map: parse map_extra field and expose in MapSpec

Bloom filter maps were introduced in Linux 5.16 and use the map_extra field to
specify the number of hash functions (lower 4 bits, 1-15 range). Parse this
field from the ELF and pass it through to the kernel.

Fixes #669

Signed-off-by: nikos.nikolakakis <[email protected]>
Co-authored-by: Timo Beckers <[email protected]>

Author: NoNickeD

elf_reader.go

@@ -877,6 +877,7 @@ func mapSpecFromBTF(es *elfSection, vs *btf.VarSecinfo, def *btf.Struct, spec *b
 		mapType            MapType
 		flags, maxEntries  uint32
 		pinType            PinType
+		mapExtra           uint64
 		innerMapSpec       *MapSpec
 		contents           []MapKV
 		err                error
@@ -1035,7 +1036,10 @@ func mapSpecFromBTF(es *elfSection, vs *btf.VarSecinfo, def *btf.Struct, spec *b
 			}
 
 		case "map_extra":
-			return nil, fmt.Errorf("BTF map definition: field %s: %w", member.Name, ErrNotSupported)
+			mapExtra, err = uintFromBTF(member.Type)
+			if err != nil {
+				return nil, fmt.Errorf("resolving map_extra: %w", err)
+			}
 
 		default:
 			return nil, fmt.Errorf("unrecognized field %s in BTF map definition", member.Name)
@@ -1067,6 +1071,7 @@ func mapSpecFromBTF(es *elfSection, vs *btf.VarSecinfo, def *btf.Struct, spec *b
 		InnerMap:   innerMapSpec,
 		Contents:   contents,
 		Tags:       slices.Clone(v.Tags),
+		MapExtra:   mapExtra,
 	}, nil
 }
 

map.go

@@ -78,6 +78,11 @@ type MapSpec struct {
 	// InnerMap is used as a template for ArrayOfMaps and HashOfMaps
 	InnerMap *MapSpec
 
+	// MapExtra is an opaque field whose meaning is map-specific.
+	//
+	// Available from 5.16.
+	MapExtra uint64
+
 	// Extra trailing bytes found in the ELF map definition when using structs
 	// larger than libbpf's bpf_map_def. nil if no trailing bytes were present.
 	// Must be nil or empty before instantiating the MapSpec into a Map.
@@ -534,6 +539,7 @@ func (spec *MapSpec) createMap(inner *sys.FD) (_ *Map, err error) {
 		MaxEntries: spec.MaxEntries,
 		MapFlags:   spec.Flags,
 		NumaNode:   spec.NumaNode,
+		MapExtra:   spec.MapExtra,
 	}
 
 	if inner != nil {

map_test.go

@@ -93,6 +93,7 @@ func TestMapSpecCopy(t *testing.T) {
 		1,
 		[]MapKV{{1, 2}}, // Can't copy Contents, use value types
 		nil,             // InnerMap
+		0,               // MapExtra
 		bytes.NewReader(nil),
 		&btf.Int{},
 		&btf.Int{},