runtime: unwind stack barriers when writing above the current frame

Stack barriers assume that writes through pointers to frames above the
current frame will get write barriers, and hence these frames do not
need to be re-scanned to pick up these changes. For normal writes,
this is true. However, there are places in the runtime that use
typedmemmove to potentially write through pointers to higher frames
(such as mapassign1). Currently, typedmemmove does not execute write
barriers if the destination is on the stack. If there's a stack
barrier between the current frame and the frame being modified with
typedmemmove, and the stack barrier is not otherwise hit, it's
possible that the garbage collector will never see the updated pointer
and incorrectly reclaim the object.

Fix this by making heapBitsBulkBarrier (which lies behind typedmemmove
and its variants) detect when the destination is in the stack and
unwind stack barriers up to the point, forcing mark termination to
later rescan the effected frame and collect these pointers.

Fixes #11084. Might be related to #10240, #10541, #10941, #11023,
 #11027 and possibly others.

Change-Id: I323d6cd0f1d29fa01f8fc946f4b90e04ef210efd
Reviewed-on: https://go-review.googlesource.com/10791
Reviewed-by: Russ Cox <[email protected]>

Author: aclements

src/runtime/mbitmap.go

@@ -367,7 +367,34 @@ func heapBitsBulkBarrier(p, size uintptr) {
 	if (p|size)&(ptrSize-1) != 0 {
 		throw("heapBitsBulkBarrier: unaligned arguments")
 	}
-	if !writeBarrierEnabled || !inheap(p) {
+	if !writeBarrierEnabled {
+		return
+	}
+	if !inheap(p) {
+		// If p is on the stack and in a higher frame than the
+		// caller, we either need to execute write barriers on
+		// it (which is what happens for normal stack writes
+		// through pointers to higher frames), or we need to
+		// force the mark termination stack scan to scan the
+		// frame containing p.
+		//
+		// Executing write barriers on p is complicated in the
+		// general case because we either need to unwind the
+		// stack to get the stack map, or we need the type's
+		// bitmap, which may be a GC program.
+		//
+		// Hence, we opt for forcing the re-scan to scan the
+		// frame containing p, which we can do by simply
+		// unwinding the stack barriers between the current SP
+		// and p's frame.
+		gp := getg().m.curg
+		if gp.stack.lo <= p && p < gp.stack.hi {
+			// Run on the system stack to give it more
+			// stack space.
+			systemstack(func() {
+				gcUnwindBarriers(gp, p)
+			})
+		}
 		return
 	}
 

src/runtime/mgcmark.go

@@ -539,7 +539,11 @@ func gcRemoveStackBarriers(gp *g) {
 
 // gcRemoveStackBarrier removes a single stack barrier. It is the
 // inverse operation of gcInstallStackBarrier.
+//
+// This is nosplit to ensure gp's stack does not move.
+//
 //go:nowritebarrier
+//go:nosplit
 func gcRemoveStackBarrier(gp *g, stkbar stkbar) {
 	if debugStackBarrier {
 		print("remove stack barrier at ", hex(stkbar.savedLRPtr), " with ", hex(stkbar.savedLRVal), ", goid=", gp.goid, "\n")
@@ -568,15 +572,21 @@ func gcPrintStkbars(stkbar []stkbar) {
 	print("]")
 }
 
-// gcSkipBarriers marks all stack barriers up to sp as hit. This is
-// used during stack unwinding for panic/recover. This must run on the
-// system stack to ensure gp's stack does not get copied.
-func gcSkipBarriers(gp *g, sp uintptr) {
+// gcUnwindBarriers marks all stack barriers up the frame containing
+// sp as hit and removes them. This is used during stack unwinding for
+// panic/recover and by heapBitsBulkBarrier to force stack re-scanning
+// when its destination is on the stack.
+//
+// This is nosplit to ensure gp's stack does not move.
+//
+//go:nosplit
+func gcUnwindBarriers(gp *g, sp uintptr) {
 	// On LR machines, if there is a stack barrier on the return
 	// from the frame containing sp, this will mark it as hit even
 	// though it isn't, but it's okay to be conservative.
 	before := gp.stkbarPos
 	for int(gp.stkbarPos) < len(gp.stkbar) && gp.stkbar[gp.stkbarPos].savedLRPtr < sp {
+		gcRemoveStackBarrier(gp, gp.stkbar[gp.stkbarPos])
 		gp.stkbarPos++
 	}
 	if debugStackBarrier && gp.stkbarPos != before {

src/runtime/panic1.go

@@ -29,7 +29,7 @@ func recovery(gp *g) {
 	// Make the deferproc for this d return again,
 	// this time returning 1.  The calling function will
 	// jump to the standard return epilogue.
-	gcSkipBarriers(gp, sp)
+	gcUnwindBarriers(gp, sp)
 	gp.sched.sp = sp
 	gp.sched.pc = pc
 	gp.sched.lr = 0