nfsd: -EINVAL on invalid anonuid/gid instead of silent failure

If we're going to refuse to accept these it would be polite of us to at
least say so....

This introduces a slight complication since we need to grandfather in
exportfs's ill-advised use of -1 uid and gid on its test_export.

If it turns out there are other users passing down -1 we may need to
do something else.

Best might be to drop the checks entirely, but I'm not sure if other
parts of the kernel might assume that a task can't run as uid or gid -1.

Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

fs/nfsd/export.c

@@ -579,17 +579,26 @@ static int svc_export_parse(struct cache_detail *cd, char *mesg, int mlen)
 				   exp.ex_uuid);
 		if (err)
 			goto out4;
+		/*
+		 * No point caching this if it would immediately expire.
+		 * Also, this protects exportfs's dummy export from the
+		 * anon_uid/anon_gid checks:
+		 */
+		if (exp.h.expiry_time < seconds_since_boot())
+			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:
 		 */
+		err = -EINVAL;
 		if (!uid_valid(exp.ex_anon_uid))
 			goto out4;
 		if (!gid_valid(exp.ex_anon_gid))
 			goto out4;
+		err = 0;
 	}
 
 	expp = svc_export_lookup(&exp);