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
diff --git a/elf/build_id_dir.go b/elf/build_id_dir.go
new file mode 100644
index 0000000..5fb7dda
--- /dev/null
+++ b/elf/build_id_dir.go
@@ -0,0 +1,172 @@
+// Copyright 2024 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 (
+ "io/fs"
+ "os"
+ "path/filepath"
+ "strings"
+ "sync"
+ "time"
+)
+
+func UpdateBuildIdDir(path string) error {
+ path = filepath.Clean(path)
+ buildIdPath := path + "/.build-id"
+
+ // Collect the list of files and build-id symlinks. If the symlinks are
+ // up to date (newer than the symbol files), there is nothing to do.
+ var buildIdFiles, symbolFiles []string
+ var buildIdMtime, symbolsMtime time.Time
+ filepath.WalkDir(path, func(path string, entry fs.DirEntry, err error) error {
+ if entry == nil || entry.IsDir() {
+ return nil
+ }
+ info, err := entry.Info()
+ if err != nil {
+ return err
+ }
+ mtime := info.ModTime()
+ if strings.HasPrefix(path, buildIdPath) {
+ if buildIdMtime.Compare(mtime) < 0 {
+ buildIdMtime = mtime
+ }
+ buildIdFiles = append(buildIdFiles, path)
+ } else {
+ if symbolsMtime.Compare(mtime) < 0 {
+ symbolsMtime = mtime
+ }
+ symbolFiles = append(symbolFiles, path)
+ }
+ return nil
+ })
+ if symbolsMtime.Compare(buildIdMtime) < 0 {
+ return nil
+ }
+
+ // Collect build-id -> file mapping from ELF files in the symbols directory.
+ concurrency := 8
+ done := make(chan error)
+ buildIdToFile := make(map[string]string)
+ var mu sync.Mutex
+ for i := 0; i != concurrency; i++ {
+ go func(paths []string) {
+ for _, path := range paths {
+ id, err := Identifier(path, true)
+ if err != nil {
+ done <- err
+ return
+ }
+ if id == "" {
+ continue
+ }
+ mu.Lock()
+ oldPath := buildIdToFile[id]
+ if oldPath == "" || oldPath > path {
+ buildIdToFile[id] = path
+ }
+ mu.Unlock()
+ }
+ done <- nil
+ }(symbolFiles[len(symbolFiles)*i/concurrency : len(symbolFiles)*(i+1)/concurrency])
+ }
+
+ // Collect previously generated build-id -> file mapping from the .build-id directory.
+ // We will use this for incremental updates. If we see anything in the .build-id
+ // directory that we did not expect, we'll delete it and start over.
+ prevBuildIdToFile := make(map[string]string)
+out:
+ for _, buildIdFile := range buildIdFiles {
+ if !strings.HasSuffix(buildIdFile, ".debug") {
+ prevBuildIdToFile = nil
+ break
+ }
+ buildId := buildIdFile[len(buildIdPath)+1 : len(buildIdFile)-6]
+ for i, ch := range buildId {
+ if i == 2 {
+ if ch != '/' {
+ prevBuildIdToFile = nil
+ break out
+ }
+ } else {
+ if (ch < '0' || ch > '9') && (ch < 'a' || ch > 'f') {
+ prevBuildIdToFile = nil
+ break out
+ }
+ }
+ }
+ target, err := os.Readlink(buildIdFile)
+ if err != nil || !strings.HasPrefix(target, "../../") {
+ prevBuildIdToFile = nil
+ break
+ }
+ prevBuildIdToFile[buildId[0:2]+buildId[3:]] = path + target[5:]
+ }
+ if prevBuildIdToFile == nil {
+ err := os.RemoveAll(buildIdPath)
+ if err != nil {
+ return err
+ }
+ prevBuildIdToFile = make(map[string]string)
+ }
+
+ // Wait for build-id collection from ELF files to finish.
+ for i := 0; i != concurrency; i++ {
+ err := <-done
+ if err != nil {
+ return err
+ }
+ }
+
+ // Delete old symlinks.
+ for id, _ := range prevBuildIdToFile {
+ if buildIdToFile[id] == "" {
+ symlinkDir := buildIdPath + "/" + id[:2]
+ symlinkPath := symlinkDir + "/" + id[2:] + ".debug"
+ if err := os.Remove(symlinkPath); err != nil {
+ return err
+ }
+ }
+ }
+
+ // Add new symlinks and update changed symlinks.
+ for id, path := range buildIdToFile {
+ prevPath := prevBuildIdToFile[id]
+ if prevPath == path {
+ continue
+ }
+ symlinkDir := buildIdPath + "/" + id[:2]
+ symlinkPath := symlinkDir + "/" + id[2:] + ".debug"
+ if prevPath == "" {
+ if err := os.MkdirAll(symlinkDir, 0755); err != nil {
+ return err
+ }
+ } else {
+ if err := os.Remove(symlinkPath); err != nil {
+ return err
+ }
+ }
+
+ target, err := filepath.Rel(symlinkDir, path)
+ if err != nil {
+ return err
+ }
+ if err := os.Symlink(target, symlinkPath); err != nil {
+ return err
+ }
+ }
+ return nil
+}