iobuf.c:186,191 - iobuf.c.orig:186,197
  
  	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",