The r241025 fixed the case when a binary, executed from nullfs mount,

was still possible to open for write from the lower filesystem.  There
is a symmetric situation where the binary could already has file
descriptors opened for write, but it can be executed from the nullfs
overlay.

Handle the issue by passing one v_writecount reference to the lower
vnode if nullfs vnode has non-zero v_writecount.  Note that only one
write reference can be donated, since nullfs only keeps one use
reference on the lower vnode.  Always use the lower vnode v_writecount
for the checks.

Introduce the VOP_GET_WRITECOUNT to read v_writecount, which is
currently always bypassed to the lower vnode, and VOP_ADD_WRITECOUNT
to manipulate the v_writecount value, which manages a single bypass
reference to the lower vnode.  Caling the VOPs instead of directly
accessing v_writecount provide the fix described in the previous
paragraph.

Tested by:	pho
MFC after:	3 weeks

1 files changed, 21 insertions, 0 deletions

diff --git a/sys/fs/nullfs/null_vnops.c b/sys/fs/nullfs/null_vnops.c
index 94bfea8..f530ed2 100644
--- a/sys/fs/nullfs/null_vnops.c
+++ b/sys/fs/nullfs/null_vnops.c
@@ -329,6 +329,26 @@ null_bypass(struct vop_generic_args *ap)
 	return (error);
 }
 
+static int
+null_add_writecount(struct vop_add_writecount_args *ap)
+{
+	struct vnode *lvp, *vp;
+	int error;
+
+	vp = ap->a_vp;
+	lvp = NULLVPTOLOWERVP(vp);
+	KASSERT(vp->v_writecount + ap->a_inc >= 0, ("wrong writecount inc"));
+	if (vp->v_writecount > 0 && vp->v_writecount + ap->a_inc == 0)
+		error = VOP_ADD_WRITECOUNT(lvp, -1);
+	else if (vp->v_writecount == 0 && vp->v_writecount + ap->a_inc > 0)
+		error = VOP_ADD_WRITECOUNT(lvp, 1);
+	else
+		error = 0;
+	if (error == 0)
+		vp->v_writecount += ap->a_inc;
+	return (error);
+}
+
 /*
  * We have to carry on the locking protocol on the null layer vnodes
  * as we progress through the tree. We also have to enforce read-only
@@ -826,4 +846,5 @@ struct vop_vector null_vnodeops = {
 	.vop_unlock =		null_unlock,
 	.vop_vptocnp =		null_vptocnp,
 	.vop_vptofh =		null_vptofh,
+	.vop_add_writecount =	null_add_writecount,
 };