diff --git a/src/runtime/malloc.go b/src/runtime/malloc.go
index 2da694d14a..0fbf45f897 100644
--- a/src/runtime/malloc.go
+++ b/src/runtime/malloc.go
@@ -604,7 +604,7 @@ func mallocinit() {
 			a, size := sysReserveAligned(unsafe.Pointer(p), arenaSize, heapArenaBytes)
 			if a != nil {
 				mheap_.arena.init(uintptr(a), size)
-				p = uintptr(a) + size // For hint below
+				p = mheap_.arena.end // For hint below
 				break
 			}
 		}
@@ -1423,6 +1423,13 @@ type linearAlloc struct {
 }
 
 func (l *linearAlloc) init(base, size uintptr) {
+	if base+size < base {
+		// Chop off the last byte. The runtime isn't prepared
+		// to deal with situations where the bounds could overflow.
+		// Leave that memory reserved, though, so we don't map it
+		// later.
+		size -= 1
+	}
 	l.next, l.mapped = base, base
 	l.end = base + size
 }