runtime: print a message on bad morestack

If morestack runs on the g0 or gsignal stack, it currently performs
some abort operation that typically produces a signal (e.g., it does
an INT $3 on x86). This is useful if you're running in a debugger, but
if you're not, the runtime tries to trap this signal, which is likely
to send the program into a deeper spiral of collapse and lead to very
confusing diagnostic output.

Help out people trying to debug without a debugger by making morestack
print an informative message before blowing up.

Change-Id: I2814c64509b137bfe20a00091d8551d18c2c4749
Reviewed-on: https://go-review.googlesource.com/31133
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rick Hudson <rlh@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/runtime/asm_386.s

@@ -354,13 +354,15 @@ TEXT runtime·morestack(SB),NOSPLIT,$0-0
 	MOVL	g_m(BX), BX
 	MOVL	m_g0(BX), SI
 	CMPL	g(CX), SI
-	JNE	2(PC)
+	JNE	3(PC)
+	CALL	runtime·badmorestackg0(SB)
 	INT	$3
 
 	// Cannot grow signal stack.
 	MOVL	m_gsignal(BX), SI
 	CMPL	g(CX), SI
-	JNE	2(PC)
+	JNE	3(PC)
+	CALL	runtime·badmorestackgsignal(SB)
 	INT	$3
 
 	// Called from f.

src/runtime/asm_amd64.s

@@ -331,13 +331,15 @@ TEXT runtime·morestack(SB),NOSPLIT,$0-0
 	MOVQ	g_m(BX), BX
 	MOVQ	m_g0(BX), SI
 	CMPQ	g(CX), SI
-	JNE	2(PC)
+	JNE	3(PC)
+	CALL	runtime·badmorestackg0(SB)
 	INT	$3
 
 	// Cannot grow signal stack (m->gsignal).
 	MOVQ	m_gsignal(BX), SI
 	CMPQ	g(CX), SI
-	JNE	2(PC)
+	JNE	3(PC)
+	CALL	runtime·badmorestackgsignal(SB)
 	INT	$3
 
 	// Called from f.

src/runtime/asm_amd64p32.s

@@ -249,13 +249,15 @@ TEXT runtime·morestack(SB),NOSPLIT,$0-0
 	// Cannot grow scheduler stack (m->g0).
 	MOVL	m_g0(BX), SI
 	CMPL	g(CX), SI
-	JNE	2(PC)
+	JNE	3(PC)
+	CALL	runtime·badmorestackg0(SB)
 	MOVL	0, AX
 
 	// Cannot grow signal stack (m->gsignal).
 	MOVL	m_gsignal(BX), SI
 	CMPL	g(CX), SI
-	JNE	2(PC)
+	JNE	3(PC)
+	CALL	runtime·badmorestackgsignal(SB)
 	MOVL	0, AX
 
 	// Called from f.

src/runtime/asm_arm.s

@@ -281,12 +281,16 @@ TEXT runtime·morestack(SB),NOSPLIT,$-4-0
 	MOVW	g_m(g), R8
 	MOVW	m_g0(R8), R4
 	CMP	g, R4
-	BL.EQ	runtime·abort(SB)
+	BNE	3(PC)
+	BL	runtime·badmorestackg0(SB)
+	B	runtime·abort(SB)
 
 	// Cannot grow signal stack (m->gsignal).
 	MOVW	m_gsignal(R8), R4
 	CMP	g, R4
-	BL.EQ	runtime·abort(SB)
+	BNE	3(PC)
+	BL	runtime·badmorestackgsignal(SB)
+	B	runtime·abort(SB)
 
 	// Called from f.
 	// Set g->sched to context in f.

src/runtime/asm_arm64.s

@@ -256,13 +256,15 @@ TEXT runtime·morestack(SB),NOSPLIT,$-8-0
 	MOVD	g_m(g), R8
 	MOVD	m_g0(R8), R4
 	CMP	g, R4
-	BNE	2(PC)
+	BNE	3(PC)
+	BL	runtime·badmorestackg0(SB)
 	B	runtime·abort(SB)
 
 	// Cannot grow signal stack (m->gsignal).
 	MOVD	m_gsignal(R8), R4
 	CMP	g, R4
-	BNE	2(PC)
+	BNE	3(PC)
+	BL	runtime·badmorestackgsignal(SB)
 	B	runtime·abort(SB)
 
 	// Called from f.

src/runtime/asm_mips64x.s

@@ -231,12 +231,14 @@ TEXT runtime·morestack(SB),NOSPLIT,$-8-0
 	// Cannot grow scheduler stack (m->g0).
 	MOVV	g_m(g), R7
 	MOVV	m_g0(R7), R8
-	BNE	g, R8, 2(PC)
+	BNE	g, R8, 3(PC)
+	JAL	runtime·badmorestackg0(SB)
 	JAL	runtime·abort(SB)
 
 	// Cannot grow signal stack (m->gsignal).
 	MOVV	m_gsignal(R7), R8
-	BNE	g, R8, 2(PC)
+	BNE	g, R8, 3(PC)
+	JAL	runtime·badmorestackgsignal(SB)
 	JAL	runtime·abort(SB)
 
 	// Called from f.

src/runtime/asm_ppc64x.s

@@ -284,13 +284,15 @@ TEXT runtime·morestack(SB),NOSPLIT|NOFRAME,$0-0
 	MOVD	g_m(g), R7
 	MOVD	m_g0(R7), R8
 	CMP	g, R8
-	BNE	2(PC)
+	BNE	3(PC)
+	BL	runtime·badmorestackg0(SB)
 	BL	runtime·abort(SB)
 
 	// Cannot grow signal stack (m->gsignal).
 	MOVD	m_gsignal(R7), R8
 	CMP	g, R8
-	BNE	2(PC)
+	BNE	3(PC)
+	BL	runtime·badmorestackgsignal(SB)
 	BL	runtime·abort(SB)
 
 	// Called from f.

src/runtime/asm_s390x.s

@@ -279,13 +279,15 @@ TEXT runtime·morestack(SB),NOSPLIT|NOFRAME,$0-0
 	// Cannot grow scheduler stack (m->g0).
 	MOVD	g_m(g), R7
 	MOVD	m_g0(R7), R8
-	CMPBNE	g, R8, 2(PC)
+	CMPBNE	g, R8, 3(PC)
+	BL	runtime·badmorestackg0(SB)
 	BL	runtime·abort(SB)
 
 	// Cannot grow signal stack (m->gsignal).
 	MOVD	m_gsignal(R7), R8
 	CMP	g, R8
-	BNE	2(PC)
+	BNE	3(PC)
+	BL	runtime·badmorestackgsignal(SB)
 	BL	runtime·abort(SB)
 
 	// Called from f.

src/runtime/proc.go

@@ -381,6 +381,24 @@ func badreflectcall() {
 	panic(plainError("arg size to reflect.call more than 1GB"))
 }
 
+var badmorestackg0Msg = "fatal: morestack on g0\n"
+
+//go:nosplit
+//go:nowritebarrierrec
+func badmorestackg0() {
+	sp := stringStructOf(&badmorestackg0Msg)
+	write(2, sp.str, int32(sp.len))
+}
+
+var badmorestackgsignalMsg = "fatal: morestack on gsignal\n"
+
+//go:nosplit
+//go:nowritebarrierrec
+func badmorestackgsignal() {
+	sp := stringStructOf(&badmorestackgsignalMsg)
+	write(2, sp.str, int32(sp.len))
+}
+
 func lockedOSThread() bool {
 	gp := getg()
 	return gp.lockedm != nil && gp.m.lockedg != nil