runtime: fix infinite callstack of cgo on arm64

This change adds CFA information to the assembly function 'crosscall1'
and reorgnizes its code to establish well-formed prologue and epilogue.
It will fix an infinite callstack issue when debugging cgo program with
GDB on arm64.

Brief root cause analysis:

GDB's aarch64 unwinder parses prologue to determine current frame's size
and previous PC&SP if CFA information is not available.

The unwinder parses the prologue of 'crosscall1' to determine a frame size
of 0x10, then turns to its next frame trying to compute its previous PC&SP
as they are not saved on current frame's stack as per its 'traditional frame
unwind' rules, which ends up getting an endless frame chain like:
    [callee]  : pc:<pc0>, sp:<sp0>
    crosscall1: pc:<pc1>, sp:<sp0>+0x10
    [caller]  : pc:<pc1>, sp:<sp0>+0x10+0x10
    [caller]  : pc:<pc1>, sp:<sp0>+0x10+0x10+0x10
    ...
GDB fails to detect the 'caller' frame is same as 'crosscall1' and terminate
unwinding since SP increases everytime.

Fixes #37238
Change-Id: Ia6bd8555828541a3a61f7dc9b94dfa00775ec52a
Reviewed-on: https://go-review.googlesource.com/c/go/+/226999
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

1 files changed, 80 insertions, 0 deletions

diff --git a/src/runtime/runtime-gdb_test.go b/src/runtime/runtime-gdb_test.go
index 5dbe4bf88a..4639e2fcb8 100644
--- a/src/runtime/runtime-gdb_test.go
+++ b/src/runtime/runtime-gdb_test.go
@@ -607,3 +607,83 @@ func TestGdbPanic(t *testing.T) {
 		}
 	}
 }
+
+const InfCallstackSource = `
+package main
+import "C"
+import "time"
+
+func loop() {
+        for i := 0; i < 1000; i++ {
+                time.Sleep(time.Millisecond*5)
+        }
+}
+
+func main() {
+        go loop()
+        time.Sleep(time.Second * 1)
+}
+`
+// TestGdbInfCallstack tests that gdb can unwind the callstack of cgo programs
+// on arm64 platforms without endless frames of function 'crossfunc1'.
+// https://golang.org/issue/37238
+func TestGdbInfCallstack(t *testing.T) {
+	checkGdbEnvironment(t)
+
+	testenv.MustHaveCGO(t)
+	if runtime.GOARCH != "arm64" {
+		t.Skip("skipping infinite callstack test on non-arm64 arches")
+	}
+
+	t.Parallel()
+	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(InfCallstackSource), 0644)
+	if err != nil {
+		t.Fatalf("failed to create file: %v", err)
+	}
+	cmd := exec.Command(testenv.GoToolPath(t), "build", "-o", "a.exe", "main.go")
+	cmd.Dir = dir
+	out, err := testenv.CleanCmdEnv(cmd).CombinedOutput()
+	if err != nil {
+		t.Fatalf("building source %v\n%s", err, out)
+	}
+
+	// Execute gdb commands.
+	// 'setg_gcc' is the first point where we can reproduce the issue with just one 'run' command.
+	args := []string{"-nx", "-batch",
+		"-iex", "add-auto-load-safe-path " + filepath.Join(runtime.GOROOT(), "src", "runtime"),
+		"-ex", "set startup-with-shell off",
+		"-ex", "break setg_gcc",
+		"-ex", "run",
+		"-ex", "backtrace 3",
+		"-ex", "disable 1",
+		"-ex", "continue",
+		filepath.Join(dir, "a.exe"),
+	}
+	got, _ := exec.Command("gdb", args...).CombinedOutput()
+
+	// Check that the backtrace matches
+	// We check the 3 inner most frames only as they are present certainly, according to gcc_<OS>_arm64.c
+	bt := []string{
+		`setg_gcc`,
+		`crosscall1`,
+		`threadentry`,
+	}
+	for i, name := range bt {
+		s := fmt.Sprintf("#%v.*%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))
+		}
+	}
+}