runtime: scan write barrier buffer conservatively

    
    In gccgo, we insert the write barriers in the frontend, and so we
    cannot completely prevent write barriers on stack writes. So it
    is possible for a bad pointer appearing in the write barrier
    buffer. When flushing the write barrier, treat it the same as
    sacnning the stack. In particular, don't mark a pointer if it
    does not point to an allocated object. We already have similar
    logic in greyobject. With this, hopefully, we can prevent an
    unallocated object from being marked completely.
    
    Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/190599

From-SVN: r274598

1 files changed, 8 insertions, 1 deletions

diff --git a/libgo/go/runtime/mwbbuf.go b/libgo/go/runtime/mwbbuf.go
index 4c875ff..a27406e 100644
--- a/libgo/go/runtime/mwbbuf.go
+++ b/libgo/go/runtime/mwbbuf.go
@@ -285,10 +285,17 @@ func wbBufFlush1(_p_ *p) {
 			// path to reduce the rate of flushes?
 			continue
 		}
-		obj, span, objIndex := findObject(ptr, 0, 0, false)
+		obj, span, objIndex := findObject(ptr, 0, 0, !usestackmaps)
 		if obj == 0 {
 			continue
 		}
+		if span.isFree(objIndex) {
+			// For gccgo, it is possible that we have a write barrier
+			// writing to unintialized stack memory. So we could see
+			// a bad pointer in the write barrier buffer. Don't mark
+			// it in this case.
+			continue
+		}
 		// TODO: Consider making two passes where the first
 		// just prefetches the mark bits.
 		mbits := span.markBitsForIndex(objIndex)