Move ELF build-id reader into a separate library.

Bug: 328702178
Change-Id: I188a8d20d22e67e4f0c7e3441e3781fff369c828

3 files changed, 298 insertions, 0 deletions

diff --git a/elf/Android.bp b/elf/Android.bp
new file mode 100644
index 000000000..6450be137
--- /dev/null
+++ b/elf/Android.bp
@@ -0,0 +1,28 @@
+// Copyright 2016 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 {
+    default_applicable_licenses: ["Android-Apache-2.0"],
+}
+
+bootstrap_go_package {
+    name: "soong-elf",
+    pkgPath: "android/soong/elf",
+    srcs: [
+        "elf.go",
+    ],
+    testSrcs: [
+        "elf_test.go",
+    ],
+}
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
+}
diff --git a/elf/elf_test.go b/elf/elf_test.go
new file mode 100644
index 000000000..a22077090
--- /dev/null
+++ b/elf/elf_test.go
@@ -0,0 +1,152 @@
+// 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 (
+	"bytes"
+	"debug/elf"
+	"encoding/binary"
+	"reflect"
+	"testing"
+)
+
+func Test_elfIdentifierFromReaderAt_BadElfFile(t *testing.T) {
+	tests := []struct {
+		name     string
+		contents string
+	}{
+		{
+			name:     "empty",
+			contents: "",
+		},
+		{
+			name:     "text",
+			contents: "#!/bin/bash\necho foobar",
+		},
+		{
+			name:     "empty elf",
+			contents: emptyElfFile(),
+		},
+		{
+			name:     "short section header",
+			contents: shortSectionHeaderElfFile(),
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			buf := bytes.NewReader([]byte(tt.contents))
+			_, err := elfIdentifierFromReaderAt(buf, "<>", false)
+			if err == nil {
+				t.Errorf("expected error reading bad elf file without allowMissing")
+			}
+			_, err = elfIdentifierFromReaderAt(buf, "<>", true)
+			if err != nil {
+				t.Errorf("expected no error reading bad elf file with allowMissing, got %q", err.Error())
+			}
+		})
+	}
+}
+
+func Test_readNote(t *testing.T) {
+	note := []byte{
+		0x04, 0x00, 0x00, 0x00,
+		0x10, 0x00, 0x00, 0x00,
+		0x03, 0x00, 0x00, 0x00,
+		0x47, 0x4e, 0x55, 0x00,
+		0xca, 0xaf, 0x44, 0xd2, 0x82, 0x78, 0x68, 0xfe, 0xc0, 0x90, 0xa3, 0x43, 0x85, 0x36, 0x6c, 0xc7,
+	}
+
+	descs, err := readNote(bytes.NewBuffer(note), binary.LittleEndian)
+	if err != nil {
+		t.Fatalf("unexpected error in readNote: %s", err)
+	}
+
+	expectedDescs := map[string][]byte{
+		"GNU\x00": []byte{0xca, 0xaf, 0x44, 0xd2, 0x82, 0x78, 0x68, 0xfe, 0xc0, 0x90, 0xa3, 0x43, 0x85, 0x36, 0x6c, 0xc7},
+	}
+
+	if !reflect.DeepEqual(descs, expectedDescs) {
+		t.Errorf("incorrect return, want %#v got %#v", expectedDescs, descs)
+	}
+}
+
+// emptyElfFile returns an elf file header with no program headers or sections.
+func emptyElfFile() string {
+	ident := [elf.EI_NIDENT]byte{}
+	identBuf := bytes.NewBuffer(ident[0:0:elf.EI_NIDENT])
+	binary.Write(identBuf, binary.LittleEndian, []byte("\x7fELF"))
+	binary.Write(identBuf, binary.LittleEndian, elf.ELFCLASS64)
+	binary.Write(identBuf, binary.LittleEndian, elf.ELFDATA2LSB)
+	binary.Write(identBuf, binary.LittleEndian, elf.EV_CURRENT)
+	binary.Write(identBuf, binary.LittleEndian, elf.ELFOSABI_LINUX)
+	binary.Write(identBuf, binary.LittleEndian, make([]byte, 8))
+
+	header := elf.Header64{
+		Ident:     ident,
+		Type:      uint16(elf.ET_EXEC),
+		Machine:   uint16(elf.EM_X86_64),
+		Version:   uint32(elf.EV_CURRENT),
+		Entry:     0,
+		Phoff:     uint64(binary.Size(elf.Header64{})),
+		Shoff:     uint64(binary.Size(elf.Header64{})),
+		Flags:     0,
+		Ehsize:    uint16(binary.Size(elf.Header64{})),
+		Phentsize: 0x38,
+		Phnum:     0,
+		Shentsize: 0x40,
+		Shnum:     0,
+		Shstrndx:  0,
+	}
+
+	buf := &bytes.Buffer{}
+	binary.Write(buf, binary.LittleEndian, header)
+	return buf.String()
+}
+
+// shortSectionHeader returns an elf file header with a section header that extends past the end of
+// the file.
+func shortSectionHeaderElfFile() string {
+	ident := [elf.EI_NIDENT]byte{}
+	identBuf := bytes.NewBuffer(ident[0:0:elf.EI_NIDENT])
+	binary.Write(identBuf, binary.LittleEndian, []byte("\x7fELF"))
+	binary.Write(identBuf, binary.LittleEndian, elf.ELFCLASS64)
+	binary.Write(identBuf, binary.LittleEndian, elf.ELFDATA2LSB)
+	binary.Write(identBuf, binary.LittleEndian, elf.EV_CURRENT)
+	binary.Write(identBuf, binary.LittleEndian, elf.ELFOSABI_LINUX)
+	binary.Write(identBuf, binary.LittleEndian, make([]byte, 8))
+
+	header := elf.Header64{
+		Ident:     ident,
+		Type:      uint16(elf.ET_EXEC),
+		Machine:   uint16(elf.EM_X86_64),
+		Version:   uint32(elf.EV_CURRENT),
+		Entry:     0,
+		Phoff:     uint64(binary.Size(elf.Header64{})),
+		Shoff:     uint64(binary.Size(elf.Header64{})),
+		Flags:     0,
+		Ehsize:    uint16(binary.Size(elf.Header64{})),
+		Phentsize: 0x38,
+		Phnum:     0,
+		Shentsize: 0x40,
+		Shnum:     1,
+		Shstrndx:  0,
+	}
+
+	buf := &bytes.Buffer{}
+	binary.Write(buf, binary.LittleEndian, header)
+	binary.Write(buf, binary.LittleEndian, []byte{0})
+	return buf.String()
+}