Better x and y validation, report 400 on errors

Report 400 Bad Request on query errors instead of reporting 200 OK
(which can be cached).

Add some additional validation for the 'x' and 'y' parameters, to catch
underflow (test with `x=-10` for example). Also fix a typo in the error
message and include more details (the actual error).

2 files changed, 16 insertions, 8 deletions

diff --git a/graph.php b/graph.php
index 3e33b92..2d62d92 100644
--- a/graph.php
+++ b/graph.php
@@ -6,16 +6,24 @@ require_once 'inc/collectd.inc.php';
 
 $plugin = validate_get(GET('p'), 'plugin');
 $type = validate_get(GET('t'), 'type');
-$width = empty($_GET['x']) ? $CONFIG['width'] : $_GET['x'];
-$height = empty($_GET['y']) ? $CONFIG['height'] : $_GET['y'];
-
-if (validate_get(GET('h'), 'host') === NULL) {
-        error_log('CGP Error: plugin contains unknown characters');
+$width = GET('x') ? filter_input(INPUT_GET, 'x', FILTER_VALIDATE_INT, array(
+        'min_range' => 10,
+        'max_range' => $CONFIG['max-width']
+)) : $CONFIG['width'];
+$height = GET('y') ? filter_input(INPUT_GET, 'y', FILTER_VALIDATE_INT, array(
+        'min_range' => 10,
+        'max_range' => $CONFIG['max-height']
+)) : $CONFIG['height'];
+
+if ($width === NULL || $height === NULL) {
+        error_log(sprintf('Invalid image dimension, x="%s", y="%s"',
+                urlencode(GET('x')),
+                urlencode(GET('y'))));
         error_image();
 }
 
-if ($width > $CONFIG['max-width'] || $height > $CONFIG['max-height']) {
-        error_log('Resquested image is too large. Please configure max-width and max-height.');
+if (validate_get(GET('h'), 'host') === NULL) {
+        error_log('Invalid host: "' . urlencode(GET('h')) . '"');
         error_image();
 }
 
diff --git a/inc/functions.inc.php b/inc/functions.inc.php
index c32b3ef..3664d78 100644
--- a/inc/functions.inc.php
+++ b/inc/functions.inc.php
@@ -55,7 +55,7 @@ function crc32hex($str) {
 }
 
 function error_image() {
-        header("Content-Type: image/png");
+        header("Content-Type: image/png", true, 400);
         readfile('layout/error.png');
         exit;
 }