Unfortunately, as useful as they really are, GCC's nested functions are usually done via trampolines, which means an executable stack. That's not really a safe thing, when dealing with critical systems, like NASA regularly do.
This isn't the case for Ada on GCC, so the support is there, it just hasn't been extended to C:
> The use of trampolines requires an executable stack, which is a security risk. To avoid this problem, GCC also supports another strategy: using descriptors for nested functions. Under this model, taking the address of a nested function results in a pointer to a non-executable function descriptor object. Initializing the static chain from the descriptor is handled at indirect call sites.
> On some targets, including HPPA and IA-64, function descriptors may be mandated by the ABI or be otherwise handled in a target-specific way by the back end in its code generation strategy for indirect calls. GCC also provides its own generic descriptor implementation to support the -fno-trampolines option. In this case runtime detection of function descriptors at indirect call sites relies on descriptor pointers being tagged with a bit that is never set in bare function addresses. Since GCC’s generic function descriptors are not ABI-compliant, this option is typically used only on a per-language basis (notably by Ada) or when it can otherwise be applied to the whole program.
> For languages other than Ada, the -ftrampolines and -fno-trampolines options currently have no effect, and trampolines are always generated on platforms that need them for nested functions.
You can actually do it in C, too! It just requires setting up a few macros, and getting it right requires a fairly decent understanding of the underlying hardware. (via TARGET_CUSTOM_FUNCTION_DESCRIPTORS).
This only a problem if you pass the nested function as a function pointer and you have a machine with a no execute stack.
The processors I program for don't have no execute stacks so the use of a trampoline is of no consequence.
Also my memory was that no execute stacks were heralded as the solution to stack smashing attacks. No execute stacks problem solved. And turned out if you're vulnerable to stack smashing no execute stacks won't save you.
So I feel we threw out something good, nested functions for some cargo cult security. And since no one uses them the reactionary luddites on WG14 are free to hope they go away.
