nfsd: return better errors to exportfs

Someone noticed exportfs happily accepted exports that would later be
rejected when mountd tried to give them to the kernel.  Fix this.

This is a regression from 4c1e1b34
"nfsd: Store ex_anon_uid and ex_anon_gid as kuids and kgids".

Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: stable@vger.kernel.org
Reported-by: Yin.JianHong <jiyin@redhat.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

fs/nfsd/export.c

@@ -536,16 +536,12 @@ static int svc_export_parse(struct cache_detail *cd, char *mesg, int mlen)
 		if (err)
 			goto out3;
 		exp.ex_anon_uid= make_kuid(&init_user_ns, an_int);
-		if (!uid_valid(exp.ex_anon_uid))
-			goto out3;
 
 		/* anon gid */
 		err = get_int(&mesg, &an_int);
 		if (err)
 			goto out3;
 		exp.ex_anon_gid= make_kgid(&init_user_ns, an_int);
-		if (!gid_valid(exp.ex_anon_gid))
-			goto out3;
 
 		/* fsid */
 		err = get_int(&mesg, &an_int);
@@ -583,6 +579,17 @@ static int svc_export_parse(struct cache_detail *cd, char *mesg, int mlen)
 				   exp.ex_uuid);
 		if (err)
 			goto out4;
+		/*
+		 * For some reason exportfs has been passing down an
+		 * invalid (-1) uid & gid on the "dummy" export which it
+		 * uses to test export support.  To make sure exportfs
+		 * sees errors from check_export we therefore need to
+		 * delay these checks till after check_export:
+		 */
+		if (!uid_valid(exp.ex_anon_uid))
+			goto out4;
+		if (!gid_valid(exp.ex_anon_gid))
+			goto out4;
 	}
 
 	expp = svc_export_lookup(&exp);