btf: Add BTF layout header parsing

In kernel 7.1 the BTF header for BTF v1 was extended to now include
two optional fields. These fields hold a offset and length for BTF
layout information, a new BTF feature. When we encountered a BTF header
with these fields set to non-zero values, we would fail to parse the
header and thus fail to parse the BTF data altogether.

This commit adds support for parsing the new header fields, and ignores
them for now. Support for the BTF layout feature will be added in the
future, but this change ensures the latest version of the library
stays compatible with the latest kernels.

Signed-off-by: Dylan Reimerink <dylan.reimerink@isovalent.com>

Author: dylandreimerink

btf/btf.go

@@ -204,7 +204,7 @@ func loadRawSpec(btf []byte, base *Spec) (*Spec, error) {
 		baseStrings = base.strings
 	}
 
-	header, bo, err := parseBTFHeader(btf)
+	header, _, bo, err := parseBTFHeader(btf)
 	if err != nil {
 		return nil, fmt.Errorf("parsing .BTF header: %v", err)
 	}

btf/btf_types.go

@@ -84,14 +84,19 @@ type btfHeader struct {
 	StringLen uint32
 }
 
+type btfLayout struct {
+	Off uint32
+	Len uint32
+}
+
 // parseBTFHeader parses the header of the .BTF section.
-func parseBTFHeader(buf []byte) (*btfHeader, binary.ByteOrder, error) {
+func parseBTFHeader(buf []byte) (*btfHeader, *btfLayout, binary.ByteOrder, error) {
 	var header btfHeader
 	var bo binary.ByteOrder
 	for _, order := range []binary.ByteOrder{binary.LittleEndian, binary.BigEndian} {
 		n, err := binary.Decode(buf, order, &header)
 		if err != nil {
-			return nil, nil, fmt.Errorf("read header: %v", err)
+			return nil, nil, nil, fmt.Errorf("read header: %v", err)
 		}
 
 		if header.Magic != btfMagic {
@@ -104,33 +109,43 @@ func parseBTFHeader(buf []byte) (*btfHeader, binary.ByteOrder, error) {
 	}
 
 	if bo == nil {
-		return nil, nil, fmt.Errorf("no valid BTF header")
+		return nil, nil, nil, fmt.Errorf("no valid BTF header")
 	}
 
 	if header.Version != 1 {
-		return nil, nil, fmt.Errorf("unexpected version %v", header.Version)
+		return nil, nil, nil, fmt.Errorf("unexpected version %v", header.Version)
 	}
 
 	if header.Flags != 0 {
-		return nil, nil, fmt.Errorf("unsupported flags %v", header.Flags)
+		return nil, nil, nil, fmt.Errorf("unsupported flags %v", header.Flags)
 	}
 
 	remainder := int64(header.HdrLen) - int64(binary.Size(&header))
 	if remainder < 0 {
-		return nil, nil, errors.New("header length shorter than btfHeader size")
+		return nil, nil, nil, errors.New("header length shorter than minimum BTF header size")
 	}
 
 	if len(buf) < int(remainder) {
-		return nil, nil, errors.New("header length exceeds available data")
+		return nil, nil, nil, errors.New("header length exceeds available data")
+	}
+
+	var layout btfLayout
+	if remainder >= int64(binary.Size(&btfLayout{})) {
+		n, err := binary.Decode(buf, bo, &layout)
+		if err != nil {
+			return nil, nil, nil, fmt.Errorf("read layout offset and length: %v", err)
+		}
+		buf = buf[n:]
+		remainder -= int64(n)
 	}
 
 	for _, b := range buf[:remainder] {
 		if b != 0 {
-			return nil, nil, errors.New("header contains non-zero trailer")
+			return nil, nil, nil, errors.New("header contains non-zero trailer")
 		}
 	}
 
-	return &header, bo, nil
+	return &header, &layout, bo, nil
 }
 
 // btfType is equivalent to struct btf_type in Documentation/bpf/btf.rst.