nfsd: introduce export flag for v4 pseudoroot

NFSv4 differs from v2 and v3 in that it presents a single unified
filesystem tree, whereas v2 and v3 exported multiple filesystem (whose
roots could be found using a separate mount protocol).

Our original NFSv4 server implementation asked the administrator to
designate a single filesystem as the NFSv4 root, then to mount
filesystems they wished to export underneath.  (Often using bind mounts
of already-existing filesystems.)

This was conceptually simple, and allowed easy implementation, but
created a serious obstacle to upgrading between v2/v3: since the paths
to v4 filesystems were different, administrators would have to adjust
all the paths in client-side mount commands when switching to v4.

Various workarounds are possible.  For example, the administrator could
export "/" and designate it as the v4 root.  However, the security risks
of that approach are obvious, and in any case we shouldn't be requiring
the administrator to take extra steps to fix this problem; instead, the
server should present consistent paths across different versions by
default.

These patches take a modified version of that approach: we provide a new
export option which exports only a subset of a filesystem.  With this
flag, it becomes safe for mountd to export "/" by default, with no need
for additional configuration.

We begin just by defining the new flag.

Signed-off-by: Steve Dickson <steved@redhat.com>
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>

2 files changed, 11 insertions, 0 deletions

diff --git a/fs/nfsd/export.c b/fs/nfsd/export.c
index cb3dae2fcd86..c64d55f319bd 100644
--- a/fs/nfsd/export.c
+++ b/fs/nfsd/export.c
@@ -1425,6 +1425,7 @@ static struct flags {
         { NFSEXP_CROSSMOUNT, {"crossmnt", ""}},
         { NFSEXP_NOSUBTREECHECK, {"no_subtree_check", ""}},
         { NFSEXP_NOAUTHNLM, {"insecure_locks", ""}},
+        { NFSEXP_V4ROOT, {"v4root", ""}},
 #ifdef MSNFS
         { NFSEXP_MSNFS, {"msnfs", ""}},
 #endif
diff --git a/include/linux/nfsd/export.h b/include/linux/nfsd/export.h
index 4cafbe1255f0..41f0d4e25374 100644
--- a/include/linux/nfsd/export.h
+++ b/include/linux/nfsd/export.h
@@ -39,6 +39,16 @@
 #define NFSEXP_FSID             0x2000
 #define NFSEXP_CROSSMOUNT       0x4000
 #define NFSEXP_NOACL            0x8000  /* reserved for possible ACL related use */
+/*
+ * The NFSEXP_V4ROOT flag causes the kernel to give access only to NFSv4
+ * clients, and only to the single directory that is the root of the
+ * export; further lookup and readdir operations are treated as if every
+ * subdirectory was a mountpoint, and ignored if they are not themselves
+ * exported.  This is used by nfsd and mountd to construct the NFSv4
+ * pseudofilesystem, which provides access only to paths leading to each
+ * exported filesystem.
+ */
+#define NFSEXP_V4ROOT           0x10000
 /* All flags that we claim to support.  (Note we don't support NOACL.) */
 #define NFSEXP_ALLFLAGS         0x7E3F