runtime: add GC testing helpers for regabi signature fuzzer

This CL adds a set of helper functions for testing GC interactions.
These are intended for use in the regabi signature fuzzer, but are
generally useful for GC tests, so we make them generally available to
runtime tests.

These provide:

1. An easy way to force stack movement, for testing stack copying.

2. A simple and robust way to check the reachability of a set of
pointers.

3. A way to check what general category of memory a pointer points to,
mostly so tests can make sure they're testing what they mean to.

For #40724, but generally useful.

Change-Id: I15d33ccb3f5a792c0472a19c2cc9a8b4a9356a66
Reviewed-on: https://go-review.googlesource.com/c/go/+/305330
Trust: Austin Clements <austin@google.com>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Michael Pratt <mpratt@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>

src/runtime/export_test.go

@@ -1244,3 +1244,24 @@ func FinalizerGAsleep() bool {
 	unlock(&finlock)
 	return result
 }
+
+// For GCTestMoveStackOnNextCall, it's important not to introduce an
+// extra layer of call, since then there's a return before the "real"
+// next call.
+var GCTestMoveStackOnNextCall = gcTestMoveStackOnNextCall
+
+// For GCTestIsReachable, it's important that we do this as a call so
+// escape analysis can see through it.
+func GCTestIsReachable(ptrs ...unsafe.Pointer) (mask uint64) {
+	return gcTestIsReachable(ptrs...)
+}
+
+// For GCTestPointerClass, it's important that we do this as a call so
+// escape analysis can see through it.
+//
+// This is nosplit because gcTestPointerClass is.
+//
+//go:nosplit
+func GCTestPointerClass(p unsafe.Pointer) string {
+	return gcTestPointerClass(p)
+}

src/runtime/gc_test.go

@@ -202,6 +202,81 @@ func TestGcZombieReporting(t *testing.T) {
 	}
 }
 
+func TestGCTestMoveStackOnNextCall(t *testing.T) {
+	t.Parallel()
+	var onStack int
+	runtime.GCTestMoveStackOnNextCall()
+	moveStackCheck(t, &onStack, uintptr(unsafe.Pointer(&onStack)))
+}
+
+// This must not be inlined because the point is to force a stack
+// growth check and move the stack.
+//
+//go:noinline
+func moveStackCheck(t *testing.T, new *int, old uintptr) {
+	// new should have been updated by the stack move;
+	// old should not have.
+
+	// Capture new's value before doing anything that could
+	// further move the stack.
+	new2 := uintptr(unsafe.Pointer(new))
+
+	t.Logf("old stack pointer %x, new stack pointer %x", old, new2)
+	if new2 == old {
+		// Check that we didn't screw up the test's escape analysis.
+		if cls := runtime.GCTestPointerClass(unsafe.Pointer(new)); cls != "stack" {
+			t.Fatalf("test bug: new (%#x) should be a stack pointer, not %s", new2, cls)
+		}
+		// This was a real failure.
+		t.Fatal("stack did not move")
+	}
+}
+
+func TestGCTestIsReachable(t *testing.T) {
+	var all, half []unsafe.Pointer
+	var want uint64
+	for i := 0; i < 16; i++ {
+		// The tiny allocator muddies things, so we use a
+		// scannable type.
+		p := unsafe.Pointer(new(*int))
+		all = append(all, p)
+		if i%2 == 0 {
+			half = append(half, p)
+			want |= 1 << i
+		}
+	}
+
+	got := runtime.GCTestIsReachable(all...)
+	if want != got {
+		t.Fatalf("did not get expected reachable set; want %b, got %b", want, got)
+	}
+	runtime.KeepAlive(half)
+}
+
+var pointerClassSink *int
+var pointerClassData = 42
+
+func TestGCTestPointerClass(t *testing.T) {
+	t.Parallel()
+	check := func(p unsafe.Pointer, want string) {
+		t.Helper()
+		got := runtime.GCTestPointerClass(p)
+		if got != want {
+			// Convert the pointer to a uintptr to avoid
+			// escaping it.
+			t.Errorf("for %#x, want class %s, got %s", uintptr(p), want, got)
+		}
+	}
+	var onStack int
+	var notOnStack int
+	pointerClassSink = &notOnStack
+	check(unsafe.Pointer(&onStack), "stack")
+	check(unsafe.Pointer(&notOnStack), "heap")
+	check(unsafe.Pointer(&pointerClassSink), "bss")
+	check(unsafe.Pointer(&pointerClassData), "data")
+	check(nil, "other")
+}
+
 func BenchmarkSetTypePtr(b *testing.B) {
 	benchSetType(b, new(*byte))
 }

src/runtime/mgc.go

@@ -2339,3 +2339,99 @@ func fmtNSAsMS(buf []byte, ns uint64) []byte {
 	}
 	return itoaDiv(buf, x, dec)
 }
+
+// Helpers for testing GC.
+
+// gcTestMoveStackOnNextCall causes the stack to be moved on a call
+// immediately following the call to this. It may not work correctly
+// if any other work appears after this call (such as returning).
+// Typically the following call should be marked go:noinline so it
+// performs a stack check.
+func gcTestMoveStackOnNextCall() {
+	gp := getg()
+	gp.stackguard0 = getcallersp()
+}
+
+// gcTestIsReachable performs a GC and returns a bit set where bit i
+// is set if ptrs[i] is reachable.
+func gcTestIsReachable(ptrs ...unsafe.Pointer) (mask uint64) {
+	// This takes the pointers as unsafe.Pointers in order to keep
+	// them live long enough for us to attach specials. After
+	// that, we drop our references to them.
+
+	if len(ptrs) > 64 {
+		panic("too many pointers for uint64 mask")
+	}
+
+	// Block GC while we attach specials and drop our references
+	// to ptrs. Otherwise, if a GC is in progress, it could mark
+	// them reachable via this function before we have a chance to
+	// drop them.
+	semacquire(&gcsema)
+
+	// Create reachability specials for ptrs.
+	specials := make([]*specialReachable, len(ptrs))
+	for i, p := range ptrs {
+		lock(&mheap_.speciallock)
+		s := (*specialReachable)(mheap_.specialReachableAlloc.alloc())
+		unlock(&mheap_.speciallock)
+		s.special.kind = _KindSpecialReachable
+		if !addspecial(p, &s.special) {
+			throw("already have a reachable special (duplicate pointer?)")
+		}
+		specials[i] = s
+		// Make sure we don't retain ptrs.
+		ptrs[i] = nil
+	}
+
+	semrelease(&gcsema)
+
+	// Force a full GC and sweep.
+	GC()
+
+	// Process specials.
+	for i, s := range specials {
+		if !s.done {
+			printlock()
+			println("runtime: object", i, "was not swept")
+			throw("IsReachable failed")
+		}
+		if s.reachable {
+			mask |= 1 << i
+		}
+		lock(&mheap_.speciallock)
+		mheap_.specialReachableAlloc.free(unsafe.Pointer(s))
+		unlock(&mheap_.speciallock)
+	}
+
+	return mask
+}
+
+// gcTestPointerClass returns the category of what p points to, one of:
+// "heap", "stack", "data", "bss", "other". This is useful for checking
+// that a test is doing what it's intended to do.
+//
+// This is nosplit simply to avoid extra pointer shuffling that may
+// complicate a test.
+//
+//go:nosplit
+func gcTestPointerClass(p unsafe.Pointer) string {
+	p2 := uintptr(noescape(p))
+	gp := getg()
+	if gp.stack.lo <= p2 && p2 < gp.stack.hi {
+		return "stack"
+	}
+	if base, _, _ := findObject(p2, 0, 0); base != 0 {
+		return "heap"
+	}
+	for _, datap := range activeModules() {
+		if datap.data <= p2 && p2 < datap.edata || datap.noptrdata <= p2 && p2 < datap.enoptrdata {
+			return "data"
+		}
+		if datap.bss <= p2 && p2 < datap.ebss || datap.noptrbss <= p2 && p2 <= datap.enoptrbss {
+			return "bss"
+		}
+	}
+	KeepAlive(p)
+	return "other"
+}

src/runtime/mgcsweep.go

@@ -385,16 +385,24 @@ func (s *mspan) sweep(preserve bool) bool {
 					siter.unlinkAndNext()
 					freeSpecial(special, unsafe.Pointer(p), size)
 				} else {
-					// This is profile record, but the object has finalizers (so kept alive).
-					// Keep special record.
+					// The object has finalizers, so we're keeping it alive.
+					// All other specials only apply when an object is freed,
+					// so just keep the special record.
 					siter.next()
 				}
 			}
 		} else {
-			// object is still live: keep special record
+			// object is still live
+			if siter.s.kind == _KindSpecialReachable {
+				special := siter.unlinkAndNext()
+				(*specialReachable)(unsafe.Pointer(special)).reachable = true
+				freeSpecial(special, unsafe.Pointer(p), size)
+			} else {
+				// keep special record
 				siter.next()
 			}
 		}
+	}
 	if hadSpecials && s.specials == nil {
 		spanHasNoSpecials(s)
 	}

src/runtime/mheap.go

@@ -212,6 +212,7 @@ type mheap struct {
 	cachealloc            fixalloc // allocator for mcache*
 	specialfinalizeralloc fixalloc // allocator for specialfinalizer*
 	specialprofilealloc   fixalloc // allocator for specialprofile*
+	specialReachableAlloc fixalloc // allocator for specialReachable
 	speciallock           mutex    // lock for special record allocators.
 	arenaHintAlloc        fixalloc // allocator for arenaHints
 
@@ -703,6 +704,7 @@ func (h *mheap) init() {
 	h.cachealloc.init(unsafe.Sizeof(mcache{}), nil, nil, &memstats.mcache_sys)
 	h.specialfinalizeralloc.init(unsafe.Sizeof(specialfinalizer{}), nil, nil, &memstats.other_sys)
 	h.specialprofilealloc.init(unsafe.Sizeof(specialprofile{}), nil, nil, &memstats.other_sys)
+	h.specialReachableAlloc.init(unsafe.Sizeof(specialReachable{}), nil, nil, &memstats.other_sys)
 	h.arenaHintAlloc.init(unsafe.Sizeof(arenaHint{}), nil, nil, &memstats.other_sys)
 
 	// Don't zero mspan allocations. Background sweeping can
@@ -1649,6 +1651,9 @@ func (list *mSpanList) takeAll(other *mSpanList) {
 const (
 	_KindSpecialFinalizer = 1
 	_KindSpecialProfile   = 2
+	// _KindSpecialReachable is a special used for tracking
+	// reachability during testing.
+	_KindSpecialReachable = 3
 	// Note: The finalizer special must be first because if we're freeing
 	// an object, a finalizer special will cause the freeing operation
 	// to abort, and we want to keep the other special records around
@@ -1854,6 +1859,14 @@ func setprofilebucket(p unsafe.Pointer, b *bucket) {
 	}
 }
 
+// specialReachable tracks whether an object is reachable on the next
+// GC cycle. This is used by testing.
+type specialReachable struct {
+	special   special
+	done      bool
+	reachable bool
+}
+
 // specialsIter helps iterate over specials lists.
 type specialsIter struct {
 	pprev **special
@@ -1898,6 +1911,10 @@ func freeSpecial(s *special, p unsafe.Pointer, size uintptr) {
 		lock(&mheap_.speciallock)
 		mheap_.specialprofilealloc.free(unsafe.Pointer(sp))
 		unlock(&mheap_.speciallock)
+	case _KindSpecialReachable:
+		sp := (*specialReachable)(unsafe.Pointer(s))
+		sp.done = true
+		// The creator frees these.
 	default:
 		throw("bad special kind")
 		panic("not reached")