Allow GIFs missing both Global and Local Colormaps to still decode (#494)

Ensure that a GIF without any Global or Local color tables is still
decoded by libgd.

GIF89a spec indicates conforming image files need not have
Global or Local color tables at all.

Spec recommends creating custom color map in that situation, and
that at least Black+White as first two entries, to ensure B&W images
are decoded.

Some commonly used single-pixel GIFs found around the web are
undecoded by libgd otherwise. Test case has been included.

References:
https://www.w3.org/Graphics/GIF/spec-gif89a.txt
http://probablyprogramming.com/2009/03/15/the-tiniest-gif-ever

src/gd_gif_in.c

@@ -315,8 +315,10 @@ BGD_DECLARE(gdImagePtr) gdImageCreateFromGifCtx(gdIOCtxPtr fd)
 			ReadImage(im, fd, width, height, localColorMap, BitSet(buf[8], INTERLACE), &ZeroDataBlock);
 		} else {
 			if(!haveGlobalColormap) {
-				gdImageDestroy(im);
-				return 0;
+				// Still a valid gif, apply simple default palette as per spec
+				ColorMap[CM_RED][1] = 0xff;
+				ColorMap[CM_GREEN][1] = 0xff;
+				ColorMap[CM_BLUE][1] = 0xff;
 			}
 
 			ReadImage(im, fd, width, height, ColorMap, BitSet(buf[8], INTERLACE), &ZeroDataBlock);

tests/gdimagecrop/CMakeLists.txt

@@ -7,4 +7,10 @@ LIST(APPEND TESTS_FILES
 	bug00486
 )
 
+IF(PNG_FOUND)
+LIST(APPEND TESTS_FILES
+	bug00486
+)
+ENDIF(PNG_FOUND)
+
 ADD_GD_TESTS()

tests/gif/.gitignore

@@ -8,6 +8,7 @@
 /bug00499
 /gif_im2im
 /gif_null
+/gif_nocolormaps
 /ossfuzz5700
 /php_bug_75571
 /uninitialized_memory_read

tests/gif/CMakeLists.txt

@@ -4,6 +4,7 @@ LIST(APPEND TESTS_FILES
 	bug00227
 	bug00499
 	gif_null
+	gif_nocolormaps
 	ossfuzz5700
 	php_bug_75571
 	uninitialized_memory_read

tests/gif/Makemodule.am

@@ -4,6 +4,7 @@ libgd_test_programs += \
 	gif/bug00227 \
 	gif/bug00499 \
 	gif/gif_null \
+	gif/gif_nocolormaps \
 	gif/ossfuzz5700 \
 	gif/php_bug_75571 \
 	gif/uninitialized_memory_read \
@@ -27,6 +28,7 @@ EXTRA_DIST += \
 	gif/bug00060.gif \
 	gif/bug00066.gif \
 	gif/bug00066_exp.png \
+	gif/gif_nocolormaps.gif \
 	gif/ossfuzz5700.gif \
 	gif/php_bug_75571.gif \
 	gif/unitialized_memory_read.gif

tests/gif/gif_nocolormaps.c

@@ -0,0 +1,25 @@
+/**
+ * Ensure that a GIF without any Global or Local color tables is still decoded
+ *
+ * GIF89a spec indicates conforming image files need not have Global or Local color tables at all
+ *
+ * See also:	https://www.w3.org/Graphics/GIF/spec-gif89a.txt
+ *		http://probablyprogramming.com/2009/03/15/the-tiniest-gif-ever
+ */
+
+#include "gd.h"
+#include "gdtest.h"
+
+int main()
+{
+	gdImagePtr im;
+	FILE *fp;
+
+	fp = gdTestFileOpen2("gif", "gif_nocolormaps.gif");
+	gdTestAssert(fp != NULL);
+	im = gdImageCreateFromGif(fp);
+	gdTestAssert(im != NULL);
+	fclose(fp);
+
+	return gdNumFailures();
+}