add read loops to gifload

and check for error in ppnmload.

ChangeLog

@@ -8,7 +8,7 @@
 - hide info messages you could get with some older glibs [kleisauke]
 - fix --no-strip on dzsave with icc-profiles [altert]
 - better GraphicsMagick image write [bfriesen]
-- Add missing read loops to spng, heif and ppm load [kleisauke]
+- Add missing read loops to spng, heif, giflib and ppm load [kleisauke]
 
 6/9/20 started 8.10.2
 - update magicksave/load profile handling [kelilevi]

libvips/foreign/gifload.c

@@ -351,20 +351,34 @@ vips_foreign_load_gif_close_giflib( VipsForeignLoadGif *gif )
 
 /* Callback from the gif loader.
  *
- * Read up to len bytes into buffer, return number of bytes read, 0 for EOF.
+ * Read up to len bytes into buffer, return number of bytes read. This is
+ * called by giflib exactly as fread, so it does not distinguish between EOF
+ * and read error.
  */
 static int
 vips_giflib_read( GifFileType *file, GifByteType *buf, int n )
 {
 	VipsForeignLoadGif *gif = (VipsForeignLoadGif *) file->UserData;
 
-	gint64 read;
+	int to_read;
 
-	read = vips_source_read( gif->source, buf, n );
+	to_read = n;
-	if( read == 0 )
+	while( to_read > 0 ) {
+		gint64 bytes_read;
+
+		bytes_read = vips_source_read( gif->source, buf, n );
+		if( bytes_read == 0 )
 			gif->eof = TRUE;
+		if( bytes_read <= 0 )
+			return( -1 );
+		if( bytes_read > INT_MAX ) 
+			return( -1 );
 
-	return( (int) read );
+		to_read -= bytes_read;
+		buf += bytes_read;
+	}
+
+	return( (int) n );
 }
 
 /* Open any underlying file resource, then giflib.

libvips/foreign/ppmload.c

@@ -462,10 +462,12 @@ vips_foreign_load_ppm_generate_binary( VipsRegion *or,
 
 		n_bytes = sizeof_line;
 		while( n_bytes > 0 ) {
-			size_t bytes_read;
+			gint64 bytes_read;
 
 			bytes_read = vips_source_read( ppm->source, 
 				q, sizeof_line );
+			if( bytes_read < 0 ) 
+				return( -1 );
 			if( bytes_read == 0 ) {
 				vips_error( class->nickname, 
 					"%s", _( "file truncated" ) );