runtime: use traceback to traverse defer structures

This makes the GC and the stack copying agree about how
to interpret the defer structures. Previously, only the stack
copying treated them precisely.
This removes an untyped memory allocation and fixes
at least three copystack bugs.

To make sure the GC can find the deferred argument
frame until it has been copied, keep a Defer on the defer list
during its execution.

In addition to making it possible to remove the untyped
memory allocation, keeping the Defer on the list fixes
two races between copystack and execution of defers
(in both gopanic and Goexit). The problem is that once
the defer has been taken off the list, a stack copy that
happens before the deferred arguments have been copied
back to the stack will not update the arguments correctly.
The new tests TestDeferPtrsPanic and TestDeferPtrsGoexit
(variations on the existing TestDeferPtrs) pass now but
failed before this CL.

In addition to those fixes, keeping the Defer on the list
helps correct a dangling pointer error during copystack.
The traceback routines walk the Defer chain to provide
information about where a panic may resume execution.
When the executing Defer was not on the Defer chain
but instead linked from the Panic chain, the traceback
had to walk the Panic chain too. But Panic structs are
on the stack and being updated by copystack.
Traceback's use of the Panic chain while copystack is
updating those structs means that it can follow an
updated pointer and find itself reading from the new stack.
The new stack is usually all zeros, so it sees an incorrect
early end to the chain. The new TestPanicUseStack makes
this happen at tip and dies when adjustdefers finds an
unexpected argp. The new StackCopyPoison mode
causes an earlier bad dereference instead.
By keeping the Defer on the list, traceback can avoid
walking the Panic chain at all,  making it okay for copystack
to update the Panics.

We'd have the same problem for any Defers on the stack.
There was only one: gopanic's dabort. Since we are not
taking the executing Defer off the chain, we can use it
to do what dabort was doing, and then there are no
Defers on the stack ever, so it is okay for traceback to use
the Defer chain even while copystack is executing:
copystack cannot modify the Defer chain.

LGTM=khr
R=khr
CC=dvyukov, golang-codereviews, iant, rlh
https://golang.org/cl/141490043

src/runtime/heapdump.c

@@ -432,7 +432,7 @@ dumpgoroutine(G *gp)
 		dumpint((uintptr)gp);
 		dumpint((uintptr)p->arg.type);
 		dumpint((uintptr)p->arg.data);
-		dumpint((uintptr)p->defer);
+		dumpint(0); // was p->defer, no longer recorded
 		dumpint((uintptr)p->link);
 	}
 }

src/runtime/malloc.go

@@ -209,6 +209,16 @@ func mallocgc(size uintptr, typ *_type, flags int) unsafe.Pointer {
 		goto marked
 	}
 
+	// If allocating a defer+arg block, now that we've picked a malloc size
+	// large enough to hold everything, cut the "asked for" size down to
+	// just the defer header, so that the GC bitmap will record the arg block
+	// as containing nothing at all (as if it were unused space at the end of
+	// a malloc block caused by size rounding).
+	// The defer arg areas are scanned as part of scanstack.
+	if typ == deferType {
+		size0 = unsafe.Sizeof(_defer{})
+	}
+
 	// From here till marked label marking the object as allocated
 	// and storing type info in the GC bitmap.
 	{

src/runtime/mgc0.c

@@ -713,6 +713,7 @@ scanstack(G *gp)
 
 	fn = scanframe;
 	runtime·gentraceback(~(uintptr)0, ~(uintptr)0, 0, gp, 0, nil, 0x7fffffff, &fn, nil, false);
+	runtime·tracebackdefers(gp, &fn, nil);
 }
 
 // The gp has been moved to a gc safepoint. If there is gcphase specific

src/runtime/panic.c

@@ -18,7 +18,8 @@ uint32 runtime·panicking;
 static Mutex paniclk;
 
 void
-runtime·deferproc_m(void) {
+runtime·deferproc_m(void)
+{
 	int32 siz;
 	FuncVal *fn;
 	uintptr argp;
@@ -35,7 +36,7 @@ runtime·deferproc_m(void) {
 	d->fn = fn;
 	d->pc = callerpc;
 	d->argp = argp;
-	runtime·memmove(d->args, (void*)argp, siz);
+	runtime·memmove(d+1, (void*)argp, siz);
 }
 
 // Unwind the stack after a deferred function calls recover

src/runtime/panic.go

@@ -90,19 +90,31 @@ func deferproc(siz int32, fn *funcval) { // arguments of fn follow fn
 	// been set and must not be clobbered.
 }
 
-// Each P holds pool for defers with arg sizes 8, 24, 40, 56 and 72 bytes.
-// Memory block is 40 (24 for 32 bits) bytes larger due to Defer header.
-// This maps exactly to malloc size classes.
+// Small malloc size classes >= 16 are the multiples of 16: 16, 32, 48, 64, 80, 96, 112, 128, 144, ...
+// Each P holds a pool for defers with small arg sizes.
+// Assign defer allocations to pools by rounding to 16, to match malloc size classes.
+
+const (
+	deferHeaderSize = unsafe.Sizeof(_defer{})
+	minDeferAlloc   = (deferHeaderSize + 15) &^ 15
+	minDeferArgs    = minDeferAlloc - deferHeaderSize
+)
 
 // defer size class for arg size sz
 //go:nosplit
 func deferclass(siz uintptr) uintptr {
-	return (siz + 7) >> 4
+	if siz <= minDeferArgs {
+		return 0
+	}
+	return (siz - minDeferArgs + 15) / 16
 }
 
 // total size of memory block for defer with arg size sz
 func totaldefersize(siz uintptr) uintptr {
-	return (unsafe.Sizeof(_defer{}) - unsafe.Sizeof(_defer{}.args)) + round(siz, ptrSize)
+	if siz <= minDeferArgs {
+		return minDeferAlloc
+	}
+	return deferHeaderSize + siz
 }
 
 // Ensure that defer arg sizes that map to the same defer size class
@@ -130,6 +142,21 @@ func testdefersizes() {
 	}
 }
 
+// The arguments associated with a deferred call are stored
+// immediately after the _defer header in memory.
+//go:nosplit
+func deferArgs(d *_defer) unsafe.Pointer {
+	return add(unsafe.Pointer(d), unsafe.Sizeof(*d))
+}
+
+var deferType *_type // type of _defer struct
+
+func init() {
+	var x interface{}
+	x = (*_defer)(nil)
+	deferType = (*(**ptrtype)(unsafe.Pointer(&x))).elem
+}
+
 // Allocate a Defer, usually using per-P pool.
 // Each defer must be released with freedefer.
 // Note: runs on M stack
@@ -145,12 +172,11 @@ func newdefer(siz int32) *_defer {
 		}
 	}
 	if d == nil {
-		// deferpool is empty or just a big defer
+		// Allocate new defer+args.
 		total := goroundupsize(totaldefersize(uintptr(siz)))
-		d = (*_defer)(mallocgc(total, conservative, 0))
+		d = (*_defer)(mallocgc(total, deferType, 0))
 	}
 	d.siz = siz
-	d.special = false
 	gp := mp.curg
 	d.link = gp._defer
 	gp._defer = d
@@ -162,18 +188,14 @@ func newdefer(siz int32) *_defer {
 // The defer cannot be used after this call.
 //go:nosplit
 func freedefer(d *_defer) {
-	if d.special {
-		return
-	}
 	sc := deferclass(uintptr(d.siz))
 	if sc < uintptr(len(p{}.deferpool)) {
 		mp := acquirem()
 		pp := mp.p
+		*d = _defer{}
 		d.link = pp.deferpool[sc]
 		pp.deferpool[sc] = d
 		releasem(mp)
-		// No need to wipe out pointers in argp/pc/fn/args,
-		// because we empty the pool before GC.
 	}
 }
 
@@ -207,7 +229,7 @@ func deferreturn(arg0 uintptr) {
 	// won't know the form of the arguments until the jmpdefer can
 	// flip the PC over to fn.
 	mp := acquirem()
-	memmove(unsafe.Pointer(argp), unsafe.Pointer(&d.args), uintptr(d.siz))
+	memmove(unsafe.Pointer(argp), deferArgs(d), uintptr(d.siz))
 	fn := d.fn
 	gp._defer = d.link
 	freedefer(d)
@@ -227,8 +249,9 @@ func Goexit() {
 	gp := getg()
 	for gp._defer != nil {
 		d := gp._defer
+		d.started = true
+		reflectcall(unsafe.Pointer(d.fn), deferArgs(d), uint32(d.siz), uint32(d.siz))
 		gp._defer = d.link
-		reflectcall(unsafe.Pointer(d.fn), unsafe.Pointer(&d.args), uint32(d.siz), uint32(d.siz))
 		freedefer(d)
 		// Note: we ignore recovers here because Goexit isn't a panic
 	}
@@ -258,55 +281,58 @@ func gopanic(e interface{}) {
 		gothrow("panic on m stack")
 	}
 	var p _panic
-	var dabort _defer
 	p.arg = e
 	p.link = gp._panic
 	gp._panic = (*_panic)(noescape(unsafe.Pointer(&p)))
 
-	fn := abortpanic
-	dabort.fn = *(**funcval)(unsafe.Pointer(&fn))
-	dabort.siz = ptrSize
-	dabort.args[0] = noescape((unsafe.Pointer)(&p)) // TODO(khr): why do I need noescape here?
-	dabort.argp = _NoArgs
-	dabort.special = true
-
 	for {
 		d := gp._defer
 		if d == nil {
 			break
 		}
-		// take defer off list in case of recursive panic
-		gp._defer = d.link
-		argp := unsafe.Pointer(d.argp) // must be pointer so it gets adjusted during stack copy
-		pc := d.pc
 
-		// The deferred function may cause another panic,
-		// so reflectcall may not return. Set up a defer
-		// to mark this panic aborted if that happens.
-		dabort.link = gp._defer
-		gp._defer = (*_defer)(noescape(unsafe.Pointer(&dabort)))
-		p._defer = d
+		// If defer was started by earlier panic or Goexit (and, since we're back here, that triggered a new panic),
+		// take defer off list. The earlier panic or Goexit will not continue running.
+		if d.started {
+			if d._panic != nil {
+				d._panic.aborted = true
+			}
+			gp._defer = d.link
+			freedefer(d)
+			continue
+		}
+
+		// Mark defer as started, but keep on list, so that traceback
+		// can find and update the defer's argument frame if stack growth
+		// or a garbage collection hapens before reflectcall starts executing d.fn.
+		d.started = true
+
+		// Record the panic that is running the defer.
+		// If there is a new panic during the deferred call, that panic
+		// will find d in the list and will mark d._panic (this panic) aborted.
+		d._panic = (*_panic)(noescape((unsafe.Pointer)(&p)))
 
 		p.argp = unsafe.Pointer(getargp(0))
-		reflectcall(unsafe.Pointer(d.fn), unsafe.Pointer(&d.args), uint32(d.siz), uint32(d.siz))
+		reflectcall(unsafe.Pointer(d.fn), deferArgs(d), uint32(d.siz), uint32(d.siz))
 		p.argp = nil
 
-		// reflectcall did not panic. Remove dabort.
-		if gp._defer != &dabort {
+		// reflectcall did not panic. Remove d.
+		if gp._defer != d {
 			gothrow("bad defer entry in panic")
 		}
-		gp._defer = dabort.link
+		gp._defer = d.link
 
 		// trigger shrinkage to test stack copy.  See stack_test.go:TestStackPanic
 		//GC()
 
+		pc := d.pc
+		argp := unsafe.Pointer(d.argp) // must be pointer so it gets adjusted during stack copy
 		freedefer(d)
 		if p.recovered {
 			gp._panic = p.link
 			// Aborted panics are marked but remain on the g.panic list.
-			// Remove them from the list and free the associated defers.
+			// Remove them from the list.
 			for gp._panic != nil && gp._panic.aborted {
-				freedefer(gp._panic._defer)
 				gp._panic = gp._panic.link
 			}
 			if gp._panic == nil { // must be done with signal
@@ -342,10 +368,6 @@ func getargp(x int) uintptr {
 	return uintptr(noescape(unsafe.Pointer(&x)))
 }
 
-func abortpanic(p *_panic) {
-	p.aborted = true
-}
-
 // The implementation of the predeclared function recover.
 // Cannot split the stack because it needs to reliably
 // find the stack segment of its caller.

src/runtime/proc.go

@@ -39,6 +39,12 @@ func main() {
 	// to preserve the lock.
 	lockOSThread()
 
+	if g.m != &m0 {
+		gothrow("runtime.main not on m0")
+	}
+
+	runtime_init() // must be before defer
+
 	// Defer unlock so that runtime.Goexit during init does the unlock too.
 	needUnlock := true
 	defer func() {
@@ -47,11 +53,6 @@ func main() {
 		}
 	}()
 
-	if g.m != &m0 {
-		gothrow("runtime.main not on m0")
-	}
-
-	runtime_init()
 	memstats.enablegc = true // now that runtime is initialized, GC is okay
 
 	main_init()

src/runtime/runtime.h

@@ -638,12 +638,12 @@ void    runtime·gcphasework(G*);
 struct Defer
 {
 	int32	siz;
-	bool	special;	// not part of defer frame
+	bool	started;
 	uintptr	argp;		// where args were copied from
 	uintptr	pc;
 	FuncVal*	fn;
+	Panic*	panic;	// panic that is running defer
 	Defer*	link;
-	void*	args[1];	// padded to actual size
 };
 
 // argp used in Defer structs when there is no argp.
@@ -657,7 +657,6 @@ struct Panic
 	void*	argp;	// pointer to arguments of deferred call run during panic; cannot move - known to liblink
 	Eface	arg;		// argument to panic
 	Panic*	link;		// link to earlier panic
-	Defer*	defer;		// current executing defer
 	bool	recovered;	// whether this panic is over
 	bool	aborted;	// the panic was aborted
 };

src/runtime/stack.c

@@ -23,6 +23,7 @@ enum
 	StackDebug = 0,
 	StackFromSystem = 0,	// allocate stacks from system memory instead of the heap
 	StackFaultOnFree = 0,	// old stacks are mapped noaccess to detect use after free
+	StackPoisonCopy = 0,	// fill stack that should not be accessed with garbage, to detect bad dereferences during copy
 
 	StackCache = 1,
 };
@@ -353,6 +354,24 @@ struct AdjustInfo {
 	uintptr delta;  // ptr distance from old to new stack (newbase - oldbase)
 };
 
+// Adjustpointer checks whether *vpp is in the old stack described by adjinfo.
+// If so, it rewrites *vpp to point into the new stack.
+static void
+adjustpointer(AdjustInfo *adjinfo, void *vpp)
+{
+	byte **pp, *p;
+	
+	pp = vpp;
+	p = *pp;
+	if(StackDebug >= 4)
+		runtime·printf("        %p:%p\n", pp, p);
+	if(adjinfo->old.lo <= (uintptr)p && (uintptr)p < adjinfo->old.hi) {
+		*pp = p + adjinfo->delta;
+		if(StackDebug >= 3)
+			runtime·printf("        adjust ptr %p: %p -> %p\n", pp, p, *pp);
+	}
+}
+
 // bv describes the memory starting at address scanp.
 // Adjust any pointers contained therein.
 static void
@@ -447,6 +466,11 @@ adjustframe(Stkframe *frame, void *arg)
 	uintptr targetpc;
 
 	adjinfo = arg;
+	targetpc = frame->continpc;
+	if(targetpc == 0) {
+		// Frame is dead.
+		return true;
+	}
 	f = frame->fn;
 	if(StackDebug >= 2)
 		runtime·printf("    adjusting %s frame=[%p,%p] pc=%p continpc=%p\n", runtime·funcname(f), frame->sp, frame->fp, frame->pc, frame->continpc);
@@ -456,11 +480,6 @@ adjustframe(Stkframe *frame, void *arg)
 		// have full GC info for it (because it is written in asm).
 		return true;
 	}
-	targetpc = frame->continpc;
-	if(targetpc == 0) {
-		// Frame is dead.
-		return true;
-	}
 	if(targetpc != f->entry)
 		targetpc--;
 	pcdata = runtime·pcdatavalue(f, PCDATA_StackMapIndex, targetpc);
@@ -495,103 +514,53 @@ adjustframe(Stkframe *frame, void *arg)
 			runtime·printf("      args\n");
 		adjustpointers((byte**)frame->argp, &bv, adjinfo, nil);
 	}
+	
 	return true;
 }
 
 static void
 adjustctxt(G *gp, AdjustInfo *adjinfo)
 {
-	if(adjinfo->old.lo <= (uintptr)gp->sched.ctxt && (uintptr)gp->sched.ctxt < adjinfo->old.hi)
-		gp->sched.ctxt = (byte*)gp->sched.ctxt + adjinfo->delta;
+	adjustpointer(adjinfo, &gp->sched.ctxt);
 }
 
 static void
 adjustdefers(G *gp, AdjustInfo *adjinfo)
 {
-	Defer *d, **dp;
-	Func *f;
-	FuncVal *fn;
-	StackMap *stackmap;
-	BitVector bv;
+	Defer *d;
+	bool (*cb)(Stkframe*, void*);
 
-	for(dp = &gp->defer, d = *dp; d != nil; dp = &d->link, d = *dp) {
-		if(adjinfo->old.lo <= (uintptr)d && (uintptr)d < adjinfo->old.hi) {
-			// The Defer record is on the stack.  Its fields will
-			// get adjusted appropriately.
-			// This only happens for runtime.main and runtime.gopanic now,
-			// but a compiler optimization could do more of this.
-			// If such an optimization were introduced, Defer.argp should
-			// change to have pointer type so that it will be updated by
-			// the stack copying. Today both of those on-stack defers
-			// set argp = NoArgs, so no adjustment is necessary.
-			*dp = (Defer*)((byte*)d + adjinfo->delta);
-			continue;
-		}
-		if(d->argp == NoArgs)
-			continue;
-		if(d->argp < adjinfo->old.lo || adjinfo->old.hi <= d->argp) {
-			runtime·printf("runtime: adjustdefers argp=%p stk=%p %p\n", d->argp, adjinfo->old.lo, adjinfo->old.hi);
-			runtime·throw("adjustdefers: unexpected argp");
-		}
-		d->argp += adjinfo->delta;
-		fn = d->fn;
-		if(fn == nil) {
-			// Defer of nil function.  It will panic when run.  See issue 8047.
-			continue;
-		}
-		f = runtime·findfunc((uintptr)fn->fn);
-		if(f == nil)
-			runtime·throw("can't adjust unknown defer");
-		if(StackDebug >= 4)
-			runtime·printf("  checking defer %s\n", runtime·funcname(f));
-		// Defer's FuncVal might be on the stack
-		if(adjinfo->old.lo <= (uintptr)fn && (uintptr)fn < adjinfo->old.hi) {
-			if(StackDebug >= 3)
-				runtime·printf("    adjust defer fn %s\n", runtime·funcname(f));
-			d->fn = (FuncVal*)((byte*)fn + adjinfo->delta);
-		} else {
-			// deferred function's args might point into the stack.
-			if(StackDebug >= 3)
-				runtime·printf("    adjust deferred args for %s\n", runtime·funcname(f));
-			stackmap = runtime·funcdata(f, FUNCDATA_ArgsPointerMaps);
-			if(stackmap == nil)
-				runtime·throw("runtime: deferred function has no arg ptr map");
-			bv = runtime·stackmapdata(stackmap, 0);
-			adjustpointers(d->args, &bv, adjinfo, f);
-		}
-		// The FuncVal may have pointers in it, but fortunately for us
-		// the compiler won't put pointers into the stack in a
-		// heap-allocated FuncVal.
-		// One day if we do need to check this, we can use the gc bits in the
-		// heap to do the right thing (although getting the size will be expensive).
+	// Adjust defer argument blocks the same way we adjust active stack frames.
+	cb = adjustframe;
+	runtime·tracebackdefers(gp, &cb, adjinfo);
+
+	// Adjust pointers in the Defer structs.
+	// Defer structs themselves are never on the stack.
+	for(d = gp->defer; d != nil; d = d->link) {
+		adjustpointer(adjinfo, &d->fn);
+		adjustpointer(adjinfo, &d->argp);
+		adjustpointer(adjinfo, &d->panic);
 	}
 }
 
 static void
 adjustpanics(G *gp, AdjustInfo *adjinfo)
 {
-	// Panic structs are all on the stack
-	// and are adjusted by stack copying.
-	// The only pointer we need to update is gp->panic, the head of the list.
-	if(adjinfo->old.lo <= (uintptr)gp->panic && (uintptr)gp->panic < adjinfo->old.hi)
-		gp->panic = (Panic*)((byte*)gp->panic + adjinfo->delta);
+	// Panics are on stack and already adjusted.
+	// Update pointer to head of list in G.
+	adjustpointer(adjinfo, &gp->panic);
 }
 
 static void
 adjustsudogs(G *gp, AdjustInfo *adjinfo)
 {
 	SudoG *s;
-	byte *e;
 
 	// the data elements pointed to by a SudoG structure
 	// might be in the stack.
 	for(s = gp->waiting; s != nil; s = s->waitlink) {
-		e = s->elem;
-		if(adjinfo->old.lo <= (uintptr)e && (uintptr)e < adjinfo->old.hi)
-			s->elem = e + adjinfo->delta;
-		e = (byte*)s->selectdone;
-		if(adjinfo->old.lo <= (uintptr)e && (uintptr)e < adjinfo->old.hi)
-			s->selectdone = (uint32*)(e + adjinfo->delta);
+		adjustpointer(adjinfo, &s->elem);
+		adjustpointer(adjinfo, &s->selectdone);
 	}
 }
 
@@ -604,6 +573,7 @@ copystack(G *gp, uintptr newsize)
 	AdjustInfo adjinfo;
 	uint32 oldstatus;
 	bool (*cb)(Stkframe*, void*);
+	byte *p, *ep;
 
 	if(gp->syscallsp != 0)
 		runtime·throw("stack growth not allowed in system call");
@@ -614,6 +584,12 @@ copystack(G *gp, uintptr newsize)
 
 	// allocate new stack
 	new = runtime·stackalloc(newsize);
+	if(StackPoisonCopy) {
+		p = (byte*)new.lo;
+		ep = (byte*)new.hi;
+		while(p < ep)
+			*p++ = 0xfd;
+	}
 
 	if(StackDebug >= 1)
 		runtime·printf("copystack gp=%p [%p %p %p]/%d -> [%p %p %p]/%d\n", gp, old.lo, old.hi-used, old.hi, (int32)(old.hi-old.lo), new.lo, new.hi-used, new.hi, (int32)newsize);
@@ -631,6 +607,12 @@ copystack(G *gp, uintptr newsize)
 	adjustsudogs(gp, &adjinfo);
 	
 	// copy the stack to the new location
+	if(StackPoisonCopy) {
+		p = (byte*)new.lo;
+		ep = (byte*)new.hi;
+		while(p < ep)
+			*p++ = 0xfb;
+	}
 	runtime·memmove((byte*)new.hi - used, (byte*)old.hi - used, used);
 
 	oldstatus = runtime·readgstatus(gp);
@@ -648,6 +630,12 @@ copystack(G *gp, uintptr newsize)
 	runtime·casgstatus(gp, Gcopystack, oldstatus); // oldstatus is Gwaiting or Grunnable
 
 	// free old stack
+	if(StackPoisonCopy) {
+		p = (byte*)old.lo;
+		ep = (byte*)old.hi;
+		while(p < ep)
+			*p++ = 0xfc;
+	}
 	runtime·stackfree(old);
 }
 

src/runtime/stack_test.go

@@ -141,7 +141,7 @@ func growStack() {
 	GC()
 }
 
-// This function is not an anonimous func, so that the compiler can do escape
+// This function is not an anonymous func, so that the compiler can do escape
 // analysis and place x on stack (and subsequently stack growth update the pointer).
 func growStackIter(p *int, n int) {
 	if n == 0 {
@@ -230,13 +230,101 @@ func TestDeferPtrs(t *testing.T) {
 	growStack()
 }
 
-// use about n KB of stack
-func useStack(n int) {
+type bigBuf [4 * 1024]byte
+
+// TestDeferPtrsGoexit is like TestDeferPtrs but exercises the possibility that the
+// stack grows as part of starting the deferred function. It calls Goexit at various
+// stack depths, forcing the deferred function (with >4kB of args) to be run at
+// the bottom of the stack. The goal is to find a stack depth less than 4kB from
+// the end of the stack. Each trial runs in a different goroutine so that an earlier
+// stack growth does not invalidate a later attempt.
+func TestDeferPtrsGoexit(t *testing.T) {
+	for i := 0; i < 100; i++ {
+		c := make(chan int, 1)
+		go testDeferPtrsGoexit(c, i)
+		if n := <-c; n != 42 {
+			t.Fatalf("defer's stack references were not adjusted appropriately (i=%d n=%d)", i, n)
+		}
+	}
+}
+
+func testDeferPtrsGoexit(c chan int, i int) {
+	var y int
+	defer func() {
+		c <- y
+	}()
+	defer setBig(&y, 42, bigBuf{})
+	useStackAndCall(i, Goexit)
+}
+
+func setBig(p *int, x int, b bigBuf) {
+	*p = x
+}
+
+// TestDeferPtrsPanic is like TestDeferPtrsGoexit, but it's using panic instead
+// of Goexit to run the Defers. Those two are different execution paths
+// in the runtime.
+func TestDeferPtrsPanic(t *testing.T) {
+	for i := 0; i < 100; i++ {
+		c := make(chan int, 1)
+		go testDeferPtrsGoexit(c, i)
+		if n := <-c; n != 42 {
+			t.Fatalf("defer's stack references were not adjusted appropriately (i=%d n=%d)", i, n)
+		}
+	}
+}
+
+func testDeferPtrsPanic(c chan int, i int) {
+	var y int
+	defer func() {
+		if recover() == nil {
+			c <- -1
+			return
+		}
+		c <- y
+	}()
+	defer setBig(&y, 42, bigBuf{})
+	useStackAndCall(i, func() { panic(1) })
+}
+
+// TestPanicUseStack checks that a chain of Panic structs on the stack are
+// updated correctly if the stack grows during the deferred execution that
+// happens as a result of the panic.
+func TestPanicUseStack(t *testing.T) {
+	pc := make([]uintptr, 10000)
+	defer func() {
+		recover()
+		Callers(0, pc) // force stack walk
+		useStackAndCall(100, func() {
+			defer func() {
+				recover()
+				Callers(0, pc) // force stack walk
+				useStackAndCall(200, func() {
+					defer func() {
+						recover()
+						Callers(0, pc) // force stack walk
+					}()
+					panic(3)
+				})
+			}()
+			panic(2)
+		})
+	}()
+	panic(1)
+}
+
+// use about n KB of stack and call f
+func useStackAndCall(n int, f func()) {
 	if n == 0 {
+		f()
 		return
 	}
 	var b [1024]byte // makes frame about 1KB
-	useStack(n - 1 + int(b[99]))
+	useStackAndCall(n-1+int(b[99]), f)
+}
+
+func useStack(n int) {
+	useStackAndCall(n, func() {})
 }
 
 func growing(c chan int, done chan struct{}) {

src/runtime/traceback.go

@@ -45,8 +45,36 @@ var (
 	externalthreadhandlerp uintptr // initialized elsewhere
 )
 
-// System-specific hook. See traceback_windows.go
-var systraceback func(*_func, *stkframe, *g, bool, func(*stkframe, unsafe.Pointer) bool, unsafe.Pointer) (changed, aborted bool)
+// Traceback over the deferred function calls.
+// Report them like calls that have been invoked but not started executing yet.
+func tracebackdefers(gp *g, callback func(*stkframe, unsafe.Pointer) bool, v unsafe.Pointer) {
+	var frame stkframe
+	for d := gp._defer; d != nil; d = d.link {
+		fn := d.fn
+		if fn == nil {
+			// Defer of nil function. Args don't matter.
+			frame.pc = 0
+			frame.fn = nil
+			frame.argp = 0
+			frame.arglen = 0
+			frame.argmap = nil
+		} else {
+			frame.pc = uintptr(fn.fn)
+			f := findfunc(frame.pc)
+			if f == nil {
+				print("runtime: unknown pc in defer ", hex(frame.pc), "\n")
+				gothrow("unknown pc")
+			}
+			frame.fn = f
+			frame.argp = uintptr(deferArgs(d))
+			setArgInfo(&frame, f, true)
+		}
+		frame.continpc = frame.pc
+		if !callback((*stkframe)(noescape(unsafe.Pointer(&frame))), v) {
+			return
+		}
+	}
+}
 
 // Generic traceback.  Handles runtime stack prints (pcbuf == nil),
 // the runtime.Callers function (pcbuf != nil), as well as the garbage
@@ -81,15 +109,11 @@ func gentraceback(pc0 uintptr, sp0 uintptr, lr0 uintptr, gp *g, skip int, pcbuf
 	waspanic := false
 	wasnewproc := false
 	printing := pcbuf == nil && callback == nil
-	panic := gp._panic
 	_defer := gp._defer
 
 	for _defer != nil && uintptr(_defer.argp) == _NoArgs {
 		_defer = _defer.link
 	}
-	for 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.
@@ -187,25 +211,7 @@ func gentraceback(pc0 uintptr, sp0 uintptr, lr0 uintptr, gp *g, skip int, pcbuf
 			if usesLR {
 				frame.argp += ptrSize
 			}
-			frame.arglen = uintptr(f.args)
-			if callback != nil && f.args == _ArgsSizeUnknown {
-				// Extract argument bitmaps for reflect stubs from the calls they made to reflect.
-				switch gofuncname(f) {
-				case "reflect.makeFuncStub", "reflect.methodValueCall":
-					arg0 := frame.sp
-					if usesLR {
-						arg0 += ptrSize
-					}
-					fn := *(**[2]uintptr)(unsafe.Pointer(arg0))
-					if fn[0] != f.entry {
-						print("runtime: confused by ", gofuncname(f), "\n")
-						gothrow("reflect mismatch")
-					}
-					bv := (*bitvector)(unsafe.Pointer(fn[1]))
-					frame.arglen = uintptr(bv.n / 2 * ptrSize)
-					frame.argmap = bv
-				}
-			}
+			setArgInfo(&frame, f, callback != nil)
 		}
 
 		// Determine function SP where deferproc would find its arguments.
@@ -246,19 +252,14 @@ func gentraceback(pc0 uintptr, sp0 uintptr, lr0 uintptr, gp *g, skip int, pcbuf
 		// returns; everything live at earlier deferprocs is still live at that one.
 		frame.continpc = frame.pc
 		if waspanic {
-			if panic != nil && panic._defer.argp == sparg {
-				frame.continpc = panic._defer.pc
-			} else if _defer != nil && _defer.argp == sparg {
+			if _defer != nil && _defer.argp == sparg {
 				frame.continpc = _defer.pc
 			} else {
 				frame.continpc = 0
 			}
 		}
 
-		// Unwind our local panic & defer stacks past this frame.
-		for panic != nil && (panic._defer == nil || panic._defer.argp == sparg || panic._defer.argp == _NoArgs) {
-			panic = panic.link
-		}
+		// Unwind our local defer stack past this frame.
 		for _defer != nil && (_defer.argp == sparg || _defer.argp == _NoArgs) {
 			_defer = _defer.link
 		}
@@ -403,25 +404,37 @@ func gentraceback(pc0 uintptr, sp0 uintptr, lr0 uintptr, gp *g, skip int, pcbuf
 		if _defer != nil {
 			print("runtime: g", gp.goid, ": leftover defer argp=", hex(_defer.argp), " pc=", hex(_defer.pc), "\n")
 		}
-		if panic != nil {
-			print("runtime: g", gp.goid, ": leftover panic argp=", hex(panic._defer.argp), " pc=", hex(panic._defer.pc), "\n")
-		}
 		for _defer = gp._defer; _defer != nil; _defer = _defer.link {
 			print("\tdefer ", _defer, " argp=", hex(_defer.argp), " pc=", hex(_defer.pc), "\n")
 		}
-		for panic = gp._panic; panic != nil; panic = panic.link {
-			print("\tpanic ", panic, " defer ", panic._defer)
-			if panic._defer != nil {
-				print(" argp=", hex(panic._defer.argp), " pc=", hex(panic._defer.pc))
-			}
-			print("\n")
-		}
-		gothrow("traceback has leftover defers or panics")
+		gothrow("traceback has leftover defers")
 	}
 
 	return n
 }
 
+func setArgInfo(frame *stkframe, f *_func, needArgMap bool) {
+	frame.arglen = uintptr(f.args)
+	if needArgMap && f.args == _ArgsSizeUnknown {
+		// Extract argument bitmaps for reflect stubs from the calls they made to reflect.
+		switch gofuncname(f) {
+		case "reflect.makeFuncStub", "reflect.methodValueCall":
+			arg0 := frame.sp
+			if usesLR {
+				arg0 += ptrSize
+			}
+			fn := *(**[2]uintptr)(unsafe.Pointer(arg0))
+			if fn[0] != f.entry {
+				print("runtime: confused by ", gofuncname(f), "\n")
+				gothrow("reflect mismatch")
+			}
+			bv := (*bitvector)(unsafe.Pointer(fn[1]))
+			frame.arglen = uintptr(bv.n / 2 * ptrSize)
+			frame.argmap = bv
+		}
+	}
+}
+
 func printcreatedby(gp *g) {
 	// Show what created goroutine, except main goroutine (goid 1).
 	pc := gp.gopc