From c97fa9a3a61d94d95a81e94a3781768df5dad92a Mon Sep 17 00:00:00 2001
From: Francesco Abbate <francesco.bbt@gmail.com>
Date: Tue, 20 Apr 2021 21:54:05 +0200
Subject: [PATCH] Fix bug with wrong hashing of rencache commands

The command in rencache.c (Command struct + variable length text) was
incorrectly sized. As a result some bites at the end of the command payload
were uninitialized and was causing hash to randomly change forcing the
system to redraw many more areas.

Since the text of the command for DRAW_TEXT commands is written beginning
at the offset of the text field we compute the bare size of the command as
offsetof(Command, text) instead of sizeof(Command).
---
 src/rencache.c | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/src/rencache.c b/src/rencache.c
index e9796f81..3d607094 100644
--- a/src/rencache.c
+++ b/src/rencache.c
@@ -1,3 +1,4 @@
+#include <stddef.h>
 #include <stdint.h>
 #include <stdio.h>
 #include "rencache.h"
@@ -11,6 +12,7 @@
 #define CELLS_Y 50
 #define CELL_SIZE 96
 #define COMMAND_BUF_SIZE (1024 * 512)
+#define COMMAND_BARE_SIZE offsetof(Command, text)
 
 enum { FREE_FONT, SET_CLIP, DRAW_TEXT, DRAW_RECT, DRAW_TEXT_SUBPIXEL };
 
@@ -91,7 +93,7 @@ static Command* push_command(int type, int size) {
     return NULL;
   }
   command_buf_idx = n;
-  memset(cmd, 0, sizeof(Command));
+  memset(cmd, 0, COMMAND_BARE_SIZE);
   cmd->type = type;
   cmd->size = size;
   return cmd;
@@ -114,20 +116,20 @@ void rencache_show_debug(bool enable) {
 
 
 void rencache_free_font(RenFont *font) {
-  Command *cmd = push_command(FREE_FONT, sizeof(Command));
+  Command *cmd = push_command(FREE_FONT, COMMAND_BARE_SIZE);
   if (cmd) { cmd->font = font; }
 }
 
 
 void rencache_set_clip_rect(RenRect rect) {
-  Command *cmd = push_command(SET_CLIP, sizeof(Command));
+  Command *cmd = push_command(SET_CLIP, COMMAND_BARE_SIZE);
   if (cmd) { cmd->rect = intersect_rects(rect, screen_rect); }
 }
 
 
 void rencache_draw_rect(RenRect rect, RenColor color) {
   if (!rects_overlap(screen_rect, rect)) { return; }
-  Command *cmd = push_command(DRAW_RECT, sizeof(Command));
+  Command *cmd = push_command(DRAW_RECT, COMMAND_BARE_SIZE);
   if (cmd) {
     cmd->rect = rect;
     cmd->color = color;
@@ -148,7 +150,7 @@ int rencache_draw_text(RenFont *font,
 
   if (rects_overlap(screen_rect, rect)) {
     int sz = strlen(text) + 1;
-    Command *cmd = push_command(draw_subpixel ? DRAW_TEXT_SUBPIXEL : DRAW_TEXT, sizeof(Command) + sz);
+    Command *cmd = push_command(draw_subpixel ? DRAW_TEXT_SUBPIXEL : DRAW_TEXT, COMMAND_BARE_SIZE + sz);
     if (cmd) {
       memcpy(cmd->text, text, sz);
       cmd->color = color;