Generate .build-id directory tree after every build.
The .build-id directory tree is as described here: https://fedoraproject.org/wiki/RolandMcGrath/BuildID Tools such as llvm-symbolizer understand this tree and can use it to look up symbols by build ID. This CL adds a post-build step that updates the .build-id directory under symbols after every build if necessary to contain symlinks to the corresponding symbols file. I also considered adding this as a build step after copying the symbol file to the symbols directory. However this would be complicated by the fact that many binaries have the same build-id such as the various copies of libc++ for the different APEXes. As a result it would be difficult to implement a .build-id updater that produces a correct symlink tree while taking into account concurrent updates to the same symlink by different build steps. This situation is resolved in the .build-id directory generator by having the symlink point to the lexically smallest path out of all paths with the same build-id. I measured the performance of the UpdateBuildIdDir function by writing a standalone program that just calls the function and running it over a copy of the symbols directory for a specific target. On my machine the execution time of the program when simulating various scenarios was as follows: Null build [1]: 36.2 ms ± 3.4 ms Initial build [2]: 162.3 ms ± 6.6 ms Single update [3]: 128.0 ms ± 6.1 ms Invalid .build-id dir [4]: 222.6 ms ± 8.6 ms This is with some improvements that have been contributed to the Go standard library [5,6]; without those improvements a null build is 37.9 ms ± 4.4 ms and a single update is 143.9 ms ± 4.5 ms. [1] hyperfine './update-build-id ~/2/test-symbols2/' [2] hyperfine -p 'rm -rf ~/2/test-symbols2/.build-id' './update-build-id ~/2/test-symbols2/' [3] hyperfine -p 'dd if=/dev/urandom of=$HOME/2/test-symbols2/system/bin/init conv=notrunc seek=808 bs=1 count=16' './update-build-id ~/2/test-symbols2/' [4] hyperfine -p 'touch ~/2/test-symbols2/.build-id/corrupt; sleep 0.1; touch ~/2/test-symbols2/system/bin/init' './update-build-id ~/2/test-symbols2/' [5] https://go.dev/cl/570877 [6] https://go.dev/cl/571436 Bug: 328702178 Change-Id: I8fc0ea81bd31ec80d6b912ba477e2e24b6b05f68
This commit is contained in:
Peter Collingbourne
@@ -20,6 +20,7 @@ bootstrap_go_package {
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name: "soong-elf",
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pkgPath: "android/soong/elf",
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srcs: [
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"build_id_dir.go",
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"elf.go",
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],
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testSrcs: [
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172
elf/build_id_dir.go
Normal file
172
elf/build_id_dir.go
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@@ -0,0 +1,172 @@
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// Copyright 2024 Google Inc. All rights reserved.
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//
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// Licensed under the Apache License, Version 2.0 (the "License");
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// you may not use this file except in compliance with the License.
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// You may obtain a copy of the License at
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//
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// http://www.apache.org/licenses/LICENSE-2.0
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//
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// Unless required by applicable law or agreed to in writing, software
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// distributed under the License is distributed on an "AS IS" BASIS,
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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// See the License for the specific language governing permissions and
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// limitations under the License.
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package elf
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import (
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"io/fs"
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"os"
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"path/filepath"
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"strings"
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"sync"
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"time"
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)
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func UpdateBuildIdDir(path string) error {
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path = filepath.Clean(path)
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buildIdPath := path + "/.build-id"
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// Collect the list of files and build-id symlinks. If the symlinks are
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// up to date (newer than the symbol files), there is nothing to do.
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var buildIdFiles, symbolFiles []string
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var buildIdMtime, symbolsMtime time.Time
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filepath.WalkDir(path, func(path string, entry fs.DirEntry, err error) error {
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if entry == nil || entry.IsDir() {
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return nil
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}
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info, err := entry.Info()
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if err != nil {
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return err
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}
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mtime := info.ModTime()
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if strings.HasPrefix(path, buildIdPath) {
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if buildIdMtime.Compare(mtime) < 0 {
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buildIdMtime = mtime
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}
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buildIdFiles = append(buildIdFiles, path)
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} else {
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if symbolsMtime.Compare(mtime) < 0 {
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symbolsMtime = mtime
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}
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symbolFiles = append(symbolFiles, path)
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}
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return nil
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})
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if symbolsMtime.Compare(buildIdMtime) < 0 {
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return nil
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}
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// Collect build-id -> file mapping from ELF files in the symbols directory.
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concurrency := 8
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done := make(chan error)
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buildIdToFile := make(map[string]string)
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var mu sync.Mutex
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for i := 0; i != concurrency; i++ {
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go func(paths []string) {
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for _, path := range paths {
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id, err := Identifier(path, true)
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if err != nil {
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done <- err
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return
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}
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if id == "" {
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continue
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}
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mu.Lock()
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oldPath := buildIdToFile[id]
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if oldPath == "" || oldPath > path {
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buildIdToFile[id] = path
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}
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mu.Unlock()
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}
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done <- nil
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}(symbolFiles[len(symbolFiles)*i/concurrency : len(symbolFiles)*(i+1)/concurrency])
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}
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// Collect previously generated build-id -> file mapping from the .build-id directory.
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// We will use this for incremental updates. If we see anything in the .build-id
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// directory that we did not expect, we'll delete it and start over.
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prevBuildIdToFile := make(map[string]string)
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out:
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for _, buildIdFile := range buildIdFiles {
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if !strings.HasSuffix(buildIdFile, ".debug") {
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prevBuildIdToFile = nil
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break
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}
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buildId := buildIdFile[len(buildIdPath)+1 : len(buildIdFile)-6]
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for i, ch := range buildId {
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if i == 2 {
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if ch != '/' {
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prevBuildIdToFile = nil
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break out
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}
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} else {
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if (ch < '0' || ch > '9') && (ch < 'a' || ch > 'f') {
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prevBuildIdToFile = nil
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break out
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}
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}
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}
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target, err := os.Readlink(buildIdFile)
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if err != nil || !strings.HasPrefix(target, "../../") {
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prevBuildIdToFile = nil
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break
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}
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prevBuildIdToFile[buildId[0:2]+buildId[3:]] = path + target[5:]
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}
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if prevBuildIdToFile == nil {
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err := os.RemoveAll(buildIdPath)
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if err != nil {
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return err
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}
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prevBuildIdToFile = make(map[string]string)
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}
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// Wait for build-id collection from ELF files to finish.
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for i := 0; i != concurrency; i++ {
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err := <-done
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if err != nil {
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return err
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}
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}
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// Delete old symlinks.
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for id, _ := range prevBuildIdToFile {
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if buildIdToFile[id] == "" {
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symlinkDir := buildIdPath + "/" + id[:2]
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symlinkPath := symlinkDir + "/" + id[2:] + ".debug"
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if err := os.Remove(symlinkPath); err != nil {
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return err
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}
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}
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}
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// Add new symlinks and update changed symlinks.
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for id, path := range buildIdToFile {
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prevPath := prevBuildIdToFile[id]
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if prevPath == path {
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continue
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}
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symlinkDir := buildIdPath + "/" + id[:2]
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symlinkPath := symlinkDir + "/" + id[2:] + ".debug"
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if prevPath == "" {
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if err := os.MkdirAll(symlinkDir, 0755); err != nil {
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return err
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}
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} else {
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if err := os.Remove(symlinkPath); err != nil {
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return err
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}
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}
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target, err := filepath.Rel(symlinkDir, path)
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if err != nil {
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return err
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}
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if err := os.Symlink(target, symlinkPath); err != nil {
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return err
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}
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}
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return nil
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}
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