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, 2 insertions, 2 deletions

diff --git a/sys/kern/vfs_vnops.c b/sys/kern/vfs_vnops.c
index 402a9d0..ea42f9d 100644
--- a/sys/kern/vfs_vnops.c
+++ b/sys/kern/vfs_vnops.c
@@ -302,7 +302,7 @@ vn_open_vnode(struct vnode *vp, int fmode, struct ucred *cred,
 		fp->f_flag |= FHASLOCK;
 	}
 	if (fmode & FWRITE) {
-		vp->v_writecount++;
+		VOP_ADD_WRITECOUNT(vp, 1);
 		CTR3(KTR_VFS, "%s: vp %p v_writecount increased to %d",
 		    __func__, vp, vp->v_writecount);
 	}
@@ -355,7 +355,7 @@ vn_close(vp, flags, file_cred, td)
 	if (flags & FWRITE) {
 		VNASSERT(vp->v_writecount > 0, vp, 
 		    ("vn_close: negative writecount"));
-		vp->v_writecount--;
+		VOP_ADD_WRITECOUNT(vp, -1);
 		CTR3(KTR_VFS, "%s: vp %p v_writecount decreased to %d",
 		    __func__, vp, vp->v_writecount);
 	}