runtime: account for missing frame pointer in preamble

If a goroutine is synchronously preempted, then taking a
frame-pointer-based stack trace at that preemption will skip PC of the
caller of the function which called into morestack. This happens because
the frame pointer is pushed to the stack after the preamble, leaving the
stack in an odd state for frame pointer unwinding.

Deal with this by marking a goroutine as synchronously preempted and
using that signal to load the missing PC from the stack. On LR platforms
this is available in gp.sched.lr. On non-LR platforms like x86, it's at
gp.sched.sp, because there are no args, no locals, and no frame pointer
pushed to the SP yet.

For #68090.

Change-Id: I73a1206d8b84eecb8a96dbe727195da30088f288
Reviewed-on: https://go-review.googlesource.com/c/go/+/684435
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Nick Ripley <nick.ripley@datadoghq.com>

src/internal/trace/testdata/testprog/stacks.go

@@ -97,6 +97,11 @@ func main() {
 		rp.Read(data[:])
 		pipeReadDone <- true
 	}()
+	go func() { // func12
+		for {
+			syncPreemptPoint()
+		}
+	}()
 
 	time.Sleep(100 * time.Millisecond)
 	runtime.GC()
@@ -127,3 +132,12 @@ func main() {
 
 	runtime.GOMAXPROCS(oldGoMaxProcs)
 }
+
+//go:noinline
+func syncPreemptPoint() {
+	if never {
+		syncPreemptPoint()
+	}
+}
+
+var never bool

src/internal/trace/trace_test.go

@@ -326,7 +326,8 @@ func TestTraceStacks(t *testing.T) {
 		const mainLine = 21
 		want := []evDesc{
 			{trace.EventStateTransition, "Goroutine Running->Runnable", []frame{
-				{"main.main", mainLine + 82},
+				{"runtime.Gosched", 0},
+				{"main.main", mainLine + 87},
 			}},
 			{trace.EventStateTransition, "Goroutine NotExist->Runnable", []frame{
 				{"main.main", mainLine + 11},
@@ -349,7 +350,7 @@ func TestTraceStacks(t *testing.T) {
 			}},
 			{trace.EventStateTransition, "Goroutine Waiting->Runnable", []frame{
 				{"runtime.chansend1", 0},
-				{"main.main", mainLine + 84},
+				{"main.main", mainLine + 89},
 			}},
 			{trace.EventStateTransition, "Goroutine Running->Waiting", []frame{
 				{"runtime.chansend1", 0},
@@ -357,7 +358,7 @@ func TestTraceStacks(t *testing.T) {
 			}},
 			{trace.EventStateTransition, "Goroutine Waiting->Runnable", []frame{
 				{"runtime.chanrecv1", 0},
-				{"main.main", mainLine + 85},
+				{"main.main", mainLine + 90},
 			}},
 			{trace.EventStateTransition, "Goroutine Running->Waiting", []frame{
 				{"runtime.selectgo", 0},
@@ -365,7 +366,7 @@ func TestTraceStacks(t *testing.T) {
 			}},
 			{trace.EventStateTransition, "Goroutine Waiting->Runnable", []frame{
 				{"runtime.selectgo", 0},
-				{"main.main", mainLine + 86},
+				{"main.main", mainLine + 91},
 			}},
 			{trace.EventStateTransition, "Goroutine Running->Waiting", []frame{
 				{"sync.(*Mutex).Lock", 0},
@@ -382,7 +383,7 @@ func TestTraceStacks(t *testing.T) {
 			{trace.EventStateTransition, "Goroutine Waiting->Runnable", []frame{
 				{"sync.(*WaitGroup).Add", 0},
 				{"sync.(*WaitGroup).Done", 0},
-				{"main.main", mainLine + 91},
+				{"main.main", mainLine + 96},
 			}},
 			{trace.EventStateTransition, "Goroutine Running->Waiting", []frame{
 				{"sync.(*Cond).Wait", 0},
@@ -402,6 +403,10 @@ func TestTraceStacks(t *testing.T) {
 				{"runtime.GOMAXPROCS", 0},
 				{"main.main", 0},
 			}},
+			{trace.EventStateTransition, "Goroutine Running->Runnable", []frame{
+				{"main.syncPreemptPoint", 0},
+				{"main.main.func12", 0},
+			}},
 		}
 		if !stress {
 			// Only check for this stack if !stress because traceAdvance alone could

src/runtime/proc.go

@@ -3307,10 +3307,10 @@ func execute(gp *g, inheritTime bool) {
 		tryRecordGoroutineProfile(gp, nil, osyield)
 	}
 
-	// Assign gp.m before entering _Grunning so running Gs have an
-	// M.
+	// Assign gp.m before entering _Grunning so running Gs have an M.
 	mp.curg = gp
 	gp.m = mp
+	gp.syncSafePoint = false // Clear the flag, which may have been set by morestack.
 	casgstatus(gp, _Grunnable, _Grunning)
 	gp.waitsince = 0
 	gp.preempt = false

src/runtime/runtime2.go

@@ -466,6 +466,7 @@ type g struct {
 	runnableTime    int64 // the amount of time spent runnable, cleared when running, only used when tracking
 	lockedm         muintptr
 	fipsIndicator   uint8
+	syncSafePoint   bool // set if g is stopped at a synchronous safe point.
 	runningCleanups atomic.Bool
 	sig             uint32
 	writebuf        []byte

src/runtime/stack.go

@@ -1115,6 +1115,9 @@ func newstack() {
 			shrinkstack(gp)
 		}
 
+		// Set a flag indicated that we've been synchronously preempted.
+		gp.syncSafePoint = true
+
 		if gp.preemptStop {
 			preemptPark(gp) // never returns
 		}

src/runtime/traceruntime.go

@@ -457,7 +457,7 @@ func (tl traceLocker) GoPreempt() {
 
 // GoStop emits a GoStop event with the provided reason.
 func (tl traceLocker) GoStop(reason traceGoStopReason) {
-	tl.eventWriter(tracev2.GoRunning, tracev2.ProcRunning).event(tracev2.EvGoStop, traceArg(trace.goStopReasons[tl.gen%2][reason]), tl.stack(1))
+	tl.eventWriter(tracev2.GoRunning, tracev2.ProcRunning).event(tracev2.EvGoStop, traceArg(trace.goStopReasons[tl.gen%2][reason]), tl.stack(0))
 }
 
 // GoPark emits a GoBlock event with the provided reason.

src/runtime/tracestack.go

@@ -109,7 +109,22 @@ func traceStack(skip int, gp *g, gen uintptr) uint64 {
 				nstk += 1 + fpTracebackPCs(unsafe.Pointer(gp.syscallbp), pcBuf[2:])
 			} else {
 				pcBuf[1] = gp.sched.pc
-				nstk += 1 + fpTracebackPCs(unsafe.Pointer(gp.sched.bp), pcBuf[2:])
+				if gp.syncSafePoint {
+					// We're stopped in morestack, which is an odd state because gp.sched.bp
+					// refers to our parent frame, since we haven't had the chance to push our
+					// frame pointer to the stack yet. If we just start walking from gp.sched.bp,
+					// we'll skip a frame as a result. Luckily, we can find the PC we want right
+					// at gp.sched.sp on non-LR platforms, and we have it directly on LR platforms.
+					// See issue go.dev/issue/68090.
+					if usesLR {
+						pcBuf[2] = gp.sched.lr
+					} else {
+						pcBuf[2] = *(*uintptr)(unsafe.Pointer(gp.sched.sp))
+					}
+					nstk += 2 + fpTracebackPCs(unsafe.Pointer(gp.sched.bp), pcBuf[3:])
+				} else {
+					nstk += 1 + fpTracebackPCs(unsafe.Pointer(gp.sched.bp), pcBuf[2:])
+				}
 			}
 		}
 	}