runtime: fix mallocgc for asan

This change finally fully fixes mallocgc for asan after the recent
refactoring. Here is everything that changed:

Fix the accounting for the alloc header; large objects don't have them.

Mask out extra bits set from unrolling the bitmap for slice backing
stores in writeHeapBitsSmall. The redzone in asan mode makes it so that
dataSize is no longer an exact multiple of typ.Size_ in this case (a
new assumption I have recently discovered) but we didn't mask out any
extra bits, so we'd accidentally set bits in other allocations. Oops.

Move the initHeapBits optimization for the 8-byte scan sizeclass on
64-bit platforms up to mallocgc, out from writeHeapBitsSmall. So, this
actually caused a problem with asan when the optimization first landed,
but we missed it. The issue was then masked once we started passing the
redzone down into writeHeapBitsSmall, since the optimization would no
longer erroneously fire on asan. What happened was that dataSize would
be 8 (because that was the user-provided alloc size) so we'd skip
writing heap bits, but it would turn out the redzone bumped the size
class, so we'd actually *have* to write the heap bits for that size
class. This is not really a problem now *but* it caused problems for me
when debugging, since I would try to remove the red zone from dataSize
and this would trigger this bug again. Ultimately, this whole situation
is confusing because the check in writeHeapBitsSmall is *not* the same
as the check in initHeapBits. By moving this check up to mallocgc, we
can make the checks align better by matching on the sizeclass, so this
should be less error-prone in the future.

Change-Id: I1e9819223be23f722f3bf21e63e812f5fb557194
Reviewed-on: https://go-review.googlesource.com/c/go/+/622041
Reviewed-by: Keith Randall <[email protected]>
Reviewed-by: Keith Randall <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

src/runtime/arena.go

@@ -798,7 +798,7 @@ func newUserArenaChunk() (unsafe.Pointer, *mspan) {
 
 	if asanenabled {
 		// TODO(mknyszek): Track individual objects.
-		rzSize := computeRZlog(span.elemsize)
+		rzSize := redZoneSize(span.elemsize)
 		span.elemsize -= rzSize
 		span.largeType.Size_ = span.elemsize
 		rzStart := span.base() + span.elemsize

src/runtime/malloc.go

@@ -1035,7 +1035,7 @@ func mallocgc(size uintptr, typ *_type, needzero bool) unsafe.Pointer {
 	// These "redzones" are marked as unaddressable.
 	var asanRZ uintptr
 	if asanenabled {
-		asanRZ = computeRZlog(size)
+		asanRZ = redZoneSize(size)
 		size += asanRZ
 	}
 
@@ -1074,10 +1074,10 @@ func mallocgc(size uintptr, typ *_type, needzero bool) unsafe.Pointer {
 		// Poison the space between the end of the requested size of x
 		// and the end of the slot. Unpoison the requested allocation.
 		frag := elemsize - size
-		if typ != nil && typ.Pointers() && !heapBitsInSpan(elemsize) {
+		if typ != nil && typ.Pointers() && !heapBitsInSpan(elemsize) && size <= maxSmallSize-mallocHeaderSize {
 			frag -= mallocHeaderSize
 		}
-		asanpoison(unsafe.Add(x, size-asanRZ), asanRZ+frag)
+		asanpoison(unsafe.Add(x, size-asanRZ), asanRZ)
 		asanunpoison(x, size-asanRZ)
 	}
 
@@ -1369,7 +1369,13 @@ func mallocgcSmallScanNoHeader(size uintptr, typ *_type, needzero bool) (unsafe.
 	if needzero && span.needzero != 0 {
 		memclrNoHeapPointers(x, size)
 	}
-	c.scanAlloc += heapSetTypeNoHeader(uintptr(x), size, typ, span)
+	if goarch.PtrSize == 8 && sizeclass == 1 {
+		// initHeapBits already set the pointer bits for the 8-byte sizeclass
+		// on 64-bit platforms.
+		c.scanAlloc += 8
+	} else {
+		c.scanAlloc += heapSetTypeNoHeader(uintptr(x), size, typ, span)
+	}
 	size = uintptr(class_to_size[sizeclass])
 
 	// Ensure that the stores above that initialize x to
@@ -2040,9 +2046,9 @@ func (p *notInHeap) add(bytes uintptr) *notInHeap {
 	return (*notInHeap)(unsafe.Pointer(uintptr(unsafe.Pointer(p)) + bytes))
 }
 
-// computeRZlog computes the size of the redzone.
+// redZoneSize computes the size of the redzone for a given allocation.
 // Refer to the implementation of the compiler-rt.
-func computeRZlog(userSize uintptr) uintptr {
+func redZoneSize(userSize uintptr) uintptr {
 	switch {
 	case userSize <= (64 - 16):
 		return 16 << 0

src/runtime/mbitmap.go

@@ -640,11 +640,6 @@ func (span *mspan) heapBitsSmallForAddr(addr uintptr) uintptr {
 //
 //go:nosplit
 func (span *mspan) writeHeapBitsSmall(x, dataSize uintptr, typ *_type) (scanSize uintptr) {
-	if goarch.PtrSize == 8 && dataSize == goarch.PtrSize {
-		// Already set by initHeapBits.
-		return
-	}
-
 	// The objects here are always really small, so a single load is sufficient.
 	src0 := readUintptr(typ.GCData)
 
@@ -654,10 +649,21 @@ func (span *mspan) writeHeapBitsSmall(x, dataSize uintptr, typ *_type) (scanSize
 	if typ.Size_ == goarch.PtrSize {
 		src = (1 << (dataSize / goarch.PtrSize)) - 1
 	} else {
+		// N.B. We rely on dataSize being an exact multiple of the type size.
+		// The alternative is to be defensive and mask out src to the length
+		// of dataSize. The purpose is to save on one additional masking operation.
+		if doubleCheckHeapSetType && !asanenabled && dataSize%typ.Size_ != 0 {
+			throw("runtime: (*mspan).writeHeapBitsSmall: dataSize is not a multiple of typ.Size_")
+		}
 		for i := typ.Size_; i < dataSize; i += typ.Size_ {
 			src |= src0 << (i / goarch.PtrSize)
 			scanSize += typ.Size_
 		}
+		if asanenabled {
+			// Mask src down to dataSize. dataSize is going to be a strange size because of
+			// the redzone required for allocations when asan is enabled.
+			src &= (1 << (dataSize / goarch.PtrSize)) - 1
+		}
 	}
 
 	// Since we're never writing more than one uintptr's worth of bits, we're either going