Re: [PATCH 3/4] x86/rust: depend on !RETHUNK

[Martin Rodriguez Reboredo <yakoyoku@gmail.com>]

On 10/23/23 14:44, Miguel Ojeda wrote:
> The Rust compiler does not support the equivalent of
> `-mfunction-return=thunk-extern` yet [1]. Thus, currently, `objtool`
> warns about it, e.g.:
> 
>      samples/rust/rust_print.o: warning: objtool: _R...init+0xa5c:
>      'naked' return found in RETHUNK build
> 
> The support in `rustc` for `-Zfunction-return` has been submitted and
> is being reviewed [2]. It adds the needed LLVM function attributes and,
> with it, I got a RETHUNK kernel build with Rust enabled that does not
> print the `objtool` related warnings, boots in QEMU and can load a kernel
> loadable module.
> 
> In any case, until proper/complete support is added to `rustc`, make it
> a hard restriction until the mitigation is in place.
> 
> This may have an impact for developers that may not need/care about the
> mitigation in the Rust side (e.g. Ubuntu offers Rust as a "technology
> preview" [3]), but given we are getting closer to having the first actual
> in-tree Rust kernel users, it seems like the right time to disallow
> it. This should also avoid confusion [4].

I'll mention that I've tested boots, both in bare metal and QEMU, with
`RUST=y` and `RETHUNK=y` and they were alright regardless of `objtool`
warnings. Although, if you had an issue in the past then I'd like to know
about it.