Re: [PATCH] gcc: Disallow trampolines when -fhardened

[Siddhesh Poyarekar <siddhesh@gotplt.org>]

On 2023-12-04 11:39, Andreas Schwab wrote:
> On Dez 04 2023, Siddhesh Poyarekar wrote:
> 
>> For hardened code in C, I think we really should look to step away from
>> nested functions instead of adding ways to continue supporting it. There's
>> probably a larger conversation to be had about the utility of nested
>> functions in general for C (and whether this GCC extension should be
>> deprecated altogether in future), but I feel like the -fhardened subset
>> gives us the opportunity to enforce at least a safe subset for now,
>> possibly extending it in future.
> 
> Nested functions by itself don't need a trampoline, only if the address
> of it is passed outside the containing function's scope (as a callback,
> for example).

Yes, that's why I said that the conversation about deprecating the C 
nested functions extension is a broader one (and hence for gcc 15) that 
will likely involve the question of whether dropping the extension 
altogether gives any benefit or if dropping support for on-stack 
trampolines is sufficient.  On-heap trampolines are maybe slightly 
better in that they don't need an executable stack, but defaulting to 
on-heap trampolines for -fhardened seems like a lost opportunity to 
enforce better user code.

Thanks,
Sid