Make the default implementation of the VOP_VPTOCNP() fail if the

directory entry, matched by the inode number, is ".".

NFSv4 client might instantiate the distinct vnodes which have the same
inode number, since single v4 export can be combined from several
filesystems on the server.  For instance, a case when the nested
server mount point is exactly one directory below the top of the
export, causes directory and its parent to have the same inode number
2.  The vop_stdvptocnp() algorithm then returns "." as the name of the
lower directory.

Filtering out the "." entry with ENOENT works around this behaviour,
the error forces getcwd(3) to fall back to usermode implementation,
which compares both st_dev and st_ino.

Based on the submission by:	rmacklem
Tested by:	rmacklem
MFC after:	1 week

1 files changed, 6 insertions, 2 deletions

diff --git a/sys/kern/vfs_default.c b/sys/kern/vfs_default.c
index 00d064e..1dd0185 100644
--- a/sys/kern/vfs_default.c
+++ b/sys/kern/vfs_default.c
@@ -856,8 +856,12 @@ vop_stdvptocnp(struct vop_vptocnp_args *ap)
 				error = ENOMEM;
 				goto out;
 			}
-			bcopy(dp->d_name, buf + i, dp->d_namlen);
-			error = 0;
+			if (dp->d_namlen == 1 && dp->d_name[0] == '.') {
+				error = ENOENT;
+			} else {
+				bcopy(dp->d_name, buf + i, dp->d_namlen);
+				error = 0;
+			}
 			goto out;
 		}
 	} while (len > 0 || !eofflag);