cmd/compile, runtime: store pointers to go:notinheap types indirectly

pointers to go:notinheap types should be treated as scalars. That
means they shouldn't be stored directly in interfaces, or directly
in reflect.Value.ptr.

Also be sure to use uintpr to compare such pointers in reflect.DeepEqual.

Fixes #42076

Change-Id: I53735f6d434e9c3108d4940bd1bae14c61ef2a74
Reviewed-on: https://go-review.googlesource.com/c/go/+/264480
Trust: Keith Randall <khr@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

2 files changed, 22 insertions, 2 deletions

diff --git a/src/reflect/deepequal.go b/src/reflect/deepequal.go
index be66464129..d951d8d999 100644
--- a/src/reflect/deepequal.go
+++ b/src/reflect/deepequal.go
@@ -35,7 +35,17 @@ func deepValueEqual(v1, v2 Value, visited map[visit]bool) bool {
 	// and it's safe and valid to get Value's internal pointer.
 	hard := func(v1, v2 Value) bool {
 		switch v1.Kind() {
-		case Map, Slice, Ptr, Interface:
+		case Ptr:
+			if v1.typ.ptrdata == 0 {
+				// go:notinheap pointers can't be cyclic.
+				// At least, all of our current uses of go:notinheap have
+				// that property. The runtime ones aren't cyclic (and we don't use
+				// DeepEqual on them anyway), and the cgo-generated ones are
+				// all empty structs.
+				return false
+			}
+			fallthrough
+		case Map, Slice, Interface:
 			// Nil pointers cannot be cyclic. Avoid putting them in the visited map.
 			return !v1.IsNil() && !v2.IsNil()
 		}
diff --git a/src/reflect/value.go b/src/reflect/value.go
index bb6371b867..24eab6a2c6 100644
--- a/src/reflect/value.go
+++ b/src/reflect/value.go
@@ -90,6 +90,7 @@ func (f flag) ro() flag {
 
 // pointer returns the underlying pointer represented by v.
 // v.Kind() must be Ptr, Map, Chan, Func, or UnsafePointer
+// if v.Kind() == Ptr, the base type must not be go:notinheap.
 func (v Value) pointer() unsafe.Pointer {
 	if v.typ.size != ptrSize || !v.typ.pointers() {
 		panic("can't call pointer on a non-pointer Value")
@@ -1453,7 +1454,16 @@ func (v Value) Pointer() uintptr {
 	// TODO: deprecate
 	k := v.kind()
 	switch k {
-	case Chan, Map, Ptr, UnsafePointer:
+	case Ptr:
+		if v.typ.ptrdata == 0 {
+			// Handle pointers to go:notinheap types directly,
+			// so we never materialize such pointers as an
+			// unsafe.Pointer. (Such pointers are always indirect.)
+			// See issue 42076.
+			return *(*uintptr)(v.ptr)
+		}
+		fallthrough
+	case Chan, Map, UnsafePointer:
 		return uintptr(v.pointer())
 	case Func:
 		if v.flag&flagMethod != 0 {