runtime: allow crash from gsignal stack

The uses of onM in dopanic/startpanic are okay even from the signal stack.

Fixes #8666.

LGTM=khr
R=khr
CC=golang-codereviews
https://golang.org/cl/134710043

src/runtime/asm_386.s

@@ -209,6 +209,23 @@ TEXT runtime·mcall(SB), NOSPLIT, $0-4
 TEXT runtime·switchtoM(SB), NOSPLIT, $0-4
 	RET
 
+// func onM_signalok(fn func())
+TEXT runtime·onM_signalok(SB), NOSPLIT, $0-4
+	get_tls(CX)
+	MOVL	g(CX), AX	// AX = g
+	MOVL	g_m(AX), BX	// BX = m
+	MOVL	m_gsignal(BX), DX	// DX = gsignal
+	CMPL	AX, DX
+	JEQ	ongsignal
+	JMP	runtime·onM(SB)
+
+ongsignal:
+	MOVL	fn+0(FP), DI	// DI = fn
+	MOVL	DI, DX
+	MOVL	0(DI), DI
+	CALL	DI
+	RET
+
 // func onM(fn func())
 TEXT runtime·onM(SB), NOSPLIT, $0-4
 	MOVL	fn+0(FP), DI	// DI = fn

src/runtime/asm_amd64.s

@@ -200,6 +200,23 @@ TEXT runtime·mcall(SB), NOSPLIT, $0-8
 TEXT runtime·switchtoM(SB), NOSPLIT, $0-8
 	RET
 
+// func onM_signalok(fn func())
+TEXT runtime·onM_signalok(SB), NOSPLIT, $0-8
+	get_tls(CX)
+	MOVQ	g(CX), AX	// AX = g
+	MOVQ	g_m(AX), BX	// BX = m
+	MOVQ	m_gsignal(BX), DX	// DX = gsignal
+	CMPQ	AX, DX
+	JEQ	ongsignal
+	JMP	runtime·onM(SB)
+
+ongsignal:
+	MOVQ	fn+0(FP), DI	// DI = fn
+	MOVQ	DI, DX
+	MOVQ	0(DI), DI
+	CALL	DI
+	RET
+
 // func onM(fn func())
 TEXT runtime·onM(SB), NOSPLIT, $0-8
 	MOVQ	fn+0(FP), DI	// DI = fn

src/runtime/asm_amd64p32.s

@@ -175,6 +175,23 @@ TEXT runtime·mcall(SB), NOSPLIT, $0-4
 TEXT runtime·switchtoM(SB), NOSPLIT, $0-4
 	RET
 
+// func onM_signalok(fn func())
+TEXT runtime·onM_signalok(SB), NOSPLIT, $0-4
+	get_tls(CX)
+	MOVL	g(CX), AX	// AX = g
+	MOVL	g_m(AX), BX	// BX = m
+	MOVL	m_gsignal(BX), DX	// DX = gsignal
+	CMPL	AX, DX
+	JEQ	ongsignal
+	JMP	runtime·onM(SB)
+
+ongsignal:
+	MOVL	fn+0(FP), DI	// DI = fn
+	MOVL	DI, DX
+	MOVL	0(DI), DI
+	CALL	DI
+	RET
+
 // func onM(fn func())
 TEXT runtime·onM(SB), NOSPLIT, $0-4
 	MOVL	fn+0(FP), DI	// DI = fn

src/runtime/asm_arm.s

@@ -202,6 +202,21 @@ TEXT runtime·switchtoM(SB),NOSPLIT,$0-4
 	BL	(R0) // clobber lr to ensure push {lr} is kept
 	RET
 
+// func onM_signalok(fn func())
+TEXT runtime·onM_signalok(SB), NOSPLIT, $-4-4
+	MOVW	g_m(g), R1
+	MOVW	m_gsignal(R1), R2
+	CMP	g, R2
+	B.EQ	ongsignal
+	B	runtime·onM(SB)
+
+ongsignal:
+	MOVW	fn+0(FP), R0
+	MOVW	R0, R7
+	MOVW	0(R0), R0
+	BL	(R0)
+	RET
+
 // func onM(fn func())
 TEXT runtime·onM(SB),NOSPLIT,$0-4
 	MOVW	fn+0(FP), R0	// R0 = fn

src/runtime/crash_test.go

@@ -175,6 +175,14 @@ func TestMainGoroutineId(t *testing.T) {
 	}
 }
 
+func TestBreakpoint(t *testing.T) {
+	output := executeTest(t, breakpointSource, nil)
+	want := "runtime.Breakpoint()"
+	if !strings.Contains(output, want) {
+		t.Fatalf("output:\n%s\n\nwant output containing: %s", output, want)
+	}
+}
+
 const crashSource = `
 package main
 
@@ -380,3 +388,11 @@ func main() {
 	panic("test")
 }
 `
+
+const breakpointSource = `
+package main
+import "runtime"
+func main() {
+	runtime.Breakpoint()
+}
+`

src/runtime/panic.go

@@ -371,7 +371,7 @@ func gorecover(argp uintptr) interface{} {
 
 //go:nosplit
 func startpanic() {
-	onM(startpanic_m)
+	onM_signalok(startpanic_m)
 }
 
 //go:nosplit
@@ -381,7 +381,7 @@ func dopanic(unused int) {
 	mp.ptrarg[0] = unsafe.Pointer(gp)
 	mp.scalararg[0] = getcallerpc((unsafe.Pointer)(&unused))
 	mp.scalararg[1] = getcallersp((unsafe.Pointer)(&unused))
-	onM(dopanic_m) // should never return
+	onM_signalok(dopanic_m) // should never return
 	*(*int)(nil) = 0
 }
 

src/runtime/proc.c

@@ -2398,6 +2398,7 @@ gfpurge(P *p)
 	runtime·unlock(&runtime·sched.gflock);
 }
 
+#pragma textflag NOSPLIT
 void
 runtime·Breakpoint(void)
 {

src/runtime/runtime.h

@@ -823,6 +823,7 @@ int32	runtime·mcount(void);
 int32	runtime·gcount(void);
 void	runtime·mcall(void(**)(G*));
 void	runtime·onM(void(**)(void));
+void	runtime·onMsignal(void(**)(void));
 uint32	runtime·fastrand1(void);
 void	runtime·rewindmorestack(Gobuf*);
 int32	runtime·timediv(int64, int32, int32*);

src/runtime/stubs.go

@@ -73,6 +73,24 @@ func mcall(fn func(*g))
 //go:noescape
 func onM(fn func())
 
+// onMsignal is like onM but is allowed to be used in code that
+// might run on the gsignal stack. Code running on a signal stack
+// may be interrupting an onM sequence on the main stack, so
+// if the onMsignal calling sequence writes to ptrarg/scalararg,
+// it must first save the old values and then restore them when
+// finished. As an exception to the rule, it is fine not to save and
+// restore the values if the program is trying to crash rather than
+// return from the signal handler.
+// Once all the runtime is written in Go, there will be no ptrarg/scalararg
+// and the distinction between onM and onMsignal (and perhaps mcall)
+// can go away.
+//
+// If onMsignal is called from a gsignal stack, it invokes fn directly,
+// without a stack switch. Otherwise onMsignal behaves like onM.
+//
+//go:noescape
+func onM_signalok(fn func())
+
 func badonm() {
 	gothrow("onM called from signal goroutine")
 }