[client] font: skip stb-incompatible UI font faces

client/CMakeLists.txt

@@ -166,6 +166,7 @@ set(SOURCES
   src/message.c
   src/config.c
   src/keybind.c
+  src/font.c
   src/util.c
   src/clipboard.c
   src/kb.c

client/include/font.h

@@ -0,0 +1,30 @@
+/**
+ * Looking Glass
+ * Copyright © 2017-2026 The Looking Glass Authors
+ * https://looking-glass.io
+ *
+ * This program is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License as published by the Free
+ * Software Foundation; either version 2 of the License, or (at your option)
+ * any later version.
+ *
+ * This program is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for
+ * more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc., 59
+ * Temple Place, Suite 330, Boston, MA 02111-1307 USA
+ */
+
+#ifndef _H_LG_FONT_
+#define _H_LG_FONT_
+
+#include <stdbool.h>
+#include <stddef.h>
+
+bool font_isStbTruetypeCompatible(
+    const void * data, size_t size, int fontIndex);
+
+#endif

client/src/font.c

@@ -0,0 +1,119 @@
+/**
+ * Looking Glass
+ * Copyright © 2017-2026 The Looking Glass Authors
+ * https://looking-glass.io
+ *
+ * This program is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License as published by the Free
+ * Software Foundation; either version 2 of the License, or (at your option)
+ * any later version.
+ *
+ * This program is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for
+ * more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc., 59
+ * Temple Place, Suite 330, Boston, MA 02111-1307 USA
+ */
+
+#include "font.h"
+
+#include <limits.h>
+#include <stdint.h>
+
+#if defined(__GNUC__)
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wunused-function"
+#endif
+#define STBTT_STATIC
+#define STB_TRUETYPE_IMPLEMENTATION
+#include "repos/cimgui/imgui/imstb_truetype.h"
+#if defined(__GNUC__)
+#pragma GCC diagnostic pop
+#endif
+
+static uint16_t font_readU16(const unsigned char * data)
+{
+  return (uint16_t)(((uint16_t)data[0] << 8) | data[1]);
+}
+
+static uint32_t font_readU32(const unsigned char * data)
+{
+  return
+    ((uint32_t)data[0] << 24) |
+    ((uint32_t)data[1] << 16) |
+    ((uint32_t)data[2] <<  8) |
+     (uint32_t)data[3];
+}
+
+static bool font_getSfntOffset(const unsigned char * data, size_t size,
+    int fontIndex, size_t * offset)
+{
+  if (!data || size < 12 || size > INT_MAX || fontIndex < 0)
+    return false;
+
+  size_t fontOffset = 0;
+  const uint32_t signature = font_readU32(data);
+  switch (signature)
+  {
+    case 0x00010000: // TrueType 1.0
+    case 0x74727565: // true
+    case 0x74797031: // typ1
+    case 0x4F54544F: // OTTO
+      if (fontIndex != 0)
+        return false;
+      break;
+
+    case 0x74746366: // ttcf
+    {
+      const uint32_t version = font_readU32(data + 4);
+      const uint32_t fontCount = font_readU32(data + 8);
+      if ((version != 0x00010000 && version != 0x00020000) ||
+          (uint32_t)fontIndex >= fontCount ||
+          fontCount > (size - 12) / 4)
+        return false;
+
+      fontOffset = font_readU32(data + 12 + (size_t)fontIndex * 4);
+      if (fontOffset > size - 12 || fontOffset > INT_MAX)
+        return false;
+      break;
+    }
+
+    default:
+      return false;
+  }
+
+  const uint16_t tableCount = font_readU16(data + fontOffset + 4);
+  if (tableCount > (size - fontOffset - 12) / 16)
+    return false;
+
+  const unsigned char * table = data + fontOffset + 12;
+  for (uint16_t i = 0; i < tableCount; ++i, table += 16)
+  {
+    const size_t tableOffset = font_readU32(table + 8);
+    const size_t tableSize   = font_readU32(table + 12);
+    if (tableOffset > size || tableSize > size - tableOffset)
+      return false;
+  }
+
+  *offset = fontOffset;
+  return true;
+}
+
+bool font_isStbTruetypeCompatible(
+    const void * data, size_t size, int fontIndex)
+{
+  size_t checkedOffset;
+  if (!font_getSfntOffset(data, size, fontIndex, &checkedOffset))
+    return false;
+
+  const int stbOffset =
+    stbtt_GetFontOffsetForIndex((const unsigned char *)data, fontIndex);
+  if (stbOffset < 0 || (size_t)stbOffset != checkedOffset)
+    return false;
+
+  stbtt_fontinfo info;
+  return stbtt_InitFont(&info, data, stbOffset);
+}

client/src/util.c

@@ -20,6 +20,7 @@
 
 #include "util.h"
 #include "main.h"
+#include "font.h"
 
 #include "common/debug.h"
 #include "common/stringutils.h"
@@ -105,6 +106,7 @@ static bool uiFontAddFace(const FcPattern * pattern, FcCharSet ** remainingPtr)
   FcChar8 * file;
   FcCharSet * charset;
   FcBool outline = FcTrue;
+  int index = 0;
 
   if (FcPatternGetString(pattern, FC_FILE, 0, &file) != FcResultMatch ||
       FcPatternGetCharSet(pattern, FC_CHARSET, 0, &charset) != FcResultMatch)
@@ -114,6 +116,9 @@ static bool uiFontAddFace(const FcPattern * pattern, FcCharSet ** remainingPtr)
       !outline)
     return true;
 
+  if (FcPatternGetInteger(pattern, FC_INDEX, 0, &index) == FcResultMatch)
+    index &= 0xFFFF;
+
   FcCharSet * covered = FcCharSetIntersect(remaining, charset);
   if (!covered)
     return false;
@@ -124,6 +129,27 @@ static bool uiFontAddFace(const FcPattern * pattern, FcCharSet ** remainingPtr)
     return true;
   }
 
+  size_t fontDataSize;
+  void * fontData = igImFileLoadToMemory(
+      (const char *)file, "rb", &fontDataSize, 0);
+  if (!fontData)
+  {
+    DEBUG_WARN("Failed to read UI font face: %s", file);
+    FcCharSetDestroy(covered);
+    return true;
+  }
+
+  const bool compatible = font_isStbTruetypeCompatible(
+      fontData, fontDataSize, index);
+  igMemFree(fontData);
+  if (!compatible)
+  {
+    DEBUG_WARN("Skipping stb-incompatible UI font face: %s (index %d)",
+        file, index);
+    FcCharSetDestroy(covered);
+    return true;
+  }
+
   struct UIFontFace * faces = realloc(UIFontFaces,
       sizeof(*UIFontFaces) * (UIFontFaceCount + 1));
   if (!faces)
@@ -144,9 +170,7 @@ static bool uiFontAddFace(const FcPattern * pattern, FcCharSet ** remainingPtr)
     return false;
   }
 
-  int index = 0;
+  face->index = index;
-  if (FcPatternGetInteger(pattern, FC_INDEX, 0, &index) == FcResultMatch)
-    face->index = index & 0xFFFF;
 
   ++UIFontFaceCount;