Allow automount of snapdir over NFS

This commit allows the NFS server to automount ZFS snapdirs.

fs/nfsd/vfs.c

@@ -112,6 +112,18 @@ nfserrno (int errno)
 	return nfserr_io;
 }
 
+static int
+is_zfs_snapdir(struct dentry *dentry)
+{
+#define	ZFSCTL_INO_SNAPDIR 0x0000FFFFFFFFFFFDULL
+
+	struct inode *inode = d_inode(dentry);
+	if (inode == NULL)
+		return 0;
+
+	return (inode->i_ino == ZFSCTL_INO_SNAPDIR);
+}
+
 /* 
  * Called from nfsd_lookup and encode_dirent. Check if we have crossed 
  * a mount point.
@@ -127,11 +139,17 @@ nfsd_cross_mnt(struct svc_rqst *rqstp, struct dentry **dpp,
 	struct path path = {.mnt = mntget(exp->ex_path.mnt),
 			    .dentry = dget(dentry)};
 	unsigned int follow_flags = 0;
+	int is_snapdir = 0;
 	int err = 0;
 
 	if (exp->ex_flags & NFSEXP_CROSSMOUNT)
 		follow_flags = LOOKUP_AUTOMOUNT;
 
+	if ((exp->ex_flags & NFSEXP_SNAPDIR) && is_zfs_snapdir(dentry)) {
+		follow_flags = LOOKUP_AUTOMOUNT;
+		is_snapdir = 1;
+	}
+
 	err = follow_down(&path, follow_flags);
 	if (err < 0)
 		goto out;
@@ -157,7 +175,8 @@ nfsd_cross_mnt(struct svc_rqst *rqstp, struct dentry **dpp,
 		path_put(&path);
 		goto out;
 	}
-	if (nfsd_v4client(rqstp) ||
+
+	if (nfsd_v4client(rqstp) || is_snapdir ||
 		(exp->ex_flags & NFSEXP_CROSSMOUNT) || EX_NOHIDE(exp2)) {
 		/* successfully crossed mount point */
 		/*
@@ -171,6 +190,7 @@ nfsd_cross_mnt(struct svc_rqst *rqstp, struct dentry **dpp,
 		*expp = exp2;
 		exp2 = exp;
 	}
+
 	path_put(&path);
 	exp_put(exp2);
 out:

include/uapi/linux/nfsd/export.h

@@ -53,9 +53,10 @@
  */
 #define	NFSEXP_V4ROOT		0x10000
 #define NFSEXP_PNFS		0x20000
+#define NFSEXP_SNAPDIR		0x40000 /* Expose the ZFS snapshot directory over NFS */
 
 /* All flags that we claim to support.  (Note we don't support NOACL.) */
-#define NFSEXP_ALLFLAGS		0x3FEFF
+#define NFSEXP_ALLFLAGS		0x7FEFF
 
 /* The flags that may vary depending on security flavor: */
 #define NFSEXP_SECINFO_FLAGS	(NFSEXP_READONLY | NFSEXP_ROOTSQUASH \