runtime: make continuation pc available to stack walk

The 'continuation pc' is where the frame will continue
execution, if anywhere. For a frame that stopped execution
due to a CALL instruction, the continuation pc is immediately
after the CALL. But for a frame that stopped execution due to
a fault, the continuation pc is the pc after the most recent CALL
to deferproc in that frame, or else 0. That is where execution
will continue, if anywhere.

The liveness information is only recorded for CALL instructions.
This change makes sure that we never look for liveness information
except for CALL instructions.

Using a valid PC fixes crashes when a garbage collection or
stack copying tries to process a stack frame that has faulted.

Record continuation pc in heapdump (format change).

Fixes #8048.

LGTM=iant, khr
R=khr, iant, dvyukov
CC=golang-codereviews, r
https://golang.org/cl/100870044

src/pkg/runtime/cgocall.c

@@ -122,7 +122,7 @@ runtime·cgocall(void (*fn)(void*), void *arg)
 	d.fn = &endcgoV;
 	d.siz = 0;
 	d.link = g->defer;
-	d.argp = (void*)-1;  // unused because unlockm never recovers
+	d.argp = NoArgs;
 	d.special = true;
 	g->defer = &d;
 	
@@ -259,7 +259,7 @@ runtime·cgocallbackg1(void)
 	d.fn = &unwindmf;
 	d.siz = 0;
 	d.link = g->defer;
-	d.argp = (void*)-1;  // unused because unwindm never recovers
+	d.argp = NoArgs;
 	d.special = true;
 	g->defer = &d;
 

src/pkg/runtime/heapdump.c

@@ -346,6 +346,7 @@ dumpframe(Stkframe *s, void *arg)
 	dumpmemrange((byte*)s->sp, s->fp - s->sp);  // frame contents
 	dumpint(f->entry);
 	dumpint(s->pc);
+	dumpint(s->continpc);
 	name = runtime·funcname(f);
 	if(name == nil)
 		name = "unknown function";

src/pkg/runtime/mgc0.c

@@ -1574,7 +1574,11 @@ scanframe(Stkframe *frame, void *wbufp)
 	bool precise;
 
 	f = frame->fn;
-	targetpc = frame->pc;
+	targetpc = frame->continpc;
+	if(targetpc == 0) {
+		// Frame is dead.
+		return true;
+	}
 	if(targetpc != f->entry)
 		targetpc--;
 	pcdata = runtime·pcdatavalue(f, PCDATA_StackMapIndex, targetpc);

src/pkg/runtime/panic.c

@@ -225,7 +225,7 @@ runtime·panic(Eface e)
 	dabort.fn = &abortpanicV;
 	dabort.siz = sizeof(&p);
 	dabort.args[0] = &p;
-	dabort.argp = (void*)-1;  // unused because abortpanic never recovers
+	dabort.argp = NoArgs;
 	dabort.special = true;
 
 	for(;;) {

src/pkg/runtime/proc.c

@@ -230,7 +230,7 @@ runtime·main(void)
 	d.fn = &initDone;
 	d.siz = 0;
 	d.link = g->defer;
-	d.argp = (void*)-1;
+	d.argp = NoArgs;
 	d.special = true;
 	g->defer = &d;
 

src/pkg/runtime/runtime.h

@@ -703,6 +703,11 @@ struct Defer
 	void*	args[1];	// padded to actual size
 };
 
+// argp used in Defer structs when there is no argp.
+// TODO(rsc): Maybe we could use nil instead, but we've always used -1
+// and I don't want to change this days before the Go 1.3 release.
+#define NoArgs ((byte*)-1)
+
 /*
  * panics
  */
@@ -724,6 +729,7 @@ struct Stkframe
 {
 	Func*	fn;	// function being run
 	uintptr	pc;	// program counter within fn
+	uintptr	continpc;	// program counter where execution can continue, or 0 if not
 	uintptr	lr;	// program counter at caller aka link register
 	uintptr	sp;	// stack pointer at pc
 	uintptr	fp;	// stack pointer at caller aka frame pointer

src/pkg/runtime/stack.c

@@ -492,10 +492,14 @@ adjustframe(Stkframe *frame, void *arg)
 	adjinfo = arg;
 	f = frame->fn;
 	if(StackDebug >= 2)
-		runtime·printf("    adjusting %s frame=[%p,%p] pc=%p\n", runtime·funcname(f), frame->sp, frame->fp, frame->pc);
+		runtime·printf("    adjusting %s frame=[%p,%p] pc=%p continpc=%p\n", runtime·funcname(f), frame->sp, frame->fp, frame->pc, frame->continpc);
 	if(f->entry == (uintptr)runtime·main)
 		return true;
-	targetpc = frame->pc;
+	targetpc = frame->continpc;
+	if(targetpc == 0) {
+		// Frame is dead.
+		return true;
+	}
 	if(targetpc != f->entry)
 		targetpc--;
 	pcdata = runtime·pcdatavalue(f, PCDATA_StackMapIndex, targetpc);

src/pkg/runtime/traceback_arm.c

@@ -8,18 +8,22 @@
 #include "funcdata.h"
 
 void runtime·sigpanic(void);
+void runtime·newproc(void);
+void runtime·deferproc(void);
 
 int32
 runtime·gentraceback(uintptr pc0, uintptr sp0, uintptr lr0, G *gp, int32 skip, uintptr *pcbuf, int32 max, bool (*callback)(Stkframe*, void*), void *v, bool printall)
 {
 	int32 i, n, nprint, line, gotraceback;
-	uintptr x, tracepc;
-	bool waspanic, printing;
+	uintptr x, tracepc, sparg;
+	bool waspanic, wasnewproc, printing;
 	Func *f, *flr;
 	Stkframe frame;
 	Stktop *stk;
 	String file;
-	
+	Panic *panic;
+	Defer *defer;
+
 	gotraceback = runtime·gotraceback(nil);
 
 	if(pc0 == ~(uintptr)0 && sp0 == ~(uintptr)0) { // Signal to fetch saved values from gp.
@@ -40,8 +44,17 @@ runtime·gentraceback(uintptr pc0, uintptr sp0, uintptr lr0, G *gp, int32 skip,
 	frame.lr = lr0;
 	frame.sp = sp0;
 	waspanic = false;
+	wasnewproc = false;
 	printing = pcbuf==nil && callback==nil;
 
+	panic = gp->panic;
+	defer = gp->defer;
+
+	while(defer != nil && defer->argp == NoArgs)
+		defer = defer->link;	
+	while(panic != nil && panic->defer == nil)
+		panic = panic->link;
+
 	// If the PC is zero, it's likely a nil function call.
 	// Start in the caller's frame.
 	if(frame.pc == 0) {
@@ -135,6 +148,47 @@ runtime·gentraceback(uintptr pc0, uintptr sp0, uintptr lr0, G *gp, int32 skip,
 			}
 		}
 
+		// Determine function SP where deferproc would find its arguments.
+		// On ARM that's just the standard bottom-of-stack plus 1 word for
+		// the saved LR. If the previous frame was a direct call to newproc/deferproc,
+		// however, the SP is three words lower than normal.
+		// If the function has no frame at all - perhaps it just started, or perhaps
+		// it is a leaf with no local variables - then we cannot possibly find its
+		// SP in a defer, and we might confuse its SP for its caller's SP, so
+		// set sparg=0 in that case.
+		sparg = 0;
+		if(frame.fp != frame.sp) {
+			sparg = frame.sp + sizeof(uintreg);
+			if(wasnewproc)
+				sparg += 3*sizeof(uintreg);
+		}
+
+		// Determine frame's 'continuation PC', where it can continue.
+		// Normally this is the return address on the stack, but if sigpanic
+		// is immediately below this function on the stack, then the frame
+		// stopped executing due to a trap, and frame.pc is probably not
+		// a safe point for looking up liveness information. In this panicking case,
+		// the function either doesn't return at all (if it has no defers or if the
+		// defers do not recover) or it returns from one of the calls to 
+		// deferproc a second time (if the corresponding deferred func recovers).
+		// It suffices to assume that the most recent deferproc is the one that
+		// returns; everything live at earlier deferprocs is still live at that one.
+		frame.continpc = frame.pc;
+		if(waspanic) {
+			if(panic != nil && panic->defer->argp == (byte*)sparg)
+				frame.continpc = (uintptr)panic->defer->pc;
+			else if(defer != nil && defer->argp == (byte*)sparg)
+				frame.continpc = (uintptr)defer->pc;
+			else
+				frame.continpc = 0;
+		}
+
+		// Unwind our local panic & defer stacks past this frame.
+		while(panic != nil && (panic->defer == nil || panic->defer->argp == (byte*)sparg || panic->defer->argp == NoArgs))
+			panic = panic->link;
+		while(defer != nil && (defer->argp == (byte*)sparg || defer->argp == NoArgs))
+			defer = defer->link;	
+
 		if(skip > 0) {
 			skip--;
 			goto skipped;
@@ -170,7 +224,7 @@ runtime·gentraceback(uintptr pc0, uintptr sp0, uintptr lr0, G *gp, int32 skip,
 				if(frame.pc > f->entry)
 					runtime·printf(" +%p", (uintptr)(frame.pc - f->entry));
 				if(m->throwing > 0 && gp == m->curg || gotraceback >= 2)
-					runtime·printf(" fp=%p", frame.fp);
+					runtime·printf(" fp=%p sp=%p", frame.fp, frame.sp);
 				runtime·printf("\n");
 				nprint++;
 			}
@@ -179,6 +233,7 @@ runtime·gentraceback(uintptr pc0, uintptr sp0, uintptr lr0, G *gp, int32 skip,
 		
 	skipped:
 		waspanic = f->entry == (uintptr)runtime·sigpanic;
+		wasnewproc = f->entry == (uintptr)runtime·newproc || f->entry == (uintptr)runtime·deferproc;
 
 		// Do not unwind past the bottom of the stack.
 		if(flr == nil)
@@ -206,6 +261,14 @@ runtime·gentraceback(uintptr pc0, uintptr sp0, uintptr lr0, G *gp, int32 skip,
 	if(pcbuf == nil && callback == nil)
 		n = nprint;
 
+	if(callback != nil && n < max && (defer != nil || panic != nil && panic->defer != nil)) {
+		if(defer != nil)
+			runtime·printf("runtime: g%D: leftover defer argp=%p pc=%p\n", gp->goid, defer->argp, defer->pc);
+		if(panic != nil && panic->defer != nil)
+			runtime·printf("runtime: g%D: leftover panic argp=%p pc=%p\n", gp->goid, panic->defer->argp, panic->defer->pc);
+		runtime·throw("traceback has leftover defers or panics");
+	}
+
 	return n;		
 }
 

src/pkg/runtime/traceback_x86.c

@@ -13,6 +13,8 @@
 #endif
 
 void runtime·sigpanic(void);
+void runtime·newproc(void);
+void runtime·deferproc(void);
 
 #ifdef GOOS_windows
 void runtime·sigtramp(void);
@@ -29,12 +31,14 @@ int32
 runtime·gentraceback(uintptr pc0, uintptr sp0, uintptr lr0, G *gp, int32 skip, uintptr *pcbuf, int32 max, bool (*callback)(Stkframe*, void*), void *v, bool printall)
 {
 	int32 i, n, nprint, line, gotraceback;
-	uintptr tracepc;
-	bool waspanic, printing;
+	uintptr tracepc, sparg;
+	bool waspanic, wasnewproc, printing;
 	Func *f, *flr;
 	Stkframe frame;
 	Stktop *stk;
 	String file;
+	Panic *panic;
+	Defer *defer;
 
 	USED(lr0);
 	
@@ -55,8 +59,16 @@ runtime·gentraceback(uintptr pc0, uintptr sp0, uintptr lr0, G *gp, int32 skip,
 	frame.pc = pc0;
 	frame.sp = sp0;
 	waspanic = false;
+	wasnewproc = false;
 	printing = pcbuf==nil && callback==nil;
-	
+	panic = gp->panic;
+	defer = gp->defer;
+
+	while(defer != nil && defer->argp == NoArgs)
+		defer = defer->link;	
+	while(panic != nil && panic->defer == nil)
+		panic = panic->link;
+
 	// If the PC is zero, it's likely a nil function call.
 	// Start in the caller's frame.
 	if(frame.pc == 0) {
@@ -119,6 +131,7 @@ runtime·gentraceback(uintptr pc0, uintptr sp0, uintptr lr0, G *gp, int32 skip,
 			
 			// Invoke callback so that stack copier sees an uncopyable frame.
 			if(callback != nil) {
+				frame.continpc = frame.pc;
 				frame.argp = nil;
 				frame.arglen = 0;
 				if(!callback(&frame, v))
@@ -194,6 +207,40 @@ runtime·gentraceback(uintptr pc0, uintptr sp0, uintptr lr0, G *gp, int32 skip,
 				frame.arglen = 0;
 			}
 		}
+		
+		// Determine function SP where deferproc would find its arguments.
+		// On x86 that's just the standard bottom-of-stack, so SP exactly.
+		// If the previous frame was a direct call to newproc/deferproc, however,
+		// the SP is two words lower than normal.
+		sparg = frame.sp;
+		if(wasnewproc)
+			sparg += 2*sizeof(uintreg);
+
+		// Determine frame's 'continuation PC', where it can continue.
+		// Normally this is the return address on the stack, but if sigpanic
+		// is immediately below this function on the stack, then the frame
+		// stopped executing due to a trap, and frame.pc is probably not
+		// a safe point for looking up liveness information. In this panicking case,
+		// the function either doesn't return at all (if it has no defers or if the
+		// defers do not recover) or it returns from one of the calls to 
+		// deferproc a second time (if the corresponding deferred func recovers).
+		// It suffices to assume that the most recent deferproc is the one that
+		// returns; everything live at earlier deferprocs is still live at that one.
+		frame.continpc = frame.pc;
+		if(waspanic) {
+			if(panic != nil && panic->defer->argp == (byte*)sparg)
+				frame.continpc = (uintptr)panic->defer->pc;
+			else if(defer != nil && defer->argp == (byte*)sparg)
+				frame.continpc = (uintptr)defer->pc;
+			else
+				frame.continpc = 0;
+		}
+
+		// Unwind our local panic & defer stacks past this frame.
+		while(panic != nil && (panic->defer == nil || panic->defer->argp == (byte*)sparg || panic->defer->argp == NoArgs))
+			panic = panic->link;
+		while(defer != nil && (defer->argp == (byte*)sparg || defer->argp == NoArgs))
+			defer = defer->link;	
 
 		if(skip > 0) {
 			skip--;
@@ -231,7 +278,7 @@ runtime·gentraceback(uintptr pc0, uintptr sp0, uintptr lr0, G *gp, int32 skip,
 				if(frame.pc > f->entry)
 					runtime·printf(" +%p", (uintptr)(frame.pc - f->entry));
 				if(m->throwing > 0 && gp == m->curg || gotraceback >= 2)
-					runtime·printf(" fp=%p", frame.fp);
+					runtime·printf(" fp=%p sp=%p", frame.fp, frame.sp);
 				runtime·printf("\n");
 				nprint++;
 			}
@@ -240,6 +287,7 @@ runtime·gentraceback(uintptr pc0, uintptr sp0, uintptr lr0, G *gp, int32 skip,
 	
 	skipped:
 		waspanic = f->entry == (uintptr)runtime·sigpanic;
+		wasnewproc = f->entry == (uintptr)runtime·newproc || f->entry == (uintptr)runtime·deferproc;
 
 		// Do not unwind past the bottom of the stack.
 		if(flr == nil)
@@ -255,7 +303,15 @@ runtime·gentraceback(uintptr pc0, uintptr sp0, uintptr lr0, G *gp, int32 skip,
 	
 	if(pcbuf == nil && callback == nil)
 		n = nprint;
-	
+
+	if(callback != nil && n < max && (defer != nil || panic != nil)) {
+		if(defer != nil)
+			runtime·printf("runtime: g%D: leftover defer argp=%p pc=%p\n", gp->goid, defer->argp, defer->pc);
+		if(panic != nil)
+			runtime·printf("runtime: g%D: leftover panic argp=%p pc=%p\n", gp->goid, panic->defer->argp, panic->defer->pc);
+		runtime·throw("traceback has leftover defers or panics");
+	}
+
 	return n;
 }
 

test/fixedbugs/issue8048.go

+// run
+
+// Copyright 2014 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Issue 8048. Incorrect handling of liveness when walking stack
+// containing faulting frame.
+
+package main
+
+import "runtime"
+
+func main() {
+	test1()
+	test2()
+	test3()
+}
+
+func test1() {
+	// test1f will panic without its own defer.
+	// The runtime.GC checks that we can walk the stack
+	// at that point and not get confused.
+	// The recover lets test1 exit normally.
+	defer func() {
+		runtime.GC()
+		recover()
+	}()
+	test1f()
+}
+
+func test1f() {
+	// Because b == false, the if does not execute,
+	// so x == nil, so the println(*x) faults reading
+	// from nil. The compiler will lay out the code
+	// so that the if body occurs above the *x,
+	// so if the liveness info at the *x is used, it will
+	// find the liveness at the call to runtime.GC.
+	// It will think y is live, but y is uninitialized,
+	// and the runtime will crash detecting a bad slice.
+	// The runtime should see that there are no defers
+	// corresponding to this panicked frame and ignore
+	// the frame entirely.
+	var x *int
+	var b bool
+	if b {
+		y := make([]int, 1)
+		runtime.GC()
+		x = &y[0]
+	}
+	println(*x)
+}
+
+func test2() {
+	// Same as test1, but the fault happens in the function with the defer.
+	// The runtime should see the defer and garbage collect the frame
+	// as if the PC were immediately after the defer statement.
+	defer func() {
+		runtime.GC()
+		recover()
+	}()
+	var x *int
+	var b bool
+	if b {
+		y := make([]int, 1)
+		runtime.GC()
+		x = &y[0]
+	}
+	println(*x)
+}
+
+func test3() {
+	// Like test1 but avoid array index, which does not
+	// move to end of function on ARM.
+	defer func() {
+		runtime.GC()
+		recover()
+	}()
+	test3setup()
+	test3f()
+}
+
+func test3setup() {
+	var x uintptr
+	var b bool
+	b = true
+	if b {
+		y := uintptr(123)
+		runtime.GC()
+		x = y
+	}
+	runtime.GC()
+	globl = x
+}
+
+var globl uintptr
+
+func test3f() {
+	var x *int
+	var b bool
+	if b {
+		y := new(int)
+		runtime.GC()
+		x = y
+	}
+	println(*x)
+}