t *testing.T) {
 		t.Fatal("store.entry(0) not found")
 	}
 
-	_, ch, err := neigh.entry(entry.Addr, "", linkRes, func(linkAddr tcpip.LinkAddress, ok bool) {
-		if !ok {
-			t.Fatal("expected successful address resolution")
-		}
-		if linkAddr != entry.LinkAddr {
-			t.Fatalf("got linkAddr = %s, want = %s", linkAddr, entry.LinkAddr)
+	_, ch, err := neigh.entry(entry.Addr, "", linkRes, func(r LinkResolutionResult) {
+		if diff := cmp.Diff(LinkResolutionResult{LinkAddress: entry.LinkAddr, Success: true}, r); diff != "" {
+			t.Fatalf("got link resolution result mismatch (-want +got):\n%s", diff)
 		}
 	})
 	if err != tcpip.ErrWouldBlock {
@@ -1539,12 +1530,9 @@ func TestNeighborCacheResolutionFailed(t *testing.T) {
 
 	// First, sanity check that resolution is working
 	{
-		_, ch, err := neigh.entry(entry.Addr, "", linkRes, func(linkAddr tcpip.LinkAddress, ok bool) {
-			if !ok {
-				t.Fatal("expected successful address resolution")
-			}
-			if linkAddr != entry.LinkAddr {
-				t.Fatalf("got linkAddr = %s, want = %s", linkAddr, entry.LinkAddr)
+		_, ch, err := neigh.entry(entry.Addr, "", linkRes, func(r LinkResolutionResult) {
+			if diff := cmp.Diff(LinkResolutionResult{LinkAddress: entry.LinkAddr, Success: true}, r); diff != "" {
+				t.Fatalf("got link resolution result mismatch (-want +got):\n%s", diff)
 			}
 		})
 		if err != tcpip.ErrWouldBlock {
@@ -1576,15 +1564,9 @@ func TestNeighborCacheResolutionFailed(t *testing.T) {
 
 	entry.Addr += "2"
 	{
-		_, ch, err := neigh.entry(entry.Addr, "", linkRes, func(linkAddr tcpip.LinkAddress, ok bool) {
-			if ok {
-				t.Error("expected unsuccessful address resolution")
-			}
-			if len(linkAddr) != 0 {
-				t.Fatalf("got linkAddr = %s, want = \"\"", linkAddr)
-			}
-			if t.Failed() {
-				t.FailNow()
+		_, ch, err := neigh.entry(entry.Addr, "", linkRes, func(r LinkResolutionResult) {
+			if diff := cmp.Diff(LinkResolutionResult{Success: false}, r); diff != "" {
+				t.Fatalf("got link resolution result mismatch (-want +got):\n%s", diff)
 			}
 		})
 		if err != tcpip.ErrWouldBlock {
@@ -1627,15 +1609,9 @@ func TestNeighborCacheResolutionTimeout(t *testing.T) {
 		t.Fatal("store.entry(0) not found")
 	}
 
-	_, ch, err := neigh.entry(entry.Addr, "", linkRes, func(linkAddr tcpip.LinkAddress, ok bool) {
-		if ok {
-			t.Error("expected unsuccessful address resolution")
-		}
-		if len(linkAddr) != 0 {
-			t.Fatalf("got linkAddr = %s, want = \"\"", linkAddr)
-		}
-		if t.Failed() {
-			t.FailNow()
+	_, ch, err := neigh.entry(entry.Addr, "", linkRes, func(r LinkResolutionResult) {
+		if diff := cmp.Diff(LinkResolutionResult{Success: false}, r); diff != "" {
+			t.Fatalf("got link resolution result mismatch (-want +got):\n%s", diff)
 		}
 	})
 	if err != tcpip.ErrWouldBlock {
@@ -1674,15 +1650,9 @@ func TestNeighborCacheRetryResolution(t *testing.T) {
 
 	// Perform address resolution with a faulty link, which will fail.
 	{
-		_, ch, err := neigh.entry(entry.Addr, "", linkRes, func(linkAddr tcpip.LinkAddress, ok bool) {
-			if ok {
-				t.Error("expected unsuccessful address resolution")
-			}
-			if len(linkAddr) != 0 {
-				t.Fatalf("got linkAddr = %s, want = \"\"", linkAddr)
-			}
-			if t.Failed() {
-				t.FailNow()
+		_, ch, err := neigh.entry(entry.Addr, "", linkRes, func(r LinkResolutionResult) {
+			if diff := cmp.Diff(LinkResolutionResult{Success: false}, r); diff != "" {
+				t.Fatalf("got link resolution result mismatch (-want +got):\n%s", diff)
 			}
 		})
 		if err != tcpip.ErrWouldBlock {
@@ -1713,9 +1683,9 @@ func TestNeighborCacheRetryResolution(t *testing.T) {
 	// Retry address resolution with a working link.
 	linkRes.dropReplies = false
 	{
-		incompleteEntry, ch, err := neigh.entry(entry.Addr, "", linkRes, func(linkAddr tcpip.LinkAddress, ok bool) {
-			if linkAddr != entry.LinkAddr {
-				t.Fatalf("got linkAddr = %s, want = %s", linkAddr, entry.LinkAddr)
+		incompleteEntry, ch, err := neigh.entry(entry.Addr, "", linkRes, func(r LinkResolutionResult) {
+			if diff := cmp.Diff(LinkResolutionResult{LinkAddress: entry.LinkAddr, Success: true}, r); diff != "" {
+				t.Fatalf("got link resolution result mismatch (-want +got):\n%s", diff)
 			}
 		})
 		if err != tcpip.ErrWouldBlock {
@@ -1772,12 +1742,9 @@ func BenchmarkCacheClear(b *testing.B) {
 				b.Fatalf("store.entry(%d) not found", i)
 			}
 
-			_, ch, err := neigh.entry(entry.Addr, "", linkRes, func(linkAddr tcpip.LinkAddress, ok bool) {
-				if !ok {
-					b.Fatal("expected successful address resolution")
-				}
-				if linkAddr != entry.LinkAddr {
-					b.Fatalf("got linkAddr = %s, want = %s", linkAddr, entry.LinkAddr)
+			_, ch, err := neigh.entry(entry.Addr, "", linkRes, func(r LinkResolutionResult) {
+				if diff := cmp.Diff(LinkResolutionResult{LinkAddress: entry.LinkAddr, Success: true}, r); diff != "" {
+					b.Fatalf("got link resolution result mismatch (-want +got):\n%s", diff)
 				}
 			})
 			if err != tcpip.ErrWouldBlock {

pkg/tcpip/stack/neighbor_entry.go

@@ -96,7 +96,7 @@ type neighborEntry struct {
 	done chan struct{}
 
 	// onResolve is called with the result of address resolution.
-	onResolve []func(tcpip.LinkAddress, bool)
+	onResolve []func(LinkResolutionResult)
 
 	isRouter bool
 	job      *tcpip.Job
@@ -143,8 +143,9 @@ func newStaticNeighborEntry(nic *NIC, addr tcpip.Address, linkAddr tcpip.LinkAdd
 //
 // Precondition: e.mu MUST be locked.
 func (e *neighborEntry) notifyCompletionLocked(succeeded bool) {
+	res := LinkResolutionResult{LinkAddress: e.neigh.LinkAddr, Success: succeeded}
 	for _, callback := range e.onResolve {
-		callback(e.neigh.LinkAddr, succeeded)
+		callback(res)
 	}
 	e.onResolve = nil
 	if ch := e.done; ch != nil {

pkg/tcpip/stack/nic.go

@@ -561,7 +561,7 @@ func (n *NIC) removeAddress(addr tcpip.Address) *tcpip.Error {
 	return tcpip.ErrBadLocalAddress
 }
 
-func (n *NIC) getNeighborLinkAddress(addr, localAddr tcpip.Address, linkRes LinkAddressResolver, onResolve func(tcpip.LinkAddress, bool)) (tcpip.LinkAddress, <-chan struct{}, *tcpip.Error) {
+func (n *NIC) getNeighborLinkAddress(addr, localAddr tcpip.Address, linkRes LinkAddressResolver, onResolve func(LinkResolutionResult)) (tcpip.LinkAddress, <-chan struct{}, *tcpip.Error) {
 	if n.neigh != nil {
 		entry, ch, err := n.neigh.entry(addr, localAddr, linkRes, onResolve)
 		return entry.LinkAddr, ch, err

pkg/tcpip/stack/route.go

@@ -347,7 +347,7 @@ func (r *Route) ResolvedFields(afterResolve func()) (RouteInfo, <-chan struct{},
 		linkAddressResolutionRequestLocalAddr = r.LocalAddress
 	}
 
-	linkAddr, ch, err := r.outgoingNIC.getNeighborLinkAddress(nextAddr, linkAddressResolutionRequestLocalAddr, r.linkRes, func(tcpip.LinkAddress, bool) {
+	linkAddr, ch, err := r.outgoingNIC.getNeighborLinkAddress(nextAddr, linkAddressResolutionRequestLocalAddr, r.linkRes, func(LinkResolutionResult) {
 		if afterResolve != nil {
 			afterResolve()
 		}

pkg/tcpip/stack/stack.go

@@ -1527,10 +1527,14 @@ func (s *Stack) AddLinkAddress(nicID tcpip.NICID, neighbor tcpip.Address, linkAd
 	return nil
 }
 
+// LinkResolutionResult is the result of a link address resolution attempt.
+type LinkResolutionResult struct {
+	LinkAddress tcpip.LinkAddress
+	Success     bool
+}
+
 // GetLinkAddress finds the link address corresponding to a neighbor's address.
 //
-// Returns a link address for the remote address, if readily available.
-//
 // Returns ErrNotSupported if the stack is not configured with a link address
 // resolver for the specified network protocol.
 //
@@ -1538,30 +1542,28 @@ func (s *Stack) AddLinkAddress(nicID tcpip.NICID, neighbor tcpip.Address, linkAd
 // with a notification channel for the caller to block on. Triggers address
 // resolution asynchronously.
 //
-// If onResolve is provided, it will be called either immediately, if
-// resolution is not required, or when address resolution is complete, with
-// the resolved link address and whether resolution succeeded. After any
-// callbacks have been called, the returned notification channel is closed.
+// onResolve will be called either immediately, if resolution is not required,
+// or when address resolution is complete, with the resolved link address and
+// whether resolution succeeded.
 //
 // If specified, the local address must be an address local to the interface
 // the neighbor cache belongs to. The local address is the source address of
 // a packet prompting NUD/link address resolution.
-//
-// TODO(gvisor.dev/issue/5151): Don't return the link address.
-func (s *Stack) GetLinkAddress(nicID tcpip.NICID, addr, localAddr tcpip.Address, protocol tcpip.NetworkProtocolNumber, onResolve func(tcpip.LinkAddress, bool)) (tcpip.LinkAddress, <-chan struct{}, *tcpip.Error) {
+func (s *Stack) GetLinkAddress(nicID tcpip.NICID, addr, localAddr tcpip.Address, protocol tcpip.NetworkProtocolNumber, onResolve func(LinkResolutionResult)) *tcpip.Error {
 	s.mu.RLock()
 	nic, ok := s.nics[nicID]
 	s.mu.RUnlock()
 	if !ok {
-		return "", nil, tcpip.ErrUnknownNICID
+		return tcpip.ErrUnknownNICID
 	}
 
 	linkRes, ok := s.linkAddrResolvers[protocol]
 	if !ok {
-		return "", nil, tcpip.ErrNotSupported
+		return tcpip.ErrNotSupported
 	}
 
-	return nic.getNeighborLinkAddress(addr, localAddr, linkRes, onResolve)
+	_, _, err := nic.getNeighborLinkAddress(addr, localAddr, linkRes, onResolve)
+	return err
 }
 
 // Neighbors returns all IP to MAC address associations.

pkg/tcpip/stack/stack_test.go

@@ -4384,10 +4384,10 @@ func TestGetLinkAddressErrors(t *testing.T) {
 		t.Fatalf("CreateNIC(%d, _) = %s", nicID, err)
 	}
 
-	if addr, _, err := s.GetLinkAddress(unknownNICID, "", "", ipv4.ProtocolNumber, nil); err != tcpip.ErrUnknownNICID {
-		t.Errorf("got s.GetLinkAddress(%d, '', '', %d, nil) = (%s, _, %s), want = (_, _, %s)", unknownNICID, ipv4.ProtocolNumber, addr, err, tcpip.ErrUnknownNICID)
+	if err := s.GetLinkAddress(unknownNICID, "", "", ipv4.ProtocolNumber, nil); err != tcpip.ErrUnknownNICID {
+		t.Errorf("got s.GetLinkAddress(%d, '', '', %d, nil) = %s, want = %s", unknownNICID, ipv4.ProtocolNumber, err, tcpip.ErrUnknownNICID)
 	}
-	if addr, _, err := s.GetLinkAddress(nicID, "", "", ipv4.ProtocolNumber, nil); err != tcpip.ErrNotSupported {
-		t.Errorf("got s.GetLinkAddress(%d, '', '', %d, nil) = (%s, _, %s), want = (_, _, %s)", unknownNICID, ipv4.ProtocolNumber, addr, err, tcpip.ErrNotSupported)
+	if err := s.GetLinkAddress(nicID, "", "", ipv4.ProtocolNumber, nil); err != tcpip.ErrNotSupported {
+		t.Errorf("got s.GetLinkAddress(%d, '', '', %d, nil) = %s, want = %s", unknownNICID, ipv4.ProtocolNumber, err, tcpip.ErrNotSupported)
 	}
 }

pkg/tcpip/tests/integration/link_resolution_test.go

@@ -406,20 +406,34 @@ func TestGetLinkAddress(t *testing.T) {
 	)
 
 	tests := []struct {
-		name             string
-		netProto         tcpip.NetworkProtocolNumber
-		remoteAddr       tcpip.Address
-		expectedLinkAddr bool
+		name       string
+		netProto   tcpip.NetworkProtocolNumber
+		remoteAddr tcpip.Address
+		expectedOk bool
 	}{
 		{
-			name:       "IPv4",
+			name:       "IPv4 resolvable",
 			netProto:   ipv4.ProtocolNumber,
 			remoteAddr: ipv4Addr2.AddressWithPrefix.Address,
+			expectedOk: true,
 		},
 		{
-			name:       "IPv6",
+			name:       "IPv6 resolvable",
 			netProto:   ipv6.ProtocolNumber,
 			remoteAddr: ipv6Addr2.AddressWithPrefix.Address,
+			expectedOk: true,
+		},
+		{
+			name:       "IPv4 not resolvable",
+			netProto:   ipv4.ProtocolNumber,
+			remoteAddr: ipv4Addr3.AddressWithPrefix.Address,
+			expectedOk: false,
+		},
+		{
+			name:       "IPv6 not resolvable",
+			netProto:   ipv6.ProtocolNumber,
+			remoteAddr: ipv6Addr3.AddressWithPrefix.Address,
+			expectedOk: false,
 		},
 	}
 
@@ -434,24 +448,18 @@ func TestGetLinkAddress(t *testing.T) {
 
 					host1Stack, _ := setupStack(t, stackOpts, host1NICID, host2NICID)
 
-					for i := 0; i < 2; i++ {
-						addr, ch, err := host1Stack.GetLinkAddress(host1NICID, test.remoteAddr, "", test.netProto, func(tcpip.LinkAddress, bool) {})
-						var want *tcpip.Error
-						if i == 0 {
-							want = tcpip.ErrWouldBlock
-						}
-						if err != want {
-							t.Fatalf("got host1Stack.GetLinkAddress(%d, %s, '', %d, _) = (%s, _, %s), want = (_, _, %s)", host1NICID, test.remoteAddr, test.netProto, addr, err, want)
-						}
-
-						if i == 0 {
-							<-ch
-							continue
-						}
-
-						if addr != linkAddr2 {
-							t.Fatalf("got addr = %s, want = %s", addr, linkAddr2)
-						}
+					ch := make(chan stack.LinkResolutionResult, 1)
+					if err := host1Stack.GetLinkAddress(host1NICID, test.remoteAddr, "", test.netProto, func(r stack.LinkResolutionResult) {
+						ch <- r
+					}); err != tcpip.ErrWouldBlock {
+						t.Fatalf("got host1Stack.GetLinkAddress(%d, %s, '', %d, _) = %s, want = %s", host1NICID, test.remoteAddr, test.netProto, err, tcpip.ErrWouldBlock)
+					}
+					wantRes := stack.LinkResolutionResult{Success: test.expectedOk}
+					if test.expectedOk {
+						wantRes.LinkAddress = linkAddr2
+					}
+					if diff := cmp.Diff(wantRes, <-ch); diff != "" {
+						t.Fatalf("link resolution result mismatch (-want +got):\n%s", diff)
 					}
 				})
 			}