diff -r eb26bc92b777 sys/src/cmd/cwfs/iobuf.c
--- a/sys/src/cmd/cwfs/iobuf.c	Sun Jan 13 17:17:11 2019 +0100
+++ b/sys/src/cmd/cwfs/iobuf.c	Sun Jan 13 21:40:51 2019 +0100
@@ -186,6 +186,12 @@
 
 	if(canqlock(p))
 		fprint(2, "buffer not locked %Z(%lld)\n", p->dev, (Wideoff)p->addr);
+
+	if((p->flags & Bmod) != 0 && p->dev->type == Devro){
+		fprint(2, "attempt to modify %Z(%lld)\n", p->dev, (Wideoff)p->addr);
+		abort();
+	}
+
 	if(p->flags & Bimm) {
 		if(!(p->flags & Bmod))
 			fprint(2, "imm and no mod %Z(%lld)\n",