A Hidden Weakness

A Hidden Weakness May 29, 2025 Categories: mozilla This is the story of a bug hunt that lasted much longer than expected, but ended with the dearest of all treasures: knowledge. Let's draw some context: the Android platform defines different API levels. Unsurprisingly, some symbols are only defined starting with a given API version. For instance, ASystemFontIterator_open is only available starting at API 29. Unconditionally trying to use ASystemFontIterator_open while targeting an API older than 29 leads to a linker error (an undefined reference), which makes sense because the symbol, well, does not exist at that API level. So a native application that wants to use this symbol can either refuse to run on older API, or use a combination of dlopen and dlsym to dynamically lookup for the symbol and provide a fallback if it does not exist. The latter approach is used on Fenix, the Android version of Firefox. Introducing __ANDROID_UNAVAILABLE_SYMBOLS_ARE_WEAK__ As an alternative to t ... Read full article.