runtime: initialize pointer bits of noscan spans

Some code paths in the runtime (cgo, heapdump) request heap bits
without first checking that the span is !noscan. Instead of trying
to find and work around all those cases, just set the pointer bits
of noscan spans correctly. It's somewhat safer than ensuring we
caught all the possible cases.

Fixes #54557
Fixes #54558

Change-Id: Ibd476e6cdea77c962e4d15aad26f29df66fd94e8
Reviewed-on: https://go-review.googlesource.com/c/go/+/425194
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

src/runtime/mbitmap.go

@@ -32,12 +32,10 @@
 // If ha.noMorePtrs[i]>>j&1 is set, the entries in ha.bitmap[8*i+j+1] and
 // beyond must all be zero until the start of the next object.
 //
-// The bitmap for noscan spans is not maintained (can be junk). Code must
-// ensure that an object is scannable before consulting its bitmap by
-// checking either the noscan bit in the span or by consulting its
-// type's information.
+// The bitmap for noscan spans is set to all zero at span allocation time.
 //
-// The bitmap for unallocated objects is also not maintained.
+// The bitmap for unallocated objects in scannable spans is not maintained
+// (can be junk).
 
 package runtime
 
@@ -722,6 +720,16 @@ func typeBitsBulkBarrier(typ *_type, dst, src, size uintptr) {
 // If this is a span of single pointer allocations, it initializes all
 // words to pointer.
 func (s *mspan) initHeapBits() {
+	if s.spanclass.noscan() {
+		// Set all the pointer bits to zero. We do this once
+		// when the span is allocated so we don't have to do it
+		// for each object allocation.
+		base := s.base()
+		size := s.npages * pageSize
+		h := writeHeapBitsForAddr(base)
+		h.flush(base, size)
+		return
+	}
 	isPtrs := goarch.PtrSize == 8 && s.elemsize == goarch.PtrSize
 	if !isPtrs {
 		return // nothing to do
@@ -873,7 +881,7 @@ func (h writeHeapBits) flush(addr, size uintptr) {
 	// Continue on writing zeros for the rest of the object.
 	// For standard use of the ptr bits this is not required, as
 	// the bits are read from the beginning of the object. Some uses,
-	// like oblets, bulk write barriers, and cgocheck, might
+	// like noscan spans, oblets, bulk write barriers, and cgocheck, might
 	// start mid-object, so these writes are still required.
 	for {
 		// Write zero bits.
@@ -942,7 +950,8 @@ func heapBitsSetType(x, size, dataSize uintptr, typ *_type) {
 		// It's one word and it has pointers, it must be a pointer.
 		// Since all allocated one-word objects are pointers
 		// (non-pointers are aggregated into tinySize allocations),
-		// initSpan sets the pointer bits for us. Nothing to do here.
+		// (*mspan).initHeapBits sets the pointer bits for us.
+		// Nothing to do here.
 		if doubleCheck {
 			h, addr := heapBitsForAddr(x, size).next()
 			if addr != x {

src/runtime/mgcmark.go

@@ -1273,6 +1273,8 @@ func scanobject(b uintptr, gcw *gcWork) {
 		throw("scanobject n == 0")
 	}
 	if s.spanclass.noscan() {
+		// Correctness-wise this is ok, but it's inefficient
+		// if noscan objects reach here.
 		throw("scanobject of a noscan object")
 	}