GCC 16 Guarantees Trampoline-Free Nested Functions Without Parent Context Access

Trampoline-Free Nested Functions in GCC 16

GCC 16 ensures that nested functions that do not access variables from their parent function will not require a run-time trampoline. This behavior was previously observed during optimization but is now guaranteed and documented, even without optimization. This change allows such functions to be safely returned from their parent and introduces warnings when attempting to return functions that access local context in simple cases.

Security Implications

The elimination of trampolines for certain nested functions addresses a security concern. Traditionally, trampolines were placed on the stack, necessitating an executable stack. A non-executable stack is a significant security feature, and this update helps maintain that security by reducing instances where an executable stack might be required. While GCC previously offered placing trampolines on the heap, this was not ideal due to higher allocation costs and potential leaks with `longjmp`.

New Built-in Functions for Wide Pointers

A patch implementing two new built-in functions has been merged into the development branch of GCC. These functions are designed to work with existing built-in `__builtin_call` and aim to improve how nested functions can access variables of their parent function without relying on trampolines. This development is part of an ongoing effort to enhance the utility and safety of nested functions in GCC.

Context Access and Callback Functions

Nested functions that do not access parent variables can still access static variables, named constants, or types (if not variably modified). This capability is useful for writing callback functions. The new built-in functions are intended to provide a more direct and compiler-supported method for nested functions to access parent context, moving beyond manual boilerplate solutions.