features: implement feature probe for StructOpsMap

With the libbpf feature probe API being reworked a while back, a probe for
StructOps maps was also implemented. Setting an invalid vmlinux type id
and checking for ENOTSUPP signals the map's availability.

Signed-off-by: Timo Beckers <timo@isovalent.com>

features/map.go

@@ -75,6 +75,10 @@ func createMapTypeAttr(mt ebpf.MapType) *sys.MapCreateAttr {
 		a.BtfKeyTypeId = 1   // BTF_KIND_INT
 		a.BtfValueTypeId = 3 // BTF_KIND_ARRAY
 		a.BtfFd = ^uint32(0)
+	case ebpf.StructOpsMap:
+		// StructOps requires setting a vmlinux type id, but id 1 will always
+		// resolve to some type of integer. This will cause ENOTSUPP.
+		a.BtfVmlinuxValueTypeId = 1
 	}
 
 	return a
@@ -100,14 +104,6 @@ func validateMaptype(mt ebpf.MapType) error {
 	if mt > mt.Max() {
 		return os.ErrInvalid
 	}
-
-	if mt == ebpf.StructOpsMap {
-		// A probe for StructOpsMap has vmlinux BTF requirements we currently
-		// cannot meet. Once we figure out how to add a working probe in this
-		// package, we can remove this check.
-		return errors.New("a probe for MapType StructOpsMap isn't implemented")
-	}
-
 	return nil
 }
 
@@ -131,6 +127,12 @@ func haveMapType(mt ebpf.MapType) error {
 			err = nil
 		}
 
+	// ENOTSUPP means the map type is at least known to the kernel.
+	case errors.Is(err, sys.ENOTSUPP):
+		if mt == ebpf.StructOpsMap {
+			err = nil
+		}
+
 	// EINVAL occurs when attempting to create a map with an unknown type.
 	// E2BIG occurs when MapCreateAttr contains non-zero bytes past the end
 	// of the struct known by the running kernel, meaning the kernel is too old

features/map_test.go

@@ -37,7 +37,7 @@ var mapTypeMinVersion = map[ebpf.MapType]string{
 	ebpf.Stack:               "4.20",
 	ebpf.SkStorage:           "5.2",
 	ebpf.DevMapHash:          "5.4",
-	ebpf.StructOpsMap:        "5.6", // requires vmlinux BTF, skip for now
+	ebpf.StructOpsMap:        "5.6",
 	ebpf.RingBuf:             "5.8",
 	ebpf.InodeStorage:        "5.10",
 	ebpf.TaskStorage:         "5.11",
@@ -55,10 +55,6 @@ func TestHaveMapType(t *testing.T) {
 		feature := fmt.Sprintf("map type %s", mt.String())
 
 		t.Run(mt.String(), func(t *testing.T) {
-			if mt == ebpf.StructOpsMap {
-				t.Skip("Test for map type StructOpsMap requires working probe")
-			}
-
 			testutils.SkipOnOldKernel(t, minVersion, feature)
 
 			if err := HaveMapType(mt); err != nil {
@@ -80,8 +76,4 @@ func TestHaveMapTypeInvalid(t *testing.T) {
 	if err := HaveMapType(ebpf.MapType(math.MaxUint32)); !errors.Is(err, os.ErrInvalid) {
 		t.Fatalf("Expected os.ErrInvalid but was: %v", err)
 	}
-
-	if err := HaveMapType(ebpf.MapType(ebpf.StructOpsMap)); err == nil {
-		t.Fatal("Expected but was nil")
-	}
 }