cmd/link: fix gdb backtrace on architectures using a link register

Also adds TestGdbBacktrace to the runtime package.

Dwarf modifications written by Bryan Chan (@bryanpkc) who is also
at IBM and covered by the same CLA.

Fixes #14628

Change-Id: I106a1f704c3745a31f29cdadb0032e3905829850
Reviewed-on: https://go-review.googlesource.com/20193
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/runtime-gdb_test.go

@@ -3,6 +3,7 @@ package runtime_test
 import (
 	"bytes"
 	"fmt"
+	"internal/testenv"
 	"io/ioutil"
 	"os"
 	"os/exec"
@@ -13,19 +14,22 @@ import (
 	"testing"
 )
 
-func checkGdbPython(t *testing.T) {
-	cmd := exec.Command("gdb", "-nx", "-q", "--batch", "-iex", "python import sys; print('go gdb python support')")
-	out, err := cmd.CombinedOutput()
-
-	if err != nil {
-		t.Skipf("skipping due to issue running gdb: %v", err)
+func checkGdbEnvironment(t *testing.T) {
+	testenv.MustHaveGoBuild(t)
+	if runtime.GOOS == "darwin" {
+		t.Skip("gdb does not work on darwin")
 	}
-	if string(out) != "go gdb python support\n" {
-		t.Skipf("skipping due to lack of python gdb support: %s", out)
+	if final := os.Getenv("GOROOT_FINAL"); final != "" && runtime.GOROOT() != final {
+		t.Skip("gdb test can fail with GOROOT_FINAL pending")
 	}
+}
 
+func checkGdbVersion(t *testing.T) {
 	// Issue 11214 reports various failures with older versions of gdb.
-	out, err = exec.Command("gdb", "--version").CombinedOutput()
+	out, err := exec.Command("gdb", "--version").CombinedOutput()
+	if err != nil {
+		t.Skipf("skipping: error executing gdb: %v", err)
+	}
 	re := regexp.MustCompile(`([0-9]+)\.([0-9]+)`)
 	matches := re.FindSubmatch(out)
 	if len(matches) < 3 {
@@ -42,6 +46,18 @@ func checkGdbPython(t *testing.T) {
 	t.Logf("gdb version %d.%d", major, minor)
 }
 
+func checkGdbPython(t *testing.T) {
+	cmd := exec.Command("gdb", "-nx", "-q", "--batch", "-iex", "python import sys; print('go gdb python support')")
+	out, err := cmd.CombinedOutput()
+
+	if err != nil {
+		t.Skipf("skipping due to issue running gdb: %v", err)
+	}
+	if string(out) != "go gdb python support\n" {
+		t.Skipf("skipping due to lack of python gdb support: %s", out)
+	}
+}
+
 const helloSource = `
 package main
 import "fmt"
@@ -57,13 +73,8 @@ func main() {
 `
 
 func TestGdbPython(t *testing.T) {
-	if runtime.GOOS == "darwin" {
-		t.Skip("gdb does not work on darwin")
-	}
-	if final := os.Getenv("GOROOT_FINAL"); final != "" && runtime.GOROOT() != final {
-		t.Skip("gdb test can fail with GOROOT_FINAL pending")
-	}
-
+	checkGdbEnvironment(t)
+	checkGdbVersion(t)
 	checkGdbPython(t)
 
 	dir, err := ioutil.TempDir("", "go-build")
@@ -162,3 +173,82 @@ func TestGdbPython(t *testing.T) {
 		t.Logf("gdb cannot backtrace for GOARCH=%s, skipped goroutine backtrace test", runtime.GOARCH)
 	}
 }
+
+const backtraceSource = `
+package main
+
+//go:noinline
+func aaa() bool { return bbb() }
+
+//go:noinline
+func bbb() bool { return ccc() }
+
+//go:noinline
+func ccc() bool { return ddd() }
+
+//go:noinline
+func ddd() bool { return f() }
+
+//go:noinline
+func eee() bool { return true }
+
+var f = eee
+
+func main() {
+	_ = aaa()
+}
+`
+
+// TestGdbBacktrace tests that gdb can unwind the stack correctly
+// using only the DWARF debug info.
+func TestGdbBacktrace(t *testing.T) {
+	checkGdbEnvironment(t)
+	checkGdbVersion(t)
+
+	dir, err := ioutil.TempDir("", "go-build")
+	if err != nil {
+		t.Fatalf("failed to create temp directory: %v", err)
+	}
+	defer os.RemoveAll(dir)
+
+	// Build the source code.
+	src := filepath.Join(dir, "main.go")
+	err = ioutil.WriteFile(src, []byte(backtraceSource), 0644)
+	if err != nil {
+		t.Fatalf("failed to create file: %v", err)
+	}
+	cmd := exec.Command("go", "build", "-o", "a.exe")
+	cmd.Dir = dir
+	out, err := testEnv(cmd).CombinedOutput()
+	if err != nil {
+		t.Fatalf("building source %v\n%s", err, out)
+	}
+
+	// Execute gdb commands.
+	args := []string{"-nx", "-batch",
+		"-ex", "break main.eee",
+		"-ex", "run",
+		"-ex", "backtrace",
+		"-ex", "continue",
+		filepath.Join(dir, "a.exe"),
+	}
+	got, _ := exec.Command("gdb", args...).CombinedOutput()
+
+	// Check that the backtrace matches the source code.
+	bt := []string{
+		"eee",
+		"ddd",
+		"ccc",
+		"bbb",
+		"aaa",
+		"main",
+	}
+	for i, name := range bt {
+		s := fmt.Sprintf("#%v.*main\\.%v", i, name)
+		re := regexp.MustCompile(s)
+		if found := re.Find(got) != nil; !found {
+			t.Errorf("could not find '%v' in backtrace", s)
+			t.Fatalf("gdb output:\n%v", string(got))
+		}
+	}
+}