runtime: top align tinyallocs in race mode

Top align allocations in tinyalloc buckets when in race mode.
This will make checkptr checks more reliable, because any code
that modifies a pointer past the end of the object will trigger
a checkptr error.

No test, because we need -race for this to actually kick in.  We could
add it to the race detector tests, but the race detector tests are all
geared towards race detector reports, not checkptr reports. Mucking
with parsing reports is more than a test is worth.

Fixes #38872

Change-Id: Ie56f0fbd1a9385539f6631fd1ac40c3de5600154
Reviewed-on: https://go-review.googlesource.com/c/go/+/315029
Trust: Keith Randall <[email protected]>
Run-TryBot: Keith Randall <[email protected]>
Reviewed-by: Cuong Manh Le <[email protected]>
Reviewed-by: Matthew Dempsky <[email protected]>
TryBot-Result: Go Bot <[email protected]>

src/runtime/export_test.go

@@ -1267,3 +1267,5 @@ func GCTestIsReachable(ptrs ...unsafe.Pointer) (mask uint64) {
 func GCTestPointerClass(p unsafe.Pointer) string {
 	return gcTestPointerClass(p)
 }
+
+const Raceenabled = raceenabled

src/runtime/malloc.go

@@ -1047,7 +1047,8 @@ func mallocgc(size uintptr, typ *_type, needzero bool) unsafe.Pointer {
 			(*[2]uint64)(x)[1] = 0
 			// See if we need to replace the existing tiny block with the new one
 			// based on amount of remaining free space.
-			if size < c.tinyoffset || c.tiny == 0 {
+			if !raceenabled && (size < c.tinyoffset || c.tiny == 0) {
+				// Note: disabled when race detector is on, see comment near end of this function.
 				c.tiny = uintptr(x)
 				c.tinyoffset = size
 			}
@@ -1165,6 +1166,22 @@ func mallocgc(size uintptr, typ *_type, needzero bool) unsafe.Pointer {
 		}
 	}
 
+	if raceenabled && noscan && dataSize < maxTinySize {
+		// Pad tinysize allocations so they are aligned with the end
+		// of the tinyalloc region. This ensures that any arithmetic
+		// that goes off the top end of the object will be detectable
+		// by checkptr (issue 38872).
+		// Note that we disable tinyalloc when raceenabled for this to work.
+		// TODO: This padding is only performed when the race detector
+		// is enabled. It would be nice to enable it if any package
+		// was compiled with checkptr, but there's no easy way to
+		// detect that (especially at compile time).
+		// TODO: enable this padding for all allocations, not just
+		// tinyalloc ones. It's tricky because of pointer maps.
+		// Maybe just all noscan objects?
+		x = add(x, size-dataSize)
+	}
+
 	return x
 }
 

src/runtime/malloc_test.go

@@ -154,6 +154,9 @@ func TestStringConcatenationAllocs(t *testing.T) {
 }
 
 func TestTinyAlloc(t *testing.T) {
+	if runtime.Raceenabled {
+		t.Skip("tinyalloc suppressed when running in race mode")
+	}
 	const N = 16
 	var v [N]unsafe.Pointer
 	for i := range v {
@@ -182,6 +185,9 @@ type obj12 struct {
 }
 
 func TestTinyAllocIssue37262(t *testing.T) {
+	if runtime.Raceenabled {
+		t.Skip("tinyalloc suppressed when running in race mode")
+	}
 	// Try to cause an alignment access fault
 	// by atomically accessing the first 64-bit
 	// value of a tiny-allocated object.