Revert "liblink, cmd/ld, runtime: remove stackguard1"

This reverts commit ab0535ae3f.

I think it will remain useful to distinguish code that must
run on a system stack from code that can run on either stack,
even if that distinction is no
longer based on the implementation language.

That is, I expect to add a //go:systemstack comment that,
in terms of the old implementation, tells the compiler,
to pretend this function was written in C.

Change-Id: I33d2ebb2f99ae12496484c6ec8ed07233d693275
Reviewed-on: https://go-review.googlesource.com/2275
Reviewed-by: Russ Cox <rsc@golang.org>

include/link.h

@@ -131,6 +131,7 @@ struct	LSym
 	short	type;
 	short	version;
 	uchar	dupok;
+	uchar	cfunc;
 	uchar	external;
 	uchar	nosplit;
 	uchar	reachable;

src/cmd/ld/lib.c

@@ -1564,3 +1564,56 @@ diag(char *fmt, ...)
 		errorexit();
 	}
 }
+
+void
+checkgo(void)
+{
+	LSym *s;
+	Reloc *r;
+	int i;
+	int changed;
+	
+	if(!debug['C'])
+		return;
+	
+	// TODO(rsc,khr): Eventually we want to get to no Go-called C functions at all,
+	// which would simplify this logic quite a bit.
+
+	// Mark every Go-called C function with cfunc=2, recursively.
+	do {
+		changed = 0;
+		for(s = ctxt->textp; s != nil; s = s->next) {
+			if(s->cfunc == 0 || (s->cfunc == 2 && s->nosplit)) {
+				for(i=0; i<s->nr; i++) {
+					r = &s->r[i];
+					if(r->sym == nil)
+						continue;
+					if((r->type == R_CALL || r->type == R_CALLARM) && r->sym->type == STEXT) {
+						if(r->sym->cfunc == 1) {
+							changed = 1;
+							r->sym->cfunc = 2;
+						}
+					}
+				}
+			}
+		}
+	}while(changed);
+
+	// Complain about Go-called C functions that can split the stack
+	// (that can be preempted for garbage collection or trigger a stack copy).
+	for(s = ctxt->textp; s != nil; s = s->next) {
+		if(s->cfunc == 0 || (s->cfunc == 2 && s->nosplit)) {
+			for(i=0; i<s->nr; i++) {
+				r = &s->r[i];
+				if(r->sym == nil)
+					continue;
+				if((r->type == R_CALL || r->type == R_CALLARM) && r->sym->type == STEXT) {
+					if(s->cfunc == 0 && r->sym->cfunc == 2 && !r->sym->nosplit)
+						print("Go %s calls C %s\n", s->name, r->sym->name);
+					else if(s->cfunc == 2 && s->nosplit && !r->sym->nosplit)
+						print("Go calls C %s calls %s\n", s->name, r->sym->name);
+				}
+			}
+		}
+	}
+}

src/cmd/ld/lib.h

@@ -183,6 +183,7 @@ uint16	be16(uchar *b);
 uint32	be32(uchar *b);
 uint64	be64(uchar *b);
 void	callgraph(void);
+void	checkgo(void);
 void	cflush(void);
 void	codeblk(int64 addr, int64 size);
 vlong	cpos(void);

src/cmd/ld/pobj.c

@@ -172,6 +172,7 @@ main(int argc, char *argv[])
 		mark(linklookup(ctxt, "runtime.read_tls_fallback", 0));
 	}
 
+	checkgo();
 	deadcode();
 	callgraph();
 	paramspace = "SP";	/* (FP) now (SP) on output */

src/liblink/obj5.c

@@ -474,7 +474,7 @@ addstacksplit(Link *ctxt, LSym *cursym)
 				p->as = AMOVW;
 				p->from.type = D_OREG;
 				p->from.reg = REGG;
-				p->from.offset = 3*ctxt->arch->ptrsize; // G.panic
+				p->from.offset = 4*ctxt->arch->ptrsize; // G.panic
 				p->to.type = D_REG;
 				p->to.reg = 1;
 			
@@ -783,7 +783,9 @@ stacksplit(Link *ctxt, Prog *p, int32 framesize, int noctxt)
 	p->as = AMOVW;
 	p->from.type = D_OREG;
 	p->from.reg = REGG;
-	p->from.offset = 2*ctxt->arch->ptrsize;	// G.stackguard
+	p->from.offset = 2*ctxt->arch->ptrsize;	// G.stackguard0
+	if(ctxt->cursym->cfunc)
+		p->from.offset = 3*ctxt->arch->ptrsize;	// G.stackguard1
 	p->to.type = D_REG;
 	p->to.reg = 1;
 	
@@ -876,7 +878,10 @@ stacksplit(Link *ctxt, Prog *p, int32 framesize, int noctxt)
 	p->as = ABL;
 	p->scond = C_SCOND_LS;
 	p->to.type = D_BRANCH;
-	p->to.sym = ctxt->symmorestack[noctxt];
+	if(ctxt->cursym->cfunc)
+		p->to.sym = linklookup(ctxt, "runtime.morestackc", 0);
+	else
+		p->to.sym = ctxt->symmorestack[noctxt];
 	
 	// BLS	start
 	p = appendp(ctxt, p);

src/liblink/obj6.c

@@ -452,7 +452,7 @@ addstacksplit(Link *ctxt, LSym *cursym)
 		p = appendp(ctxt, p);
 		p->as = AMOVQ;
 		p->from.type = D_INDIR+D_CX;
-		p->from.offset = 3*ctxt->arch->ptrsize; // G.panic
+		p->from.offset = 4*ctxt->arch->ptrsize; // G.panic
 		p->to.type = D_BX;
 		if(ctxt->headtype == Hnacl) {
 			p->as = AMOVL;
@@ -689,7 +689,9 @@ stacksplit(Link *ctxt, Prog *p, int32 framesize, int32 textarg, int noctxt, Prog
 		p->as = cmp;
 		p->from.type = D_SP;
 		indir_cx(ctxt, &p->to);
-		p->to.offset = 2*ctxt->arch->ptrsize;	// G.stackguard
+		p->to.offset = 2*ctxt->arch->ptrsize;	// G.stackguard0
+		if(ctxt->cursym->cfunc)
+			p->to.offset = 3*ctxt->arch->ptrsize;	// G.stackguard1
 	} else if(framesize <= StackBig) {
 		// large stack: SP-framesize <= stackguard-StackSmall
 		//	LEAQ -xxx(SP), AX
@@ -704,7 +706,9 @@ stacksplit(Link *ctxt, Prog *p, int32 framesize, int32 textarg, int noctxt, Prog
 		p->as = cmp;
 		p->from.type = D_AX;
 		indir_cx(ctxt, &p->to);
-		p->to.offset = 2*ctxt->arch->ptrsize;	// G.stackguard
+		p->to.offset = 2*ctxt->arch->ptrsize;	// G.stackguard0
+		if(ctxt->cursym->cfunc)
+			p->to.offset = 3*ctxt->arch->ptrsize;	// G.stackguard1
 	} else {
 		// Such a large stack we need to protect against wraparound.
 		// If SP is close to zero:
@@ -724,7 +728,9 @@ stacksplit(Link *ctxt, Prog *p, int32 framesize, int32 textarg, int noctxt, Prog
 		p = appendp(ctxt, p);
 		p->as = mov;
 		indir_cx(ctxt, &p->from);
-		p->from.offset = 2*ctxt->arch->ptrsize;	// G.stackguard
+		p->from.offset = 2*ctxt->arch->ptrsize;	// G.stackguard0
+		if(ctxt->cursym->cfunc)
+			p->from.offset = 3*ctxt->arch->ptrsize;	// G.stackguard1
 		p->to.type = D_SI;
 
 		p = appendp(ctxt, p);
@@ -765,7 +771,10 @@ stacksplit(Link *ctxt, Prog *p, int32 framesize, int32 textarg, int noctxt, Prog
 	p = appendp(ctxt, p);
 	p->as = ACALL;
 	p->to.type = D_BRANCH;
-	p->to.sym = ctxt->symmorestack[noctxt];
+	if(ctxt->cursym->cfunc)
+		p->to.sym = linklookup(ctxt, "runtime.morestackc", 0);
+	else
+		p->to.sym = ctxt->symmorestack[noctxt];
 	
 	p = appendp(ctxt, p);
 	p->as = AJMP;

src/liblink/obj8.c

@@ -335,7 +335,7 @@ addstacksplit(Link *ctxt, LSym *cursym)
 		p = appendp(ctxt, p);
 		p->as = AMOVL;
 		p->from.type = D_INDIR+D_CX;
-		p->from.offset = 3*ctxt->arch->ptrsize; // G.panic
+		p->from.offset = 4*ctxt->arch->ptrsize; // G.panic
 		p->to.type = D_BX;
 
 		p = appendp(ctxt, p);
@@ -538,7 +538,9 @@ stacksplit(Link *ctxt, Prog *p, int32 framesize, int noctxt, Prog **jmpok)
 		p->as = ACMPL;
 		p->from.type = D_SP;
 		p->to.type = D_INDIR+D_CX;
-		p->to.offset = 2*ctxt->arch->ptrsize;	// G.stackguard
+		p->to.offset = 2*ctxt->arch->ptrsize;	// G.stackguard0
+		if(ctxt->cursym->cfunc)
+			p->to.offset = 3*ctxt->arch->ptrsize;	// G.stackguard1
 	} else if(framesize <= StackBig) {
 		// large stack: SP-framesize <= stackguard-StackSmall
 		//	LEAL -(framesize-StackSmall)(SP), AX
@@ -553,7 +555,9 @@ stacksplit(Link *ctxt, Prog *p, int32 framesize, int noctxt, Prog **jmpok)
 		p->as = ACMPL;
 		p->from.type = D_AX;
 		p->to.type = D_INDIR+D_CX;
-		p->to.offset = 2*ctxt->arch->ptrsize;	// G.stackguard
+		p->to.offset = 2*ctxt->arch->ptrsize;	// G.stackguard0
+		if(ctxt->cursym->cfunc)
+			p->to.offset = 3*ctxt->arch->ptrsize;	// G.stackguard1
 	} else {
 		// Such a large stack we need to protect against wraparound
 		// if SP is close to zero.
@@ -573,7 +577,9 @@ stacksplit(Link *ctxt, Prog *p, int32 framesize, int noctxt, Prog **jmpok)
 		p->as = AMOVL;
 		p->from.type = D_INDIR+D_CX;
 		p->from.offset = 0;
-		p->from.offset = 2*ctxt->arch->ptrsize;	// G.stackguard
+		p->from.offset = 2*ctxt->arch->ptrsize;	// G.stackguard0
+		if(ctxt->cursym->cfunc)
+			p->from.offset = 3*ctxt->arch->ptrsize;	// G.stackguard1
 		p->to.type = D_SI;
 
 		p = appendp(ctxt, p);
@@ -616,7 +622,10 @@ stacksplit(Link *ctxt, Prog *p, int32 framesize, int noctxt, Prog **jmpok)
 	p = appendp(ctxt, p);
 	p->as = ACALL;
 	p->to.type = D_BRANCH;
-	p->to.sym = ctxt->symmorestack[noctxt];
+	if(ctxt->cursym->cfunc)
+		p->to.sym = linklookup(ctxt, "runtime.morestackc", 0);
+	else
+		p->to.sym = ctxt->symmorestack[noctxt];
 
 	p = appendp(ctxt, p);
 	p->as = AJMP;

src/liblink/obj9.c

@@ -492,7 +492,7 @@ addstacksplit(Link *ctxt, LSym *cursym)
 				q->as = AMOVD;
 				q->from.type = D_OREG;
 				q->from.reg = REGG;
-				q->from.offset = 3*ctxt->arch->ptrsize; // G.panic
+				q->from.offset = 4*ctxt->arch->ptrsize; // G.panic
 				q->to.type = D_REG;
 				q->to.reg = 3;
 
@@ -724,7 +724,9 @@ stacksplit(Link *ctxt, Prog *p, int32 framesize, int noctxt)
 	p->as = AMOVD;
 	p->from.type = D_OREG;
 	p->from.reg = REGG;
-	p->from.offset = 2*ctxt->arch->ptrsize;	// G.stackguard
+	p->from.offset = 2*ctxt->arch->ptrsize;	// G.stackguard0
+	if(ctxt->cursym->cfunc)
+		p->from.offset = 3*ctxt->arch->ptrsize;	// G.stackguard1
 	p->to.type = D_REG;
 	p->to.reg = 3;
 
@@ -832,7 +834,10 @@ stacksplit(Link *ctxt, Prog *p, int32 framesize, int noctxt)
 	p = appendp(ctxt, p);
 	p->as = ABL;
 	p->to.type = D_BRANCH;
-	p->to.sym = ctxt->symmorestack[noctxt];
+	if(ctxt->cursym->cfunc)
+		p->to.sym = linklookup(ctxt, "runtime.morestackc", 0);
+	else
+		p->to.sym = ctxt->symmorestack[noctxt];
 
 	// BR	start
 	p = appendp(ctxt, p);

src/liblink/objfile.c

@@ -332,6 +332,8 @@ writesym(Link *ctxt, Biobuf *b, LSym *s)
 			Bprint(ctxt->bso, "t=%d ", s->type);
 		if(s->dupok)
 			Bprint(ctxt->bso, "dupok ");
+		if(s->cfunc)
+			Bprint(ctxt->bso, "cfunc ");
 		if(s->nosplit)
 			Bprint(ctxt->bso, "nosplit ");
 		Bprint(ctxt->bso, "size=%lld value=%lld", (vlong)s->size, (vlong)s->value);
@@ -397,7 +399,7 @@ writesym(Link *ctxt, Biobuf *b, LSym *s)
 		wrint(b, s->args);
 		wrint(b, s->locals);
 		wrint(b, s->nosplit);
-		wrint(b, s->leaf);
+		wrint(b, s->leaf | s->cfunc<<1);
 		n = 0;
 		for(a = s->autom; a != nil; a = a->link)
 			n++;
@@ -641,6 +643,7 @@ overwrite:
 		s->nosplit = rdint(f);
 		v = rdint(f);
 		s->leaf = v&1;
+		s->cfunc = v&2;
 		n = rdint(f);
 		for(i=0; i<n; i++) {
 			a = emallocz(sizeof *a);
@@ -696,6 +699,8 @@ overwrite:
 			Bprint(ctxt->bso, "t=%d ", s->type);
 		if(s->dupok)
 			Bprint(ctxt->bso, "dupok ");
+		if(s->cfunc)
+			Bprint(ctxt->bso, "cfunc ");
 		if(s->nosplit)
 			Bprint(ctxt->bso, "nosplit ");
 		Bprint(ctxt->bso, "size=%lld value=%lld", (vlong)s->size, (vlong)s->value);

src/runtime/asm_386.s

@@ -20,7 +20,8 @@ TEXT runtime·rt0_go(SB),NOSPLIT,$0
 	// _cgo_init may update stackguard.
 	MOVL	$runtime·g0(SB), BP
 	LEAL	(-64*1024+104)(SP), BX
-	MOVL	BX, g_stackguard(BP)
+	MOVL	BX, g_stackguard0(BP)
+	MOVL	BX, g_stackguard1(BP)
 	MOVL	BX, (g_stack+stack_lo)(BP)
 	MOVL	SP, (g_stack+stack_hi)(BP)
 	
@@ -50,7 +51,8 @@ nocpuinfo:
 	MOVL	$runtime·g0(SB), CX
 	MOVL	(g_stack+stack_lo)(CX), AX
 	ADDL	$const__StackGuard, AX
-	MOVL	AX, g_stackguard(CX)
+	MOVL	AX, g_stackguard0(CX)
+	MOVL	AX, g_stackguard1(CX)
 
 	// skip runtime·ldt0setup(SB) and tls test after _cgo_init for non-windows
 	CMPL runtime·iswindows(SB), $0

src/runtime/asm_amd64.s

@@ -20,7 +20,8 @@ TEXT runtime·rt0_go(SB),NOSPLIT,$0
 	// _cgo_init may update stackguard.
 	MOVQ	$runtime·g0(SB), DI
 	LEAQ	(-64*1024+104)(SP), BX
-	MOVQ	BX, g_stackguard(DI)
+	MOVQ	BX, g_stackguard0(DI)
+	MOVQ	BX, g_stackguard1(DI)
 	MOVQ	BX, (g_stack+stack_lo)(DI)
 	MOVQ	SP, (g_stack+stack_hi)(DI)
 
@@ -48,7 +49,8 @@ nocpuinfo:
 	MOVQ	$runtime·g0(SB), CX
 	MOVQ	(g_stack+stack_lo)(CX), AX
 	ADDQ	$const__StackGuard, AX
-	MOVQ	AX, g_stackguard(CX)
+	MOVQ	AX, g_stackguard0(CX)
+	MOVQ	AX, g_stackguard1(CX)
 
 	CMPL	runtime·iswindows(SB), $0
 	JEQ ok

src/runtime/asm_amd64p32.s

@@ -22,7 +22,8 @@ TEXT runtime·rt0_go(SB),NOSPLIT,$0
 	// create istack out of the given (operating system) stack.
 	MOVL	$runtime·g0(SB), DI
 	LEAL	(-64*1024+104)(SP), BX
-	MOVL	BX, g_stackguard(DI)
+	MOVL	BX, g_stackguard0(DI)
+	MOVL	BX, g_stackguard1(DI)
 	MOVL	BX, (g_stack+stack_lo)(DI)
 	MOVL	SP, (g_stack+stack_hi)(DI)
 

src/runtime/asm_arm.s

@@ -32,7 +32,8 @@ TEXT runtime·rt0_go(SB),NOSPLIT,$-4
 
 	// create istack out of the OS stack
 	MOVW	$(-8192+104)(R13), R0
-	MOVW	R0, g_stackguard(g)
+	MOVW	R0, g_stackguard0(g)
+	MOVW	R0, g_stackguard1(g)
 	MOVW	R0, (g_stack+stack_lo)(g)
 	MOVW	R13, (g_stack+stack_hi)(g)
 
@@ -55,7 +56,8 @@ nocgo:
 	// update stackguard after _cgo_init
 	MOVW	(g_stack+stack_lo)(g), R0
 	ADD	$const__StackGuard, R0
-	MOVW	R0, g_stackguard(g)
+	MOVW	R0, g_stackguard0(g)
+	MOVW	R0, g_stackguard1(g)
 
 	BL	runtime·checkgoarm(SB)
 	BL	runtime·check(SB)

src/runtime/asm_ppc64x.s

@@ -22,7 +22,8 @@ TEXT runtime·rt0_go(SB),NOSPLIT,$0
 	MOVD	$runtime·g0(SB), g
 	MOVD	$(-64*1024), R31
 	ADD	R31, R1, R3
-	MOVD	R3, g_stackguard(g)
+	MOVD	R3, g_stackguard0(g)
+	MOVD	R3, g_stackguard1(g)
 	MOVD	R3, (g_stack+stack_lo)(g)
 	MOVD	R1, (g_stack+stack_hi)(g)
 

src/runtime/lock_futex.go

@@ -114,7 +114,7 @@ func unlock(l *mutex) {
 		throw("runtime·unlock: lock count")
 	}
 	if gp.m.locks == 0 && gp.preempt { // restore the preemption request in case we've cleared it in newstack
-		gp.stackguard = stackPreempt
+		gp.stackguard0 = stackPreempt
 	}
 }
 

src/runtime/lock_sema.go

@@ -115,7 +115,7 @@ func unlock(l *mutex) {
 		throw("runtime·unlock: lock count")
 	}
 	if gp.m.locks == 0 && gp.preempt { // restore the preemption request in case we've cleared it in newstack
-		gp.stackguard = stackPreempt
+		gp.stackguard0 = stackPreempt
 	}
 }
 

src/runtime/malloc.go

@@ -64,7 +64,7 @@ func mallocgc(size uintptr, typ *_type, flags uint32) unsafe.Pointer {
 		}
 		mp.mallocing = 1
 		if mp.curg != nil {
-			mp.curg.stackguard = ^uintptr(0xfff) | 0xbad
+			mp.curg.stackguard0 = ^uintptr(0xfff) | 0xbad
 		}
 	}
 
@@ -127,7 +127,7 @@ func mallocgc(size uintptr, typ *_type, flags uint32) unsafe.Pointer {
 						}
 						mp.mallocing = 0
 						if mp.curg != nil {
-							mp.curg.stackguard = mp.curg.stack.lo + _StackGuard
+							mp.curg.stackguard0 = mp.curg.stack.lo + _StackGuard
 						}
 						// Note: one releasem for the acquirem just above.
 						// The other for the acquirem at start of malloc.
@@ -319,7 +319,7 @@ marked:
 		}
 		mp.mallocing = 0
 		if mp.curg != nil {
-			mp.curg.stackguard = mp.curg.stack.lo + _StackGuard
+			mp.curg.stackguard0 = mp.curg.stack.lo + _StackGuard
 		}
 		// Note: one releasem for the acquirem just above.
 		// The other for the acquirem at start of malloc.

src/runtime/proc1.go

@@ -179,6 +179,9 @@ func mcommoninit(mp *m) {
 	sched.mcount++
 	checkmcount()
 	mpreinit(mp)
+	if mp.gsignal != nil {
+		mp.gsignal.stackguard1 = mp.gsignal.stack.lo + _StackGuard
+	}
 
 	// Add to allm so garbage collector doesn't free g->m
 	// when it is just in a register or thread-local storage.
@@ -210,7 +213,7 @@ func ready(gp *g) {
 	}
 	_g_.m.locks--
 	if _g_.m.locks == 0 && _g_.preempt { // restore the preemption request in case we've cleared it in newstack
-		_g_.stackguard = stackPreempt
+		_g_.stackguard0 = stackPreempt
 	}
 }
 
@@ -460,7 +463,7 @@ func stopg(gp *g) bool {
 			if !gp.gcworkdone {
 				gp.preemptscan = true
 				gp.preempt = true
-				gp.stackguard = stackPreempt
+				gp.stackguard0 = stackPreempt
 			}
 
 			// Unclaim.
@@ -542,7 +545,7 @@ func mquiesce(gpmaster *g) {
 				gp.gcworkdone = true // scan is a noop
 				break
 			}
-			if status == _Grunning && gp.stackguard == uintptr(stackPreempt) && notetsleep(&sched.stopnote, 100*1000) { // nanosecond arg
+			if status == _Grunning && gp.stackguard0 == uintptr(stackPreempt) && notetsleep(&sched.stopnote, 100*1000) { // nanosecond arg
 				noteclear(&sched.stopnote)
 			} else {
 				stopscanstart(gp)
@@ -701,7 +704,7 @@ func starttheworld() {
 	}
 	_g_.m.locks--
 	if _g_.m.locks == 0 && _g_.preempt { // restore the preemption request in case we've cleared it in newstack
-		_g_.stackguard = stackPreempt
+		_g_.stackguard0 = stackPreempt
 	}
 }
 
@@ -722,7 +725,8 @@ func mstart() {
 	}
 	// Initialize stack guards so that we can start calling
 	// both Go and C functions with stack growth prologues.
-	_g_.stackguard = _g_.stack.lo + _StackGuard
+	_g_.stackguard0 = _g_.stack.lo + _StackGuard
+	_g_.stackguard1 = _g_.stackguard0
 	mstart1()
 }
 
@@ -802,7 +806,7 @@ func allocm(_p_ *p) *m {
 	}
 	_g_.m.locks--
 	if _g_.m.locks == 0 && _g_.preempt { // restore the preemption request in case we've cleared it in newstack
-		_g_.stackguard = stackPreempt
+		_g_.stackguard0 = stackPreempt
 	}
 
 	return mp
@@ -879,7 +883,7 @@ func needm(x byte) {
 	_g_ := getg()
 	_g_.stack.hi = uintptr(noescape(unsafe.Pointer(&x))) + 1024
 	_g_.stack.lo = uintptr(noescape(unsafe.Pointer(&x))) - 32*1024
-	_g_.stackguard = _g_.stack.lo + _StackGuard
+	_g_.stackguard0 = _g_.stack.lo + _StackGuard
 
 	// Initialize this thread to use the m.
 	asminit()
@@ -1217,7 +1221,7 @@ func execute(gp *g) {
 	casgstatus(gp, _Grunnable, _Grunning)
 	gp.waitsince = 0
 	gp.preempt = false
-	gp.stackguard = gp.stack.lo + _StackGuard
+	gp.stackguard0 = gp.stack.lo + _StackGuard
 	_g_.m.p.schedtick++
 	_g_.m.curg = gp
 	gp.m = _g_.m
@@ -1613,7 +1617,7 @@ func reentersyscall(pc, sp uintptr) {
 	// (See details in comment above.)
 	// Catch calls that might, by replacing the stack guard with something that
 	// will trip any stack check and leaving a flag to tell newstack to die.
-	_g_.stackguard = stackPreempt
+	_g_.stackguard0 = stackPreempt
 	_g_.throwsplit = true
 
 	// Leave SP around for GC and traceback.
@@ -1644,7 +1648,7 @@ func reentersyscall(pc, sp uintptr) {
 	// Goroutines must not split stacks in Gsyscall status (it would corrupt g->sched).
 	// We set _StackGuard to StackPreempt so that first split stack check calls morestack.
 	// Morestack detects this case and throws.
-	_g_.stackguard = stackPreempt
+	_g_.stackguard0 = stackPreempt
 	_g_.m.locks--
 }
 
@@ -1682,7 +1686,7 @@ func entersyscallblock(dummy int32) {
 
 	_g_.m.locks++ // see comment in entersyscall
 	_g_.throwsplit = true
-	_g_.stackguard = stackPreempt // see comment in entersyscall
+	_g_.stackguard0 = stackPreempt // see comment in entersyscall
 
 	// Leave SP around for GC and traceback.
 	pc := getcallerpc(unsafe.Pointer(&dummy))
@@ -1748,10 +1752,10 @@ func exitsyscall(dummy int32) {
 		_g_.m.locks--
 		if _g_.preempt {
 			// restore the preemption request in case we've cleared it in newstack
-			_g_.stackguard = stackPreempt
+			_g_.stackguard0 = stackPreempt
 		} else {
 			// otherwise restore the real _StackGuard, we've spoiled it in entersyscall/entersyscallblock
-			_g_.stackguard = _g_.stack.lo + _StackGuard
+			_g_.stackguard0 = _g_.stack.lo + _StackGuard
 		}
 		_g_.throwsplit = false
 		return
@@ -1869,7 +1873,7 @@ func beforefork() {
 	// Code between fork and exec must not allocate memory nor even try to grow stack.
 	// Here we spoil g->_StackGuard to reliably detect any attempts to grow stack.
 	// runtime_AfterFork will undo this in parent process, but not in child.
-	gp.stackguard = stackFork
+	gp.stackguard0 = stackFork
 }
 
 // Called from syscall package before fork.
@@ -1883,7 +1887,7 @@ func afterfork() {
 	gp := getg().m.curg
 
 	// See the comment in beforefork.
-	gp.stackguard = gp.stack.lo + _StackGuard
+	gp.stackguard0 = gp.stack.lo + _StackGuard
 
 	hz := sched.profilehz
 	if hz != 0 {
@@ -1907,7 +1911,8 @@ func malg(stacksize int32) *g {
 		systemstack(func() {
 			newg.stack = stackalloc(uint32(stacksize))
 		})
-		newg.stackguard = newg.stack.lo + _StackGuard
+		newg.stackguard0 = newg.stack.lo + _StackGuard
+		newg.stackguard1 = ^uintptr(0)
 	}
 	return newg
 }
@@ -2003,7 +2008,7 @@ func newproc1(fn *funcval, argp *uint8, narg int32, nret int32, callerpc uintptr
 	}
 	_g_.m.locks--
 	if _g_.m.locks == 0 && _g_.preempt { // restore the preemption request in case we've cleared it in newstack
-		_g_.stackguard = stackPreempt
+		_g_.stackguard0 = stackPreempt
 	}
 	return newg
 }
@@ -2022,7 +2027,7 @@ func gfput(_p_ *p, gp *g) {
 		stackfree(gp.stack)
 		gp.stack.lo = 0
 		gp.stack.hi = 0
-		gp.stackguard = 0
+		gp.stackguard0 = 0
 	}
 
 	gp.schedlink = _p_.gfree
@@ -2068,7 +2073,7 @@ retry:
 			systemstack(func() {
 				gp.stack = stackalloc(_FixedStack)
 			})
-			gp.stackguard = gp.stack.lo + _StackGuard
+			gp.stackguard0 = gp.stack.lo + _StackGuard
 		} else {
 			if raceenabled {
 				racemalloc(unsafe.Pointer(gp.stack.lo), gp.stack.hi-gp.stack.lo)
@@ -2773,10 +2778,10 @@ func preemptone(_p_ *p) bool {
 	gp.preempt = true
 
 	// Every call in a go routine checks for stack overflow by
-	// comparing the current stack pointer to gp->stackguard.
+	// comparing the current stack pointer to gp->stackguard0.
-	// Setting gp->stackguard to StackPreempt folds
+	// Setting gp->stackguard0 to StackPreempt folds
 	// preemption into the normal stack overflow check.
-	gp.stackguard = stackPreempt
+	gp.stackguard0 = stackPreempt
 	return true
 }
 

src/runtime/runtime1.go

@@ -386,7 +386,7 @@ func releasem(mp *m) {
 	mp.locks--
 	if mp.locks == 0 && _g_.preempt {
 		// restore the preemption request in case we've cleared it in newstack
-		_g_.stackguard = stackPreempt
+		_g_.stackguard0 = stackPreempt
 	}
 }
 

src/runtime/runtime2.go

@@ -154,10 +154,14 @@ type stack struct {
 type g struct {
 	// Stack parameters.
 	// stack describes the actual stack memory: [stack.lo, stack.hi).
-	// stackguard is the stack pointer compared in the Go stack growth prologue.
+	// stackguard0 is the stack pointer compared in the Go stack growth prologue.
 	// It is stack.lo+StackGuard normally, but can be StackPreempt to trigger a preemption.
-	stack      stack   // offset known to runtime/cgo
+	// stackguard1 is the stack pointer compared in the C stack growth prologue.
-	stackguard uintptr // offset known to liblink
+	// It is stack.lo+StackGuard on g0 and gsignal stacks.
+	// It is ~0 on other goroutine stacks, to trigger a call to morestackc (and crash).
+	stack       stack   // offset known to runtime/cgo
+	stackguard0 uintptr // offset known to liblink
+	stackguard1 uintptr // offset known to liblink
 
 	_panic       *_panic // innermost panic - offset known to liblink
 	_defer       *_defer // innermost defer
@@ -171,7 +175,7 @@ type g struct {
 	waitreason   string // if status==gwaiting
 	schedlink    *g
 	issystem     bool // do not output in stack dump, ignore in deadlock detector
-	preempt      bool // preemption signal, duplicates stackguard = stackpreempt
+	preempt      bool // preemption signal, duplicates stackguard0 = stackpreempt
 	paniconfault bool // panic (instead of crash) on unexpected fault address
 	preemptscan  bool // preempted g does scan for gc
 	gcworkdone   bool // debug: cleared at begining of gc work phase cycle, set by gcphasework, tested at end of cycle

src/runtime/stack1.go

@@ -26,13 +26,13 @@ const (
 	poisonStack = uintptrMask & 0x6868686868686868
 
 	// Goroutine preemption request.
-	// Stored into g->stackguard to cause split stack check failure.
+	// Stored into g->stackguard0 to cause split stack check failure.
 	// Must be greater than any real sp.
 	// 0xfffffade in hex.
 	stackPreempt = uintptrMask & -1314
 
 	// Thread is forking.
-	// Stored into g->stackguard to cause split stack check failure.
+	// Stored into g->stackguard0 to cause split stack check failure.
 	// Must be greater than any real sp.
 	stackFork = uintptrMask & -1234
 )
@@ -566,7 +566,7 @@ func copystack(gp *g, newsize uintptr) {
 
 	// Swap out old stack for new one
 	gp.stack = new
-	gp.stackguard = new.lo + _StackGuard // NOTE: might clobber a preempt request
+	gp.stackguard0 = new.lo + _StackGuard // NOTE: might clobber a preempt request
 	gp.sched.sp = new.hi - used
 
 	// free old stack
@@ -611,7 +611,7 @@ func round2(x int32) int32 {
 func newstack() {
 	thisg := getg()
 	// TODO: double check all gp. shouldn't be getg().
-	if thisg.m.morebuf.g.stackguard == stackFork {
+	if thisg.m.morebuf.g.stackguard0 == stackFork {
 		throw("stack growth after fork")
 	}
 	if thisg.m.morebuf.g != thisg.m.curg {
@@ -674,7 +674,7 @@ func newstack() {
 		writebarrierptr_nostore((*uintptr)(unsafe.Pointer(&gp.sched.ctxt)), uintptr(gp.sched.ctxt))
 	}
 
-	if gp.stackguard == stackPreempt {
+	if gp.stackguard0 == stackPreempt {
 		if gp == thisg.m.g0 {
 			throw("runtime: preempt g0")
 		}
@@ -689,7 +689,7 @@ func newstack() {
 			gcphasework(gp)
 			casfrom_Gscanstatus(gp, _Gscanwaiting, _Gwaiting)
 			casgstatus(gp, _Gwaiting, _Grunning)
-			gp.stackguard = gp.stack.lo + _StackGuard
+			gp.stackguard0 = gp.stack.lo + _StackGuard
 			gp.preempt = false
 			gp.preemptscan = false // Tells the GC premption was successful.
 			gogo(&gp.sched)        // never return
@@ -700,7 +700,7 @@ func newstack() {
 		if thisg.m.locks != 0 || thisg.m.mallocing != 0 || thisg.m.gcing != 0 || thisg.m.p.status != _Prunning {
 			// Let the goroutine keep running for now.
 			// gp->preempt is set, so it will be preempted next time.
-			gp.stackguard = gp.stack.lo + _StackGuard
+			gp.stackguard0 = gp.stack.lo + _StackGuard
 			casgstatus(gp, _Gwaiting, _Grunning)
 			gogo(&gp.sched) // never return
 		}
@@ -804,3 +804,10 @@ func shrinkfinish() {
 		s = t
 	}
 }
+
+//go:nosplit
+func morestackc() {
+	systemstack(func() {
+		throw("attempt to execute C code on Go stack")
+	})
+}

src/runtime/stack2.go

@@ -97,7 +97,7 @@ const (
 )
 
 // Goroutine preemption request.
-// Stored into g->stackguard to cause split stack check failure.
+// Stored into g->stackguard0 to cause split stack check failure.
 // Must be greater than any real sp.
 // 0xfffffade in hex.
 const (

src/runtime/sys_plan9_386.s

@@ -148,7 +148,8 @@ TEXT runtime·tstart_plan9(SB),NOSPLIT,$0
 	MOVL	AX, (g_stack+stack_hi)(DX)
 	SUBL	$(64*1024), AX		// stack size
 	MOVL	AX, (g_stack+stack_lo)(DX)
-	MOVL	AX, g_stackguard(DX)
+	MOVL	AX, g_stackguard0(DX)
+	MOVL	AX, g_stackguard1(DX)
 
 	// Initialize procid from TOS struct.
 	MOVL	_tos(SB), AX

src/runtime/sys_plan9_amd64.s

@@ -145,7 +145,8 @@ TEXT runtime·tstart_plan9(SB),NOSPLIT,$0
 	MOVQ	AX, (g_stack+stack_hi)(DX)
 	SUBQ	$(64*1024), AX		// stack size
 	MOVQ	AX, (g_stack+stack_lo)(DX)
-	MOVQ	AX, g_stackguard(DX)
+	MOVQ	AX, g_stackguard0(DX)
+	MOVQ	AX, g_stackguard1(DX)
 
 	// Initialize procid from TOS struct.
 	MOVQ	_tos(SB), AX

src/runtime/sys_solaris_amd64.s

@@ -134,7 +134,8 @@ TEXT runtime·tstart_sysvicall(SB),NOSPLIT,$0
 	SUBQ	$(0x100000), AX		// stack size
 	MOVQ	AX, (g_stack+stack_lo)(DX)
 	ADDQ	$const__StackGuard, AX
-	MOVQ	AX, g_stackguard(DX)
+	MOVQ	AX, g_stackguard0(DX)
+	MOVQ	AX, g_stackguard1(DX)
 
 	// Someday the convention will be D is always cleared.
 	CLD

src/runtime/sys_windows_386.s

@@ -209,7 +209,8 @@ TEXT runtime·externalthreadhandler(SB),NOSPLIT,$0
 	LEAL	-8192(SP), CX
 	MOVL	CX, (g_stack+stack_lo)(SP)
 	ADDL	$const__StackGuard, CX
-	MOVL	CX, g_stackguard(SP)
+	MOVL	CX, g_stackguard0(SP)
+	MOVL	CX, g_stackguard1(SP)
 	MOVL	DX, (g_stack+stack_hi)(SP)
 
 	PUSHL	16(BP)			// arg for handler
@@ -314,7 +315,8 @@ TEXT runtime·tstart(SB),NOSPLIT,$0
 	SUBL	$(64*1024), AX		// stack size
 	MOVL	AX, (g_stack+stack_lo)(DX)
 	ADDL	$const__StackGuard, AX
-	MOVL	AX, g_stackguard(DX)
+	MOVL	AX, g_stackguard0(DX)
+	MOVL	AX, g_stackguard1(DX)
 
 	// Set up tls.
 	LEAL	m_tls(CX), SI

src/runtime/sys_windows_amd64.s

@@ -246,7 +246,8 @@ TEXT runtime·externalthreadhandler(SB),NOSPLIT,$0
 	LEAQ	-8192(SP), CX
 	MOVQ	CX, (g_stack+stack_lo)(SP)
 	ADDQ	$const__StackGuard, CX
-	MOVQ	CX, g_stackguard(SP)
+	MOVQ	CX, g_stackguard0(SP)
+	MOVQ	CX, g_stackguard1(SP)
 	MOVQ	DX, (g_stack+stack_hi)(SP)
 
 	PUSHQ	32(BP)			// arg for handler
@@ -355,7 +356,8 @@ TEXT runtime·tstart_stdcall(SB),NOSPLIT,$0
 	SUBQ	$(64*1024), AX		// stack size
 	MOVQ	AX, (g_stack+stack_lo)(DX)
 	ADDQ	$const__StackGuard, AX
-	MOVQ	AX, g_stackguard(DX)
+	MOVQ	AX, g_stackguard0(DX)
+	MOVQ	AX, g_stackguard1(DX)
 
 	// Set up tls.
 	LEAQ	m_tls(CX), SI