runtime: print a stack trace at "morestack on g0"

Error like "morestack on g0" is one of the errors that is very
hard to debug, because often it doesn't print a useful stack trace.
The runtime doesn't directly print a stack trace because it is
a bad stack state to call print. Sometimes the SIGABRT may trigger
a traceback, but sometimes not especially in a cgo binary. Even if
it triggers a traceback it often does not include the stack trace
of the bad stack.

This CL makes it explicitly print a stack trace and throw. The
idea is to have some space as an "emergency" crash stack. When the
stack is in a really bad state, we switch to the crash stack and
do a traceback.

Currently only implemented on AMD64 and ARM64.

TODO: also handle errors like "morestack on gsignal" and bad
systemstack. Also handle other architectures.

Change-Id: Ibfc397202f2bb0737c5cbe99f2763de83301c1c1
Reviewed-on: https://go-review.googlesource.com/c/go/+/419435
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Pratt <mpratt@google.com>

src/runtime/asm.s

@@ -12,3 +12,11 @@ TEXT ·sigpanic0(SB),NOSPLIT,$0-0
 // See map.go comment on the need for this routine.
 TEXT ·mapinitnoop<ABIInternal>(SB),NOSPLIT,$0-0
 	RET
+
+#ifndef GOARCH_amd64
+#ifndef GOARCH_arm64
+// stub to appease shared build mode.
+TEXT ·switchToCrashStack0<ABIInternal>(SB),NOSPLIT,$0-0
+	UNDEF
+#endif
+#endif

src/runtime/asm_amd64.s

@@ -537,6 +537,30 @@ bad:
 	CALL	AX
 	INT	$3
 
+// func switchToCrashStack0(fn func())
+TEXT runtime·switchToCrashStack0<ABIInternal>(SB), NOSPLIT, $0-8
+	MOVQ	g_m(R14), BX // curm
+
+	// set g to gcrash
+	LEAQ	runtime·gcrash(SB), R14 // g = &gcrash
+	MOVQ	BX, g_m(R14)            // g.m = curm
+	MOVQ	R14, m_g0(BX)           // curm.g0 = g
+	get_tls(CX)
+	MOVQ	R14, g(CX)
+
+	// switch to crashstack
+	MOVQ	(g_stack+stack_hi)(R14), BX
+	SUBQ	$(4*8), BX
+	MOVQ	BX, SP
+
+	// call target function
+	MOVQ	AX, DX
+	MOVQ	0(AX), AX
+	CALL	AX
+
+	// should never return
+	CALL	runtime·abort(SB)
+	UNDEF
 
 /*
  * support for morestack
@@ -551,17 +575,26 @@ bad:
 TEXT runtime·morestack(SB),NOSPLIT|NOFRAME,$0-0
 	// Cannot grow scheduler stack (m->g0).
 	get_tls(CX)
-	MOVQ	g(CX), BX
-	MOVQ	g_m(BX), BX
-	MOVQ	m_g0(BX), SI
-	CMPQ	g(CX), SI
+	MOVQ	g(CX), DI     // DI = g
+	MOVQ	g_m(DI), BX   // BX = m
+
+	// Set g->sched to context in f.
+	MOVQ	0(SP), AX // f's PC
+	MOVQ	AX, (g_sched+gobuf_pc)(DI)
+	LEAQ	8(SP), AX // f's SP
+	MOVQ	AX, (g_sched+gobuf_sp)(DI)
+	MOVQ	BP, (g_sched+gobuf_bp)(DI)
+	MOVQ	DX, (g_sched+gobuf_ctxt)(DI)
+
+	MOVQ	m_g0(BX), SI  // SI = m.g0
+	CMPQ	DI, SI
 	JNE	3(PC)
 	CALL	runtime·badmorestackg0(SB)
 	CALL	runtime·abort(SB)
 
 	// Cannot grow signal stack (m->gsignal).
 	MOVQ	m_gsignal(BX), SI
-	CMPQ	g(CX), SI
+	CMPQ	DI, SI
 	JNE	3(PC)
 	CALL	runtime·badmorestackgsignal(SB)
 	CALL	runtime·abort(SB)
@@ -573,17 +606,7 @@ TEXT runtime·morestack(SB),NOSPLIT|NOFRAME,$0-0
 	MOVQ	AX, (m_morebuf+gobuf_pc)(BX)
 	LEAQ	16(SP), AX	// f's caller's SP
 	MOVQ	AX, (m_morebuf+gobuf_sp)(BX)
-	get_tls(CX)
-	MOVQ	g(CX), SI
-	MOVQ	SI, (m_morebuf+gobuf_g)(BX)
-
-	// Set g->sched to context in f.
-	MOVQ	0(SP), AX // f's PC
-	MOVQ	AX, (g_sched+gobuf_pc)(SI)
-	LEAQ	8(SP), AX // f's SP
-	MOVQ	AX, (g_sched+gobuf_sp)(SI)
-	MOVQ	BP, (g_sched+gobuf_bp)(SI)
-	MOVQ	DX, (g_sched+gobuf_ctxt)(SI)
+	MOVQ	DI, (m_morebuf+gobuf_g)(BX)
 
 	// Call newstack on m->g0's stack.
 	MOVQ	m_g0(BX), BX

src/runtime/asm_arm64.s

@@ -262,6 +262,30 @@ noswitch:
 	SUB	$8, RSP, R29	// restore FP
 	B	(R3)
 
+// func switchToCrashStack0(fn func())
+TEXT runtime·switchToCrashStack0<ABIInternal>(SB), NOSPLIT, $0-8
+	MOVD	R0, R26    // context register
+	MOVD	g_m(g), R1 // curm
+
+	// set g to gcrash
+	MOVD	$runtime·gcrash(SB), g // g = &gcrash
+	BL	runtime·save_g(SB)         // clobbers R0
+	MOVD	R1, g_m(g)             // g.m = curm
+	MOVD	g, m_g0(R1)            // curm.g0 = g
+
+	// switch to crashstack
+	MOVD	(g_stack+stack_hi)(g), R1
+	SUB	$(4*8), R1
+	MOVD	R1, RSP
+
+	// call target function
+	MOVD	0(R26), R0
+	CALL	(R0)
+
+	// should never return
+	CALL	runtime·abort(SB)
+	UNDEF
+
 /*
  * support for morestack
  */
@@ -278,6 +302,16 @@ TEXT runtime·morestack(SB),NOSPLIT|NOFRAME,$0-0
 	// Cannot grow scheduler stack (m->g0).
 	MOVD	g_m(g), R8
 	MOVD	m_g0(R8), R4
+
+	// Called from f.
+	// Set g->sched to context in f
+	MOVD	RSP, R0
+	MOVD	R0, (g_sched+gobuf_sp)(g)
+	MOVD	R29, (g_sched+gobuf_bp)(g)
+	MOVD	LR, (g_sched+gobuf_pc)(g)
+	MOVD	R3, (g_sched+gobuf_lr)(g)
+	MOVD	R26, (g_sched+gobuf_ctxt)(g)
+
 	CMP	g, R4
 	BNE	3(PC)
 	BL	runtime·badmorestackg0(SB)
@@ -290,15 +324,6 @@ TEXT runtime·morestack(SB),NOSPLIT|NOFRAME,$0-0
 	BL	runtime·badmorestackgsignal(SB)
 	B	runtime·abort(SB)
 
-	// Called from f.
-	// Set g->sched to context in f
-	MOVD	RSP, R0
-	MOVD	R0, (g_sched+gobuf_sp)(g)
-	MOVD	R29, (g_sched+gobuf_bp)(g)
-	MOVD	LR, (g_sched+gobuf_pc)(g)
-	MOVD	R3, (g_sched+gobuf_lr)(g)
-	MOVD	R26, (g_sched+gobuf_ctxt)(g)
-
 	// Called from f.
 	// Set m->morebuf to f's callers.
 	MOVD	R3, (m_morebuf+gobuf_pc)(R8)	// f's caller's PC

src/runtime/crash_test.go

@@ -804,6 +804,14 @@ func TestG0StackOverflow(t *testing.T) {
 		if n := strings.Count(string(out), "morestack on g0\n"); n != 1 {
 			t.Fatalf("%s\n(exit status %v)", out, err)
 		}
+		if runtime.GOARCH == "amd64" || runtime.GOARCH == "arm64" {
+			// check for a stack trace
+			want := "runtime.stackOverflow"
+			if n := strings.Count(string(out), want); n < 5 {
+				t.Errorf("output does not contain %q at least 5 times:\n%s", want, out)
+			}
+			return // it's not a signal-style traceback
+		}
 		// Check that it's a signal-style traceback.
 		if runtime.GOOS != "windows" {
 			if want := "PC="; !strings.Contains(string(out), want) {

src/runtime/export_test.go

@@ -694,7 +694,7 @@ func G0StackOverflow() {
 		// The stack bounds for g0 stack is not always precise.
 		// Use an artificially small stack, to trigger a stack overflow
 		// without actually run out of the system stack (which may seg fault).
-		g0.stack.lo = sp - 4096
+		g0.stack.lo = sp - 4096 - stackSystem
 		g0.stackguard0 = g0.stack.lo + stackGuard
 		g0.stackguard1 = g0.stackguard0
 

src/runtime/proc.go

@@ -516,7 +516,20 @@ func badreflectcall() {
 //go:nosplit
 //go:nowritebarrierrec
 func badmorestackg0() {
-	writeErrStr("fatal: morestack on g0\n")
+	if !crashStackImplemented {
+		writeErrStr("fatal: morestack on g0\n")
+		return
+	}
+
+	g := getg()
+	switchToCrashStack(func() {
+		print("runtime: morestack on g0, stack [", hex(g.stack.lo), " ", hex(g.stack.hi), "], sp=", hex(g.sched.sp), ", called from\n")
+		g.m.traceback = 2 // include pc and sp in stack trace
+		traceback1(g.sched.pc, g.sched.sp, g.sched.lr, g, 0)
+		print("\n")
+
+		throw("morestack on g0")
+	})
 }
 
 //go:nosplit
@@ -530,6 +543,49 @@ func badctxt() {
 	throw("ctxt != 0")
 }
 
+// crashstack is a space that can be used as the stack when it is
+// crashing on bad stack conditions, e.g. morestack on g0.
+// gcrash is the corresponding (fake) g.
+var crashstack [16384]byte
+
+var gcrash = g{
+	stack:       stack{uintptr(unsafe.Pointer(&crashstack)), uintptr(unsafe.Pointer(&crashstack)) + unsafe.Sizeof(crashstack)},
+	stackguard0: uintptr(unsafe.Pointer(&crashstack)) + 1000,
+	stackguard1: uintptr(unsafe.Pointer(&crashstack)) + 1000,
+}
+
+var crashingG atomic.Pointer[g]
+
+// Switch to crashstack and call fn, with special handling of
+// concurrent and recursive cases.
+//
+// Nosplit as it is called in a bad stack condition (we know
+// morestack would fail).
+//
+//go:nosplit
+//go:nowritebarrierrec
+func switchToCrashStack(fn func()) {
+	me := getg()
+	if crashingG.CompareAndSwapNoWB(nil, me) {
+		switchToCrashStack0(fn) // should never return
+		abort()
+	}
+	if crashingG.Load() == me {
+		// recursive crashing. too bad.
+		writeErrStr("fatal: recursive switchToCrashStack\n")
+		abort()
+	}
+	// Another g is crashing. Give it some time, hopefully it will finish traceback.
+	usleep_no_g(100)
+	writeErrStr("fatal: concurrent switchToCrashStack\n")
+	abort()
+}
+
+const crashStackImplemented = GOARCH == "amd64" || GOARCH == "arm64"
+
+//go:noescape
+func switchToCrashStack0(func()) // in assembly
+
 func lockedOSThread() bool {
 	gp := getg()
 	return gp.lockedm != 0 && gp.m.lockedg != 0