1 files changed, 118 insertions, 0 deletions
diff --git a/elf/elf.go b/elf/elf.go
new file mode 100644
index 000000000..e84a8aeea
--- /dev/null
+++ b/elf/elf.go
@@ -0,0 +1,118 @@
+// Copyright 2022 Google Inc. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package elf
+
+import (
+	"debug/elf"
+	"encoding/binary"
+	"encoding/hex"
+	"errors"
+	"fmt"
+	"io"
+	"os"
+)
+
+const gnuBuildID = "GNU\x00"
+
+// Identifier extracts the elf build ID from an elf file.  If allowMissing is true it returns
+// an empty identifier if the file exists but the build ID note does not.
+func Identifier(filename string, allowMissing bool) (string, error) {
+	f, err := os.Open(filename)
+	if err != nil {
+		return "", fmt.Errorf("failed to open %s: %w", filename, err)
+	}
+	defer f.Close()
+
+	return elfIdentifierFromReaderAt(f, filename, allowMissing)
+}
+
+// elfIdentifierFromReaderAt extracts the elf build ID from a ReaderAt.  If allowMissing is true it
+// returns an empty identifier if the file exists but the build ID note does not.
+func elfIdentifierFromReaderAt(r io.ReaderAt, filename string, allowMissing bool) (string, error) {
+	f, err := elf.NewFile(r)
+	if err != nil {
+		if allowMissing {
+			if errors.Is(err, io.EOF) || errors.Is(err, io.ErrUnexpectedEOF) {
+				return "", nil
+			}
+			if _, ok := err.(*elf.FormatError); ok {
+				// The file was not an elf file.
+				return "", nil
+			}
+		}
+		return "", fmt.Errorf("failed to parse elf file %s: %w", filename, err)
+	}
+	defer f.Close()
+
+	buildIDNote := f.Section(".note.gnu.build-id")
+	if buildIDNote == nil {
+		if allowMissing {
+			return "", nil
+		}
+		return "", fmt.Errorf("failed to find .note.gnu.build-id in  %s", filename)
+	}
+
+	buildIDs, err := readNote(buildIDNote.Open(), f.ByteOrder)
+	if err != nil {
+		return "", fmt.Errorf("failed to read .note.gnu.build-id: %w", err)
+	}
+
+	for name, desc := range buildIDs {
+		if name == gnuBuildID {
+			return hex.EncodeToString(desc), nil
+		}
+	}
+
+	return "", nil
+}
+
+// readNote reads the contents of a note section, returning it as a map from name to descriptor.
+func readNote(note io.Reader, byteOrder binary.ByteOrder) (map[string][]byte, error) {
+	var noteHeader struct {
+		Namesz uint32
+		Descsz uint32
+		Type   uint32
+	}
+
+	notes := make(map[string][]byte)
+	for {
+		err := binary.Read(note, byteOrder, &noteHeader)
+		if err != nil {
+			if err == io.EOF {
+				return notes, nil
+			}
+			return nil, fmt.Errorf("failed to read note header: %w", err)
+		}
+
+		nameBuf := make([]byte, align4(noteHeader.Namesz))
+		err = binary.Read(note, byteOrder, &nameBuf)
+		if err != nil {
+			return nil, fmt.Errorf("failed to read note name: %w", err)
+		}
+		name := string(nameBuf[:noteHeader.Namesz])
+
+		descBuf := make([]byte, align4(noteHeader.Descsz))
+		err = binary.Read(note, byteOrder, &descBuf)
+		if err != nil {
+			return nil, fmt.Errorf("failed to read note desc: %w", err)
+		}
+		notes[name] = descBuf[:noteHeader.Descsz]
+	}
+}
+
+// align4 rounds the input up to the next multiple of 4.
+func align4(i uint32) uint32 {
+	return (i + 3) &^ 3
+}