A simple policy that both set_fs() calls need to happen within the same function body with corresponding CI test based on AST/DWARF inspection would have also prevented it. Do you really want to rely on stack unwinding/destructors for security sensitive code when stack is usually the first thing that gets controlled by the attacker? Exception handling (SEH) on Windows is an exploitation vector of it's own.
I'm talking about the general idea not specific implementation. Having something happen at function/block exit doesn't mean a runtime configurable behaviour. If you don't have exceptions, it's pretty easy to statically compile that behaviour and guarantee it rather than rely on checks.
