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// 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, ¬eHeader)
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
}
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