runtime: force segv for nil defer function to be in deferreturn()

If the defer function pointer is nil, force the seg fault to happen in deferreturn
rather than in jmpdefer. jmpdefer is used fairly infrequently now because most
functions have open-coded defers.

The open-coded defer implementation calls gentraceback() with a callback when
looking for the first open-coded defer frame. gentraceback() throws an error if it
is called with a callback on an LR architecture and jmpdefer is on the stack,
because the stack trace can be incorrect in that case - see issue #8153. So, we
want to make sure that we don't have a seg fault in jmpdefer.

Fixes #36050

Change-Id: Ie25e6f015d8eb170b40248dedeb26a37b7f9b38d
Reviewed-on: https://go-review.googlesource.com/c/go/+/210978
Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: Dan Scales <danscales@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/callers_test.go

@@ -254,9 +254,8 @@ func TestCallersDivZeroPanic(t *testing.T) {
 func TestCallersDeferNilFuncPanic(t *testing.T) {
 	// Make sure we don't have any extra frames on the stack. We cut off the check
 	// at runtime.sigpanic, because non-open-coded defers (which may be used in
-	// non-opt or race checker mode) include an extra 'jmpdefer' frame (which is
-	// where the nil pointer deref happens). We could consider hiding jmpdefer in
-	// tracebacks.
+	// non-opt or race checker mode) include an extra 'deferreturn' frame (which is
+	// where the nil pointer deref happens).
 	state := 1
 	want := []string{"runtime.Callers", "runtime_test.TestCallersDeferNilFuncPanic.func1",
 		"runtime.gopanic", "runtime.panicmem", "runtime.sigpanic"}
@@ -279,3 +278,32 @@ func TestCallersDeferNilFuncPanic(t *testing.T) {
 	// function exit, rather than at the defer statement.
 	state = 2
 }
+
+// Same test, but forcing non-open-coded defer by putting the defer in a loop.  See
+// issue #36050
+func TestCallersDeferNilFuncPanicWithLoop(t *testing.T) {
+	state := 1
+	want := []string{"runtime.Callers", "runtime_test.TestCallersDeferNilFuncPanicWithLoop.func1",
+		"runtime.gopanic", "runtime.panicmem", "runtime.sigpanic", "runtime.deferreturn", "runtime_test.TestCallersDeferNilFuncPanicWithLoop"}
+
+	defer func() {
+		if r := recover(); r == nil {
+			t.Fatal("did not panic")
+		}
+		pcs := make([]uintptr, 20)
+		pcs = pcs[:runtime.Callers(0, pcs)]
+		testCallersEqual(t, pcs, want)
+		if state == 1 {
+			t.Fatal("nil defer func panicked at defer time rather than function exit time")
+		}
+
+	}()
+
+	for i := 0; i < 1; i++ {
+		var f func()
+		defer f()
+	}
+	// Use the value of 'state' to make sure nil defer func f causes panic at
+	// function exit, rather than at the defer statement.
+	state = 2
+}

src/runtime/panic.go

@@ -561,6 +561,12 @@ func deferreturn(arg0 uintptr) {
 	d.fn = nil
 	gp._defer = d.link
 	freedefer(d)
+	// If the defer function pointer is nil, force the seg fault to happen
+	// here rather than in jmpdefer. gentraceback() throws an error if it is
+	// called with a callback on an LR architecture and jmpdefer is on the
+	// stack, because the stack trace can be incorrect in that case - see
+	// issue #8153).
+	_ = fn.fn
 	jmpdefer(fn, uintptr(unsafe.Pointer(&arg0)))
 }