runtime: add GODEBUG wbshadow for finding missing write barriers

This is the detection code. It works well enough that I know of
a handful of missing write barriers. However, those are subtle
enough that I'll address them in separate followup CLs.

GODEBUG=wbshadow=1 checks for a write that bypassed the
write barrier at the next write barrier of the same word.
If a bug can be detected in this mode it is typically easy to
understand, since the crash says quite clearly what kind of
word has missed a write barrier.

GODEBUG=wbshadow=2 adds a check of the write barrier
shadow copy during garbage collection. Bugs detected at
garbage collection can be difficult to understand, because
there is no context for what the found word means.
Typically you have to reproduce the problem with allocfreetrace=1
in order to understand the type of the badly updated word.

Change-Id: If863837308e7c50d96b5bdc7d65af4969bf53a6e
Reviewed-on: https://go-review.googlesource.com/2061Reviewed-by: Austin Clements <austin@google.com>

src/runtime/extern.go

@@ -54,6 +54,18 @@ a comma-separated list of name=val pairs. Supported names are:
 
 	scavenge: scavenge=1 enables debugging mode of heap scavenger.
 
+	wbshadow: setting wbshadow=1 enables a shadow copy of the heap
+	used to detect missing write barriers at the next write to a
+	given location. If a bug can be detected in this mode it is
+	typically easy to understand, since the crash says quite
+	clearly what kind of word has missed a write barrier.
+	Setting wbshadow=2 checks the shadow copy during garbage
+	collection as well. Bugs detected at garbage collection can be
+	difficult to understand, because there is no context for what
+	the found word means. Typically you have to reproduce the
+	problem with allocfreetrace=1 in order to understand the type
+	of the badly updated word.
+
 The GOMAXPROCS variable limits the number of operating system threads that
 can execute user-level Go code simultaneously. There is no limit to the number of threads
 that can be blocked in system calls on behalf of Go code; those do not count against

src/runtime/malloc.go

@@ -308,6 +308,10 @@ marked:
 		})
 	}
 
+	if mheap_.shadow_enabled {
+		clearshadow(uintptr(x), size)
+	}
+
 	if raceenabled {
 		racemalloc(x, size)
 	}

src/runtime/malloc1.go

@@ -223,6 +223,69 @@ func mallocinit() {
 	_g_.m.mcache = allocmcache()
 }
 
+func wbshadowinit() {
+	// Initialize write barrier shadow heap if we were asked for it
+	// and we have enough address space (not on 32-bit).
+	if debug.wbshadow == 0 {
+		return
+	}
+	if ptrSize != 8 {
+		print("runtime: GODEBUG=wbshadow=1 disabled on 32-bit system\n")
+		return
+	}
+
+	var reserved bool
+	p1 := sysReserve(nil, mheap_.arena_end-mheap_.arena_start, &reserved)
+	if p1 == nil {
+		throw("cannot map shadow heap")
+	}
+	mheap_.shadow_heap = uintptr(p1) - mheap_.arena_start
+	sysMap(p1, mheap_.arena_used-mheap_.arena_start, reserved, &memstats.other_sys)
+	memmove(p1, unsafe.Pointer(mheap_.arena_start), mheap_.arena_used-mheap_.arena_start)
+
+	mheap_.shadow_reserved = reserved
+	start := ^uintptr(0)
+	end := uintptr(0)
+	if start > uintptr(unsafe.Pointer(&noptrdata)) {
+		start = uintptr(unsafe.Pointer(&noptrdata))
+	}
+	if start > uintptr(unsafe.Pointer(&data)) {
+		start = uintptr(unsafe.Pointer(&data))
+	}
+	if start > uintptr(unsafe.Pointer(&noptrbss)) {
+		start = uintptr(unsafe.Pointer(&noptrbss))
+	}
+	if start > uintptr(unsafe.Pointer(&bss)) {
+		start = uintptr(unsafe.Pointer(&bss))
+	}
+	if end < uintptr(unsafe.Pointer(&enoptrdata)) {
+		end = uintptr(unsafe.Pointer(&enoptrdata))
+	}
+	if end < uintptr(unsafe.Pointer(&edata)) {
+		end = uintptr(unsafe.Pointer(&edata))
+	}
+	if end < uintptr(unsafe.Pointer(&enoptrbss)) {
+		end = uintptr(unsafe.Pointer(&enoptrbss))
+	}
+	if end < uintptr(unsafe.Pointer(&ebss)) {
+		end = uintptr(unsafe.Pointer(&ebss))
+	}
+	start &^= _PageSize - 1
+	end = round(end, _PageSize)
+	mheap_.data_start = start
+	mheap_.data_end = end
+	reserved = false
+	p1 = sysReserve(nil, end-start, &reserved)
+	if p1 == nil {
+		throw("cannot map shadow data")
+	}
+	mheap_.shadow_data = uintptr(p1) - start
+	sysMap(p1, end-start, reserved, &memstats.other_sys)
+	memmove(p1, unsafe.Pointer(start), end-start)
+
+	mheap_.shadow_enabled = true
+}
+
 func mHeap_SysAlloc(h *mheap, n uintptr) unsafe.Pointer {
 	if n > uintptr(h.arena_end)-uintptr(h.arena_used) {
 		// We are in 32-bit mode, maybe we didn't use all possible address space yet.
@@ -260,6 +323,9 @@ func mHeap_SysAlloc(h *mheap, n uintptr) unsafe.Pointer {
 		if raceenabled {
 			racemapshadow((unsafe.Pointer)(p), n)
 		}
+		if mheap_.shadow_enabled {
+			sysMap(unsafe.Pointer(p+mheap_.shadow_heap), n, h.shadow_reserved, &memstats.other_sys)
+		}
 
 		if uintptr(p)&(_PageSize-1) != 0 {
 			throw("misrounded allocation in MHeap_SysAlloc")

src/runtime/malloc2.go

@@ -434,6 +434,15 @@ type mheap struct {
 	arena_end      uintptr
 	arena_reserved bool
 
+	// write barrier shadow data+heap.
+	// 64-bit systems only, enabled by GODEBUG=wbshadow=1.
+	shadow_enabled  bool    // shadow should be updated and checked
+	shadow_reserved bool    // shadow memory is reserved
+	shadow_heap     uintptr // heap-addr + shadow_heap = shadow heap addr
+	shadow_data     uintptr // data-addr + shadow_data = shadow data addr
+	data_start      uintptr // start of shadowed data addresses
+	data_end        uintptr // end of shadowed data addresses
+
 	// central free lists for small size classes.
 	// the padding makes sure that the MCentrals are
 	// spaced CacheLineSize bytes apart, so that each MCentral.lock

src/runtime/mgc.go

@@ -241,8 +241,8 @@ var (
 	gccheckmarkenable = true
 )
 
-// Is address b in the known heap. If it doesn't have a valid gcmap
-// returns false. For example pointers into stacks will return false.
+// inheap reports whether b is a pointer into a (potentially dead) heap object.
+// It returns false for pointers into stack spans.
 //go:nowritebarrier
 func inheap(b uintptr) bool {
 	if b == 0 || b < mheap_.arena_start || b >= mheap_.arena_used {
@@ -557,6 +557,10 @@ func scanobject(b, n uintptr, ptrmask *uint8, wbuf *workbuf) *workbuf {
 			continue
 		}
 
+		if mheap_.shadow_enabled && debug.wbshadow >= 2 && gccheckmarkenable && checkmark {
+			checkwbshadow((*uintptr)(unsafe.Pointer(b + i)))
+		}
+
 		// Mark the object. return some important bits.
 		// We we combine the following two rotines we don't have to pass mbits or obj around.
 		var mbits markbits
@@ -575,7 +579,12 @@ func scanobject(b, n uintptr, ptrmask *uint8, wbuf *workbuf) *workbuf {
 // As a special case, scanblock(nil, 0, nil) means to scan previously queued work,
 // stopping only when no work is left in the system.
 //go:nowritebarrier
-func scanblock(b, n uintptr, ptrmask *uint8) {
+func scanblock(b0, n0 uintptr, ptrmask *uint8) {
+	// Use local copies of original parameters, so that a stack trace
+	// due to one of the throws below shows the original block
+	// base and extent.
+	b := b0
+	n := n0
 	wbuf := getpartialorempty()
 	if b != 0 {
 		wbuf = scanobject(b, n, ptrmask, wbuf)

src/runtime/mgc0.go

@@ -105,24 +105,123 @@ const (
 )
 
 func needwb() bool {
-	return gcphase == _GCmark || gcphase == _GCmarktermination
+	return gcphase == _GCmark || gcphase == _GCmarktermination || mheap_.shadow_enabled
+}
+
+// shadowptr returns a pointer to the shadow value for addr.
+//go:nosplit
+func shadowptr(addr uintptr) *uintptr {
+	var shadow *uintptr
+	if mheap_.data_start <= addr && addr < mheap_.data_end {
+		shadow = (*uintptr)(unsafe.Pointer(addr + mheap_.shadow_data))
+	} else if inheap(addr) {
+		shadow = (*uintptr)(unsafe.Pointer(addr + mheap_.shadow_heap))
+	}
+	return shadow
+}
+
+// clearshadow clears the shadow copy associated with the n bytes of memory at addr.
+func clearshadow(addr, n uintptr) {
+	if !mheap_.shadow_enabled {
+		return
+	}
+	p := shadowptr(addr)
+	if p == nil || n <= ptrSize {
+		return
+	}
+	memclr(unsafe.Pointer(p), n)
 }
 
 // NOTE: Really dst *unsafe.Pointer, src unsafe.Pointer,
 // but if we do that, Go inserts a write barrier on *dst = src.
 //go:nosplit
 func writebarrierptr(dst *uintptr, src uintptr) {
+	if !needwb() {
+		*dst = src
+		return
+	}
+
+	if src != 0 && (src < _PageSize || src == _PoisonGC || src == _PoisonStack) {
+		systemstack(func() { throw("bad pointer in write barrier") })
+	}
+
+	if mheap_.shadow_enabled {
+		systemstack(func() {
+			addr := uintptr(unsafe.Pointer(dst))
+			shadow := shadowptr(addr)
+			if shadow == nil {
+				return
+			}
+			// There is a race here but only if the program is using
+			// racy writes instead of sync/atomic. In that case we
+			// don't mind crashing.
+			if *shadow != *dst && *shadow != noShadow && istrackedptr(*dst) {
+				mheap_.shadow_enabled = false
+				print("runtime: write barrier dst=", dst, " old=", hex(*dst), " shadow=", shadow, " old=", hex(*shadow), " new=", hex(src), "\n")
+				throw("missed write barrier")
+			}
+			*shadow = src
+		})
+	}
+
 	*dst = src
-	if needwb() {
-		writebarrierptr_nostore(dst, src)
+	writebarrierptr_nostore1(dst, src)
+}
+
+// istrackedptr reports whether the pointer value p requires a write barrier
+// when stored into the heap.
+func istrackedptr(p uintptr) bool {
+	return inheap(p)
+}
+
+// checkwbshadow checks that p matches its shadow word.
+// The garbage collector calls checkwbshadow for each pointer during the checkmark phase.
+// It is only called when mheap_.shadow_enabled is true.
+func checkwbshadow(p *uintptr) {
+	addr := uintptr(unsafe.Pointer(p))
+	shadow := shadowptr(addr)
+	if shadow == nil {
+		return
+	}
+	// There is no race on the accesses here, because the world is stopped,
+	// but there may be racy writes that lead to the shadow and the
+	// heap being inconsistent. If so, we will detect that here as a
+	// missed write barrier and crash. We don't mind.
+	// Code should use sync/atomic instead of racy pointer writes.
+	if *shadow != *p && *shadow != noShadow && istrackedptr(*p) {
+		mheap_.shadow_enabled = false
+		print("runtime: checkwritebarrier p=", p, " *p=", hex(*p), " shadow=", shadow, " *shadow=", hex(*shadow), "\n")
+		throw("missed write barrier")
 	}
 }
 
+// noShadow is stored in as the shadow pointer to mark that there is no
+// shadow word recorded. It matches any actual pointer word.
+// noShadow is used when it is impossible to know the right word
+// to store in the shadow heap, such as when the real heap word
+// is being manipulated atomically.
+const noShadow uintptr = 1
+
+// writebarrierptr_noshadow records that the value in *dst
+// has been written to using an atomic operation and the shadow
+// has not been updated. (In general if dst must be manipulated
+// atomically we cannot get the right bits for use in the shadow.)
+//go:nosplit
+func writebarrierptr_noshadow(dst *uintptr) {
+	addr := uintptr(unsafe.Pointer(dst))
+	shadow := shadowptr(addr)
+	if shadow == nil {
+		return
+	}
+
+	*shadow = noShadow
+}
+
 // Like writebarrierptr, but the store has already been applied.
 // Do not reapply.
 //go:nosplit
 func writebarrierptr_nostore(dst *uintptr, src uintptr) {
-	if getg() == nil || !needwb() { // very low-level startup
+	if !needwb() {
 		return
 	}
 
@@ -130,6 +229,26 @@ func writebarrierptr_nostore(dst *uintptr, src uintptr) {
 		systemstack(func() { throw("bad pointer in write barrier") })
 	}
 
+	// Apply changes to shadow.
+	// Since *dst has been overwritten already, we cannot check
+	// whether there were any missed updates, but writebarrierptr_nostore
+	// is only rarely used (right now there is just one call, in newstack).
+	if mheap_.shadow_enabled {
+		systemstack(func() {
+			addr := uintptr(unsafe.Pointer(dst))
+			shadow := shadowptr(addr)
+			if shadow == nil {
+				return
+			}
+			*shadow = src
+		})
+	}
+
+	writebarrierptr_nostore1(dst, src)
+}
+
+//go:nosplit
+func writebarrierptr_nostore1(dst *uintptr, src uintptr) {
 	mp := acquirem()
 	if mp.inwb || mp.dying > 0 {
 		releasem(mp)

src/runtime/panic.go

@@ -177,6 +177,16 @@ func newdefer(siz int32) *_defer {
 		d = (*_defer)(mallocgc(total, deferType, 0))
 	}
 	d.siz = siz
+	if mheap_.shadow_enabled {
+		// This memory will be written directly, with no write barrier,
+		// and then scanned like stacks during collection.
+		// Unlike real stacks, it is from heap spans, so mark the
+		// shadow as explicitly unusable.
+		p := deferArgs(d)
+		for i := uintptr(0); i+ptrSize <= uintptr(siz); i += ptrSize {
+			writebarrierptr_noshadow((*uintptr)(add(p, i)))
+		}
+	}
 	gp := mp.curg
 	d.link = gp._defer
 	gp._defer = d
@@ -194,6 +204,12 @@ func freedefer(d *_defer) {
 	if d.fn != nil {
 		freedeferfn()
 	}
+	if mheap_.shadow_enabled {
+		// Undo the marking in newdefer.
+		systemstack(func() {
+			clearshadow(uintptr(deferArgs(d)), uintptr(d.siz))
+		})
+	}
 	sc := deferclass(uintptr(d.siz))
 	if sc < uintptr(len(p{}.deferpool)) {
 		mp := acquirem()

src/runtime/proc1.go

@@ -122,6 +122,7 @@ func schedinit() {
 	goargs()
 	goenvs()
 	parsedebugvars()
+	wbshadowinit()
 	gcinit()
 
 	sched.lastpoll = uint64(nanotime())

src/runtime/runtime1.go

@@ -301,18 +301,31 @@ type dbgVar struct {
 	value *int32
 }
 
-// Do we report invalid pointers found during stack or heap scans?
-//var invalidptr int32 = 1
+// TODO(rsc): Make GC respect debug.invalidptr.
+
+// Holds variables parsed from GODEBUG env var.
+var debug struct {
+	allocfreetrace int32
+	efence         int32
+	gcdead         int32
+	gctrace        int32
+	invalidptr     int32
+	scavenge       int32
+	scheddetail    int32
+	schedtrace     int32
+	wbshadow       int32
+}
 
 var dbgvars = []dbgVar{
 	{"allocfreetrace", &debug.allocfreetrace},
-	{"invalidptr", &invalidptr},
 	{"efence", &debug.efence},
-	{"gctrace", &debug.gctrace},
 	{"gcdead", &debug.gcdead},
+	{"gctrace", &debug.gctrace},
+	{"invalidptr", &debug.invalidptr},
+	{"scavenge", &debug.scavenge},
 	{"scheddetail", &debug.scheddetail},
 	{"schedtrace", &debug.schedtrace},
-	{"scavenge", &debug.scavenge},
+	{"wbshadow", &debug.wbshadow},
 }
 
 func parsedebugvars() {

src/runtime/runtime2.go

@@ -439,17 +439,6 @@ type cgomal struct {
 	alloc unsafe.Pointer
 }
 
-// Holds variables parsed from GODEBUG env var.
-type debugvars struct {
-	allocfreetrace int32
-	efence         int32
-	gctrace        int32
-	gcdead         int32
-	scheddetail    int32
-	schedtrace     int32
-	scavenge       int32
-}
-
 // Indicates to write barrier and sychronization task to preform.
 const (
 	_GCoff             = iota // GC not running, write barrier disabled
@@ -501,8 +490,6 @@ func extendRandom(r []byte, n int) {
 	}
 }
 
-var invalidptr int32
-
 /*
  * deferred subroutine calls
  */
@@ -569,7 +556,6 @@ var (
 	iscgo       bool
 	cpuid_ecx   uint32
 	cpuid_edx   uint32
-	debug       debugvars
 	signote     note
 	forcegc     forcegcstate
 	sched       schedt

src/runtime/stack1.go

@@ -389,7 +389,7 @@ func adjustpointers(scanp unsafe.Pointer, cbv *bitvector, adjinfo *adjustinfo, f
 		case _BitsPointer:
 			p := *(*unsafe.Pointer)(add(scanp, i*ptrSize))
 			up := uintptr(p)
-			if f != nil && 0 < up && up < _PageSize && invalidptr != 0 || up == poisonGC || up == poisonStack {
+			if f != nil && 0 < up && up < _PageSize && debug.invalidptr != 0 || up == poisonGC || up == poisonStack {
 				// Looks like a junk value in a pointer slot.
 				// Live analysis wrong?
 				getg().m.traceback = 2
@@ -611,13 +611,13 @@ func round2(x int32) int32 {
 func newstack() {
 	thisg := getg()
 	// TODO: double check all gp. shouldn't be getg().
-	if thisg.m.morebuf.g.stackguard0 == stackFork {
+	if thisg.m.morebuf.g.ptr().stackguard0 == stackFork {
 		throw("stack growth after fork")
 	}
-	if thisg.m.morebuf.g != thisg.m.curg {
+	if thisg.m.morebuf.g.ptr() != thisg.m.curg {
 		print("runtime: newstack called from g=", thisg.m.morebuf.g, "\n"+"\tm=", thisg.m, " m->curg=", thisg.m.curg, " m->g0=", thisg.m.g0, " m->gsignal=", thisg.m.gsignal, "\n")
 		morebuf := thisg.m.morebuf
-		traceback(morebuf.pc, morebuf.sp, morebuf.lr, morebuf.g)
+		traceback(morebuf.pc, morebuf.sp, morebuf.lr, morebuf.g.ptr())
 		throw("runtime: wrong goroutine in newstack")
 	}
 	if thisg.m.curg.throwsplit {
@@ -629,6 +629,8 @@ func newstack() {
 		print("runtime: newstack sp=", hex(gp.sched.sp), " stack=[", hex(gp.stack.lo), ", ", hex(gp.stack.hi), "]\n",
 			"\tmorebuf={pc:", hex(morebuf.pc), " sp:", hex(morebuf.sp), " lr:", hex(morebuf.lr), "}\n",
 			"\tsched={pc:", hex(gp.sched.pc), " sp:", hex(gp.sched.sp), " lr:", hex(gp.sched.lr), " ctxt:", gp.sched.ctxt, "}\n")
+
+		traceback(morebuf.pc, morebuf.sp, morebuf.lr, gp)
 		throw("runtime: stack split at bad time")
 	}
 
@@ -640,7 +642,7 @@ func newstack() {
 	thisg.m.morebuf.pc = 0
 	thisg.m.morebuf.lr = 0
 	thisg.m.morebuf.sp = 0
-	thisg.m.morebuf.g = nil
+	thisg.m.morebuf.g = 0
 
 	casgstatus(gp, _Grunning, _Gwaiting)
 	gp.waitreason = "stack growth"