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, 1 insertions, 1 deletions

diff --git a/sys/ufs/ufs/ufs_extattr.c b/sys/ufs/ufs/ufs_extattr.c
index 48b272e..d52f650 100644
--- a/sys/ufs/ufs/ufs_extattr.c
+++ b/sys/ufs/ufs/ufs_extattr.c
@@ -334,7 +334,7 @@ ufs_extattr_enable_with_open(struct ufsmount *ump, struct vnode *vp,
 		return (error);
 	}
 
-	vp->v_writecount++;
+	VOP_ADD_WRITECOUNT(vp, 1);
 	CTR3(KTR_VFS, "%s: vp %p v_writecount increased to %d", __func__, vp,
 	    vp->v_writecount);