JpegMetadataExtractor: Allow empty segments

A segment which consists only of the marker and length value
(equal to 2, the length of the value itself) appears to be
perfectly valid, and is ignored by every image viewer I tested.

Bug: T147015
Change-Id: I3124c587ccb1c457df25fd5bf7a47feab9312a38

diff --git a/includes/media/JpegMetadataExtractor.php b/includes/media/JpegMetadataExtractor.php
index 81722c6..9ad4097 100644 (file)
--- a/includes/media/JpegMetadataExtractor.php
+++ b/includes/media/JpegMetadataExtractor.php
@@ -155,7 +155,7 @@ class JpegMetadataExtractor {
                        } else {
                                // segment we don't care about, so skip
                                $size = wfUnpack( "nint", fread( $fh, 2 ), 2 );
-                               if ( $size['int'] <= 2 ) {
+                               if ( $size['int'] < 2 ) {
                                        throw new MWException( "invalid marker size in jpeg" );
                                }
                                fseek( $fh, $size['int'] - 2, SEEK_CUR );
@@ -173,9 +173,13 @@ class JpegMetadataExtractor {
         */
        private static function jpegExtractMarker( &$fh ) {
                $size = wfUnpack( "nint", fread( $fh, 2 ), 2 );
-               if ( $size['int'] <= 2 ) {
+               if ( $size['int'] < 2 ) {
                        throw new MWException( "invalid marker size in jpeg" );
                }
+               if ( $size['int'] === 2 ) {
+                       // fread( ..., 0 ) generates a warning
+                       return '';
+               }
                $segment = fread( $fh, $size['int'] - 2 );
                if ( strlen( $segment ) !== $size['int'] - 2 ) {
                        throw new MWException( "Segment shorter than expected" );