Fonts: Better assert during load when passing truncated font data or wrong data size. (#6822)

docs/CHANGELOG.txt

@@ -71,6 +71,7 @@ Other changes:
 - Fonts: 'float size_pixels' passed to AddFontXXX() functions is now rounded to lowest integer.
   This is because our layout/font system currently doesn't fully support non-integer sizes. Until
   it does, this has been a common pitfall leading to more or less subtle issues. (#3164, #3309, #6800)
+- Fonts: Better assert during load when passing truncated font data or wrong data size. (#6822)
 - InputTextMultiline: Fixed a crash pressing Down on last empty line of a multiline buffer.
   (regression from 1.89.2, only happened in some states). (#6783, #6000)
 - InputTextMultiline: Fixed Tabbing cycle leading to a situation where Enter key wouldn't

imgui.h

@@ -2877,8 +2877,8 @@ struct ImFontAtlas
     IMGUI_API ImFont*           AddFont(const ImFontConfig* font_cfg);
     IMGUI_API ImFont*           AddFontDefault(const ImFontConfig* font_cfg = NULL);
     IMGUI_API ImFont*           AddFontFromFileTTF(const char* filename, float size_pixels, const ImFontConfig* font_cfg = NULL, const ImWchar* glyph_ranges = NULL);
-    IMGUI_API ImFont*           AddFontFromMemoryTTF(void* font_data, int font_size, float size_pixels, const ImFontConfig* font_cfg = NULL, const ImWchar* glyph_ranges = NULL); // Note: Transfer ownership of 'ttf_data' to ImFontAtlas! Will be deleted after destruction of the atlas. Set font_cfg->FontDataOwnedByAtlas=false to keep ownership of your data and it won't be freed.
-    IMGUI_API ImFont*           AddFontFromMemoryCompressedTTF(const void* compressed_font_data, int compressed_font_size, float size_pixels, const ImFontConfig* font_cfg = NULL, const ImWchar* glyph_ranges = NULL); // 'compressed_font_data' still owned by caller. Compress with binary_to_compressed_c.cpp.
+    IMGUI_API ImFont*           AddFontFromMemoryTTF(void* font_data, int font_data_size, float size_pixels, const ImFontConfig* font_cfg = NULL, const ImWchar* glyph_ranges = NULL); // Note: Transfer ownership of 'ttf_data' to ImFontAtlas! Will be deleted after destruction of the atlas. Set font_cfg->FontDataOwnedByAtlas=false to keep ownership of your data and it won't be freed.
+    IMGUI_API ImFont*           AddFontFromMemoryCompressedTTF(const void* compressed_font_data, int compressed_font_data_size, float size_pixels, const ImFontConfig* font_cfg = NULL, const ImWchar* glyph_ranges = NULL); // 'compressed_font_data' still owned by caller. Compress with binary_to_compressed_c.cpp.
     IMGUI_API ImFont*           AddFontFromMemoryCompressedBase85TTF(const char* compressed_font_data_base85, float size_pixels, const ImFontConfig* font_cfg = NULL, const ImWchar* glyph_ranges = NULL);              // 'compressed_font_data_base85' still owned by caller. Compress with binary_to_compressed_c.cpp with -base85 parameter.
     IMGUI_API void              ClearInputData();           // Clear input data (all ImFontConfig structures including sizes, TTF data, glyph ranges, etc.) = all the data used to build the texture and fonts.
     IMGUI_API void              ClearTexData();             // Clear output texture data (CPU side). Saves RAM once the texture has been copied to graphics memory.

imgui_draw.cpp

@@ -2249,13 +2249,14 @@ ImFont* ImFontAtlas::AddFontFromFileTTF(const char* filename, float size_pixels,
 }
 
 // NB: Transfer ownership of 'ttf_data' to ImFontAtlas, unless font_cfg_template->FontDataOwnedByAtlas == false. Owned TTF buffer will be deleted after Build().
-ImFont* ImFontAtlas::AddFontFromMemoryTTF(void* ttf_data, int ttf_size, float size_pixels, const ImFontConfig* font_cfg_template, const ImWchar* glyph_ranges)
+ImFont* ImFontAtlas::AddFontFromMemoryTTF(void* font_data, int font_data_size, float size_pixels, const ImFontConfig* font_cfg_template, const ImWchar* glyph_ranges)
 {
     IM_ASSERT(!Locked && "Cannot modify a locked ImFontAtlas between NewFrame() and EndFrame/Render()!");
     ImFontConfig font_cfg = font_cfg_template ? *font_cfg_template : ImFontConfig();
     IM_ASSERT(font_cfg.FontData == NULL);
-    font_cfg.FontData = ttf_data;
-    font_cfg.FontDataSize = ttf_size;
+    IM_ASSERT(font_data_size > 100 && "Incorrect value for font_data_size!"); // Heuristic to prevent accidentally passing a wrong value to font_data_size.
+    font_cfg.FontData = font_data;
+    font_cfg.FontDataSize = font_data_size;
     font_cfg.SizePixels = size_pixels > 0.0f ? size_pixels : font_cfg.SizePixels;
     if (glyph_ranges)
         font_cfg.GlyphRanges = glyph_ranges;
@@ -2470,7 +2471,10 @@ static bool ImFontAtlasBuildWithStbTruetype(ImFontAtlas* atlas)
         const int font_offset = stbtt_GetFontOffsetForIndex((unsigned char*)cfg.FontData, cfg.FontNo);
         IM_ASSERT(font_offset >= 0 && "FontData is incorrect, or FontNo cannot be found.");
         if (!stbtt_InitFont(&src_tmp.FontInfo, (unsigned char*)cfg.FontData, font_offset))
+        {
+            IM_ASSERT(0 && "stbtt_InitFont(): failed to parse FontData. It is correct and complete? Check FontDataSize.");
             return false;
+        }
 
         // Measure highest codepoints
         ImFontBuildDstData& dst_tmp = dst_tmp_array[src_tmp.DstIndex];