g_vfs_open: allow only one mount per device vnode

In other words, deny multiple read-only mounts of the same device.
Shared read-only mounts should theoretically be possible, but,
unfortunately, can not be implemented correctly using current
buffer cache code/interface and results in an eventual system crash.
Also, using nullfs seems to be a more efficient way to achieve the same
goal.

This gets us back to where we were before GEOM and where other BSDs are.

Submitted by:	pjd (idea for checking for shared mounting)
Discussed with:	phk, pjd
Silence from:	fs@, geom@
MFC after:	2 weeks

1 files changed, 6 insertions, 1 deletions

diff --git a/sys/geom/geom_vfs.c b/sys/geom/geom_vfs.c
index 9d01f40..e50e17b 100644
--- a/sys/geom/geom_vfs.c
+++ b/sys/geom/geom_vfs.c
@@ -161,6 +161,10 @@ g_vfs_open(struct vnode *vp, struct g_consumer **cpp, const char *fsname, int wr
 	g_topology_assert();
 
 	*cpp = NULL;
+	bo = &vp->v_bufobj;
+	if (bo->bo_private != vp)
+		return (EBUSY);
+
 	pp = g_dev_getprovider(vp->v_rdev);
 	if (pp == NULL)
 		return (ENOENT);
@@ -176,7 +180,7 @@ g_vfs_open(struct vnode *vp, struct g_consumer **cpp, const char *fsname, int wr
 	vnode_create_vobject(vp, pp->mediasize, curthread);
 	VFS_UNLOCK_GIANT(vfslocked);
 	*cpp = cp;
-	bo = &vp->v_bufobj;
+	cp->private = vp;
 	bo->bo_ops = g_vfs_bufops;
 	bo->bo_private = cp;
 	bo->bo_bsize = pp->sectorsize;
@@ -196,5 +200,6 @@ g_vfs_close(struct g_consumer *cp)
 	gp = cp->geom;
 	bo = gp->softc;
 	bufobj_invalbuf(bo, V_SAVE, 0, 0);
+	bo->bo_private = cp->private;
 	g_wither_geom_close(gp, ENXIO);
 }