cmd/link, runtime: mark goexit as the top of the call stack

This CL adds a new attribute, TOPFRAME, which can be used to mark
functions that should be treated as being at the top of the call
stack. The function `runtime.goexit` has been marked this way on
architectures that use a link register.

This will stop programs that use DWARF to unwind the call stack
from unwinding past `runtime.goexit` on architectures that use a
link register. For example, it eliminates "corrupt stack?"
warnings when generating a backtrace that hits `runtime.goexit`
in GDB on s390x.

Similar code should be added for non-link-register architectures
(i.e. amd64, 386). They mark the top of the call stack slightly
differently to link register architectures so I haven't added
that code (they need to mark "rip" as undefined).

Fixes #24385.

Change-Id: I15b4c69ac75b491daa0acf0d981cb80eb06488de
Reviewed-on: https://go-review.googlesource.com/c/go/+/169726
Run-TryBot: Michael Munday <mike.munday@ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

1 files changed, 22 insertions, 0 deletions

diff --git a/src/runtime/runtime-gdb_test.go b/src/runtime/runtime-gdb_test.go
index d47c7c2262..8117a5c979 100644
--- a/src/runtime/runtime-gdb_test.go
+++ b/src/runtime/runtime-gdb_test.go
@@ -82,6 +82,22 @@ func checkGdbPython(t *testing.T) {
 	}
 }
 
+// checkCleanBacktrace checks that the given backtrace is well formed and does
+// not contain any error messages from GDB.
+func checkCleanBacktrace(t *testing.T, backtrace string) {
+	backtrace = strings.TrimSpace(backtrace)
+	lines := strings.Split(backtrace, "\n")
+	if len(lines) == 0 {
+		t.Fatalf("empty backtrace")
+	}
+	for i, l := range lines {
+		if !strings.HasPrefix(l, fmt.Sprintf("#%v  ", i)) {
+			t.Fatalf("malformed backtrace at line %v: %v", i, l)
+		}
+	}
+	// TODO(mundaym): check for unknown frames (e.g. "??").
+}
+
 const helloSource = `
 import "fmt"
 import "runtime"
@@ -272,6 +288,11 @@ func testGdbPython(t *testing.T, cgo bool) {
 		t.Fatalf("info locals failed: %s", bl)
 	}
 
+	// Check that the backtraces are well formed.
+	checkCleanBacktrace(t, blocks["goroutine 1 bt"])
+	checkCleanBacktrace(t, blocks["goroutine 2 bt"])
+	checkCleanBacktrace(t, blocks["goroutine 1 bt at the end"])
+
 	btGoroutine1Re := regexp.MustCompile(`(?m)^#0\s+(0x[0-9a-f]+\s+in\s+)?main\.main.+at`)
 	if bl := blocks["goroutine 1 bt"]; !btGoroutine1Re.MatchString(bl) {
 		t.Fatalf("goroutine 1 bt failed: %s", bl)
@@ -281,6 +302,7 @@ func testGdbPython(t *testing.T, cgo bool) {
 	if bl := blocks["goroutine 2 bt"]; !btGoroutine2Re.MatchString(bl) {
 		t.Fatalf("goroutine 2 bt failed: %s", bl)
 	}
+
 	btGoroutine1AtTheEndRe := regexp.MustCompile(`(?m)^#0\s+(0x[0-9a-f]+\s+in\s+)?main\.main.+at`)
 	if bl := blocks["goroutine 1 bt at the end"]; !btGoroutine1AtTheEndRe.MatchString(bl) {
 		t.Fatalf("goroutine 1 bt at the end failed: %s", bl)