cmd/ld: clear unused ctxt before morestack

For non-closure functions, the context register is uninitialized
on entry and will not be used, but morestack saves it and then the
garbage collector treats it as live. This can be a source of memory
leaks if the context register points at otherwise dead memory.
Avoid this by introducing a parallel set of morestack functions
that clear the context register, and use those for the non-closure functions.

I hope this will help with some of the finalizer flakiness, but it probably won't.

Fixes #7244.

LGTM=dvyukov
R=khr, dvyukov
CC=golang-codereviews
https://golang.org/cl/71030044

include/link.h

@@ -356,7 +356,7 @@ struct	Link
 	LSym*	sym_divu;
 	LSym*	sym_mod;
 	LSym*	sym_modu;
-	LSym*	symmorestack[10];
+	LSym*	symmorestack[20];
 	LSym*	gmsym;
 	LSym*	plan9tos;
 	Prog*	curp;

src/cmd/5g/gsubr.c

@@ -1264,6 +1264,8 @@ naddr(Node *n, Addr *a, int canemitcode)
 		break;
 	
 	case OCLOSUREVAR:
+		if(!curfn->needctxt)
+			fatal("closurevar without needctxt");
 		a->type = D_OREG;
 		a->reg = 7;
 		a->offset = n->xoffset;

src/cmd/6g/gsubr.c

@@ -1186,6 +1186,8 @@ naddr(Node *n, Addr *a, int canemitcode)
 		break;
 	
 	case OCLOSUREVAR:
+		if(!curfn->needctxt)
+			fatal("closurevar without needctxt");
 		a->type = D_DX+D_INDIR;
 		a->sym = nil;
 		a->offset = n->xoffset;

src/cmd/8g/gsubr.c

@@ -2211,6 +2211,8 @@ naddr(Node *n, Addr *a, int canemitcode)
 		break;
 
 	case OCLOSUREVAR:
+		if(!curfn->needctxt)
+			fatal("closurevar without needctxt");
 		a->type = D_DX+D_INDIR;
 		a->offset = n->xoffset;
 		a->sym = nil;

src/cmd/gc/closure.c

@@ -161,6 +161,7 @@ makeclosure(Node *func)
 	// and initialize in entry prologue.
 	body = nil;
 	offset = widthptr;
+	xfunc->needctxt = func->cvars != nil;
 	for(l=func->cvars; l; l=l->next) {
 		v = l->n;
 		if(v->op == 0)
@@ -361,6 +362,7 @@ makepartialcall(Node *fn, Type *t0, Node *meth)
 
 	// Declare and initialize variable holding receiver.
 	body = nil;
+	xfunc->needctxt = 1;
 	cv = nod(OCLOSUREVAR, N, N);
 	cv->xoffset = widthptr;
 	cv->type = rcvrtype;

src/cmd/gc/go.h

@@ -283,6 +283,7 @@ struct	Node
 	schar	likely; // likeliness of if statement
 	uchar	hasbreak;	// has break statement
 	uchar	needzero; // if it contains pointers, needs to be zeroed on function entry
+	uchar	needctxt;	// function uses context register (has closure variables)
 	uint	esc;		// EscXXX
 	int	funcdepth;
 

src/cmd/gc/pgen.c

@@ -192,6 +192,8 @@ compile(Node *fn)
 		ptxt->TEXTFLAG |= DUPOK;
 	if(fn->wrapper)
 		ptxt->TEXTFLAG |= WRAPPER;
+	if(fn->needctxt)
+		ptxt->TEXTFLAG |= NEEDCTXT;
 
 	// Clumsy but important.
 	// See test/recover.go for test cases and src/pkg/reflect/value.go

src/cmd/ld/textflag.h

@@ -19,3 +19,5 @@
 #define NOPTR	16
 // This is a wrapper function and should not count as disabling 'recover'.
 #define WRAPPER 32
+// This function uses its incoming context register.
+#define NEEDCTXT 64

src/liblink/obj5.c

@@ -194,7 +194,7 @@ prg(void)
 	return p;
 }
 
-static	Prog*	stacksplit(Link*, Prog*, int32);
+static	Prog*	stacksplit(Link*, Prog*, int32, int);
 static	void		initdiv(Link*);
 static	void	softfloat(Link*, LSym*);
 
@@ -237,8 +237,10 @@ addstacksplit(Link *ctxt, LSym *cursym)
 	
 	autosize = 0;
 
-	if(ctxt->symmorestack[0] == nil)
+	if(ctxt->symmorestack[0] == nil) {
 		ctxt->symmorestack[0] = linklookup(ctxt, "runtime.morestack", 0);
+		ctxt->symmorestack[1] = linklookup(ctxt, "runtime.morestack_noctxt", 0);
+	}
 
 	q = nil;
 	
@@ -409,7 +411,7 @@ addstacksplit(Link *ctxt, LSym *cursym)
 			}
 
 			if(!(p->reg & NOSPLIT))
-				p = stacksplit(ctxt, p, autosize); // emit split check
+				p = stacksplit(ctxt, p, autosize, !(cursym->text->from.scale&NEEDCTXT)); // emit split check
 			
 			// MOVW.W		R14,$-autosize(SP)
 			p = appendp(ctxt, p);
@@ -727,7 +729,7 @@ softfloat(Link *ctxt, LSym *cursym)
 }
 
 static Prog*
-stacksplit(Link *ctxt, Prog *p, int32 framesize)
+stacksplit(Link *ctxt, Prog *p, int32 framesize, int noctxt)
 {
 	int32 arg;
 
@@ -851,7 +853,7 @@ stacksplit(Link *ctxt, Prog *p, int32 framesize)
 	p->as = ABL;
 	p->scond = C_SCOND_LS;
 	p->to.type = D_BRANCH;
-	p->to.sym = ctxt->symmorestack[0];
+	p->to.sym = ctxt->symmorestack[noctxt];
 	
 	// BLS	start
 	p = appendp(ctxt, p);

src/liblink/obj6.c

@@ -342,20 +342,30 @@ static char*
 morename[] =
 {
 	"runtime.morestack00",
+	"runtime.morestack00_noctxt",
 	"runtime.morestack10",
+	"runtime.morestack10_noctxt",
 	"runtime.morestack01",
+	"runtime.morestack01_noctxt",
 	"runtime.morestack11",
+	"runtime.morestack11_noctxt",
 
 	"runtime.morestack8",
+	"runtime.morestack8_noctxt",
 	"runtime.morestack16",
+	"runtime.morestack16_noctxt",
 	"runtime.morestack24",
+	"runtime.morestack24_noctxt",
 	"runtime.morestack32",
+	"runtime.morestack32_noctxt",
 	"runtime.morestack40",
+	"runtime.morestack40_noctxt",
 	"runtime.morestack48",
+	"runtime.morestack48_noctxt",
 };
 
 static Prog*	load_g_cx(Link*, Prog*);
-static Prog*	stacksplit(Link*, Prog*, int32, int32, Prog**);
+static Prog*	stacksplit(Link*, Prog*, int32, int32, int, Prog**);
 static void	indir_cx(Link*, Addr*);
 
 static void
@@ -419,7 +429,7 @@ addstacksplit(Link *ctxt, LSym *cursym)
 		p = load_g_cx(ctxt, p); // load g into CX
 	}
 	if(!(cursym->text->from.scale & NOSPLIT))
-		p = stacksplit(ctxt, p, autoffset, textarg, &q); // emit split check
+		p = stacksplit(ctxt, p, autoffset, textarg, !(cursym->text->from.scale&NEEDCTXT), &q); // emit split check
 
 	if(autoffset) {
 		if(autoffset%ctxt->arch->regsize != 0)
@@ -674,7 +684,7 @@ load_g_cx(Link *ctxt, Prog *p)
 // On return, *jmpok is the instruction that should jump
 // to the stack frame allocation if no split is needed.
 static Prog*
-stacksplit(Link *ctxt, Prog *p, int32 framesize, int32 textarg, Prog **jmpok)
+stacksplit(Link *ctxt, Prog *p, int32 framesize, int32 textarg, int noctxt, Prog **jmpok)
 {
 	Prog *q, *q1;
 	uint32 moreconst1, moreconst2, i;
@@ -822,7 +832,7 @@ stacksplit(Link *ctxt, Prog *p, int32 framesize, int32 textarg, Prog **jmpok)
 	if(moreconst1 == 0 && moreconst2 == 0) {
 		p->as = ACALL;
 		p->to.type = D_BRANCH;
-		p->to.sym = ctxt->symmorestack[0];
+		p->to.sym = ctxt->symmorestack[0*2+noctxt];
 	} else
 	if(moreconst1 != 0 && moreconst2 == 0) {
 		p->as = AMOVL;
@@ -833,13 +843,13 @@ stacksplit(Link *ctxt, Prog *p, int32 framesize, int32 textarg, Prog **jmpok)
 		p = appendp(ctxt, p);
 		p->as = ACALL;
 		p->to.type = D_BRANCH;
-		p->to.sym = ctxt->symmorestack[1];
+		p->to.sym = ctxt->symmorestack[1*2+noctxt];
 	} else
 	if(moreconst1 == 0 && moreconst2 <= 48 && moreconst2%8 == 0) {
 		i = moreconst2/8 + 3;
 		p->as = ACALL;
 		p->to.type = D_BRANCH;
-		p->to.sym = ctxt->symmorestack[i];
+		p->to.sym = ctxt->symmorestack[i*2+noctxt];
 	} else
 	if(moreconst1 == 0 && moreconst2 != 0) {
 		p->as = AMOVL;
@@ -850,7 +860,7 @@ stacksplit(Link *ctxt, Prog *p, int32 framesize, int32 textarg, Prog **jmpok)
 		p = appendp(ctxt, p);
 		p->as = ACALL;
 		p->to.type = D_BRANCH;
-		p->to.sym = ctxt->symmorestack[2];
+		p->to.sym = ctxt->symmorestack[2*2+noctxt];
 	} else {
 		p->as = mov;
 		p->from.type = D_CONST;
@@ -861,7 +871,7 @@ stacksplit(Link *ctxt, Prog *p, int32 framesize, int32 textarg, Prog **jmpok)
 		p = appendp(ctxt, p);
 		p->as = ACALL;
 		p->to.type = D_BRANCH;
-		p->to.sym = ctxt->symmorestack[3];
+		p->to.sym = ctxt->symmorestack[3*2+noctxt];
 	}
 	
 	p = appendp(ctxt, p);

src/liblink/obj8.c

@@ -256,7 +256,7 @@ prg(void)
 }
 
 static Prog*	load_g_cx(Link*, Prog*);
-static Prog*	stacksplit(Link*, Prog*, int32, Prog**);
+static Prog*	stacksplit(Link*, Prog*, int32, int, Prog**);
 
 static void
 addstacksplit(Link *ctxt, LSym *cursym)
@@ -265,8 +265,10 @@ addstacksplit(Link *ctxt, LSym *cursym)
 	int32 autoffset, deltasp;
 	int a;
 
-	if(ctxt->symmorestack[0] == nil)
+	if(ctxt->symmorestack[0] == nil) {
 		ctxt->symmorestack[0] = linklookup(ctxt, "runtime.morestack", 0);
+		ctxt->symmorestack[1] = linklookup(ctxt, "runtime.morestack_noctxt", 0);
+	}
 
 	if(ctxt->headtype == Hplan9 && ctxt->plan9tos == nil)
 		ctxt->plan9tos = linklookup(ctxt, "_tos", 0);
@@ -291,7 +293,7 @@ addstacksplit(Link *ctxt, LSym *cursym)
 		p = load_g_cx(ctxt, p); // load g into CX
 	}
 	if(!(cursym->text->from.scale & NOSPLIT))
-		p = stacksplit(ctxt, p, autoffset, &q); // emit split check
+		p = stacksplit(ctxt, p, autoffset, !(cursym->text->from.scale&NEEDCTXT), &q); // emit split check
 
 	if(autoffset) {
 		p = appendp(ctxt, p);
@@ -499,7 +501,7 @@ load_g_cx(Link *ctxt, Prog *p)
 // On return, *jmpok is the instruction that should jump
 // to the stack frame allocation if no split is needed.
 static Prog*
-stacksplit(Link *ctxt, Prog *p, int32 framesize, Prog **jmpok)
+stacksplit(Link *ctxt, Prog *p, int32 framesize, int noctxt, Prog **jmpok)
 {
 	Prog *q, *q1;
 	int arg;
@@ -642,7 +644,7 @@ stacksplit(Link *ctxt, Prog *p, int32 framesize, Prog **jmpok)
 	p = appendp(ctxt, p);
 	p->as = ACALL;
 	p->to.type = D_BRANCH;
-	p->to.sym = ctxt->symmorestack[0];
+	p->to.sym = ctxt->symmorestack[noctxt];
 
 	p = appendp(ctxt, p);
 	p->as = AJMP;

src/pkg/runtime/asm_386.s

@@ -247,6 +247,10 @@ TEXT runtime·morestack(SB),NOSPLIT,$0-0
 	MOVL	$0, 0x1003	// crash if newstack returns
 	RET
 
+TEXT runtime·morestack_noctxt(SB),NOSPLIT,$0-0
+	MOVL	$0, DX
+	JMP runtime·morestack(SB)
+
 // Called from panic.  Mimics morestack,
 // reuses stack growth code to create a frame
 // with the desired args running the desired function.

src/pkg/runtime/asm_amd64.s

@@ -456,6 +456,46 @@ TEXT morestack<>(SB),NOSPLIT,$0
 	MOVQ	$runtime·morestack(SB), AX
 	JMP	AX
 
+TEXT runtime·morestack00_noctxt(SB),NOSPLIT,$0
+	MOVL	$0, DX
+	JMP	runtime·morestack00(SB)
+
+TEXT runtime·morestack01_noctxt(SB),NOSPLIT,$0
+	MOVL	$0, DX
+	JMP	runtime·morestack01(SB)
+
+TEXT runtime·morestack10_noctxt(SB),NOSPLIT,$0
+	MOVL	$0, DX
+	JMP	runtime·morestack10(SB)
+
+TEXT runtime·morestack11_noctxt(SB),NOSPLIT,$0
+	MOVL	$0, DX
+	JMP	runtime·morestack11(SB)
+
+TEXT runtime·morestack8_noctxt(SB),NOSPLIT,$0
+	MOVL	$0, DX
+	JMP	runtime·morestack8(SB)
+
+TEXT runtime·morestack16_noctxt(SB),NOSPLIT,$0
+	MOVL	$0, DX
+	JMP	runtime·morestack16(SB)
+
+TEXT runtime·morestack24_noctxt(SB),NOSPLIT,$0
+	MOVL	$0, DX
+	JMP	runtime·morestack24(SB)
+
+TEXT runtime·morestack32_noctxt(SB),NOSPLIT,$0
+	MOVL	$0, DX
+	JMP	runtime·morestack32(SB)
+
+TEXT runtime·morestack40_noctxt(SB),NOSPLIT,$0
+	MOVL	$0, DX
+	JMP	runtime·morestack40(SB)
+
+TEXT runtime·morestack48_noctxt(SB),NOSPLIT,$0
+	MOVL	$0, DX
+	JMP	runtime·morestack48(SB)
+
 // bool cas(int32 *val, int32 old, int32 new)
 // Atomically:
 //	if(*val == old){

src/pkg/runtime/asm_amd64p32.s

@@ -437,6 +437,46 @@ TEXT morestack<>(SB),NOSPLIT,$0
 	MOVL	$runtime·morestack(SB), AX
 	JMP	AX
 
+TEXT runtime·morestack00_noctxt(SB),NOSPLIT,$0
+	MOVL	$0, DX
+	JMP	runtime·morestack00(SB)
+
+TEXT runtime·morestack01_noctxt(SB),NOSPLIT,$0
+	MOVL	$0, DX
+	JMP	runtime·morestack01(SB)
+
+TEXT runtime·morestack10_noctxt(SB),NOSPLIT,$0
+	MOVL	$0, DX
+	JMP	runtime·morestack10(SB)
+
+TEXT runtime·morestack11_noctxt(SB),NOSPLIT,$0
+	MOVL	$0, DX
+	JMP	runtime·morestack11(SB)
+
+TEXT runtime·morestack8_noctxt(SB),NOSPLIT,$0
+	MOVL	$0, DX
+	JMP	runtime·morestack8(SB)
+
+TEXT runtime·morestack16_noctxt(SB),NOSPLIT,$0
+	MOVL	$0, DX
+	JMP	runtime·morestack16(SB)
+
+TEXT runtime·morestack24_noctxt(SB),NOSPLIT,$0
+	MOVL	$0, DX
+	JMP	runtime·morestack24(SB)
+
+TEXT runtime·morestack32_noctxt(SB),NOSPLIT,$0
+	MOVL	$0, DX
+	JMP	runtime·morestack32(SB)
+
+TEXT runtime·morestack40_noctxt(SB),NOSPLIT,$0
+	MOVL	$0, DX
+	JMP	runtime·morestack40(SB)
+
+TEXT runtime·morestack48_noctxt(SB),NOSPLIT,$0
+	MOVL	$0, DX
+	JMP	runtime·morestack48(SB)
+
 // bool cas(int32 *val, int32 old, int32 new)
 // Atomically:
 //	if(*val == old){

src/pkg/runtime/asm_arm.s

@@ -213,6 +213,10 @@ TEXT runtime·morestack(SB),NOSPLIT,$-4-0
 	// is still in this function, and not the beginning of the next.
 	RET
 
+TEXT runtime·morestack_noctxt(SB),NOSPLIT,$-4-0
+	MOVW	$0, R7
+	JMP runtime·morestack(SB)
+
 // Called from panic.  Mimics morestack,
 // reuses stack growth code to create a frame
 // with the desired args running the desired function.