--- kernel/bsd/vfs/vfs_vnops.c	2018/04/24 18:26:06	1.1
+++ kernel/bsd/vfs/vfs_vnops.c	2018/04/24 18:31:25	1.1.1.2
@@ -148,11 +148,21 @@ vn_open(ndp, fmode, cmode)
 		goto bad;
 	}
 	if ((fmode & O_CREAT) == 0) {
-		if (fmode & FREAD) {
-			if (error = VOP_ACCESS(vp, VREAD, cred, p))
+		if (fmode & FREAD && fmode & (FWRITE | O_TRUNC)) {
+			int err = 0;
+			if (vp->v_type == VDIR)
+				err = EISDIR;
+			else
+				err = vn_writechk(vp);
+			if (err && !(error = VOP_ACCESS(vp, VREAD, cred, p)))
+				error = err;
+			if (error || (error = VOP_ACCESS(vp, VREAD|VWRITE,
+							 cred, p)))
 				goto bad;
-		}
-		if (fmode & (FWRITE | O_TRUNC)) {
+		} else if (fmode & FREAD) {
+			if ((error = VOP_ACCESS(vp, VREAD, cred, p)))
+				goto bad;
+		} else if (fmode & (FWRITE | O_TRUNC)) {
 			if (vp->v_type == VDIR) {
 				error = EISDIR;
 				goto bad;
@@ -191,7 +201,8 @@ vn_open(ndp, fmode, cmode)
 #endif /* MACH_NBC */
 
 	if (fmode & FWRITE)
-		vp->v_writecount++;
+		if (++vp->v_writecount <= 0)
+			panic("vn_open: v_writecount");
 	return (0);
 bad:
 	vput(vp);