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

[Miguel Ojeda]

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].

Link: https://github.com/rust-lang/rust/issues/116853 [1]
Link: https://github.com/rust-lang/rust/pull/116892 [2]
Link: https://lore.kernel.org/rust-for-linux/ZSQXqX2%2Flhf5ICZP@gpd/ [3]
Link: https://lore.kernel.org/rust-for-linux/CANiq72n6DMeXQrgOzS_+3VdgNYAmpcnneAHJnZERUQhMExg+0A@xxxxxxxxxxxxxx/ [4]
Signed-off-by: Miguel Ojeda <ojeda@xxxxxxxxxx>
---
 init/Kconfig | 1 +
 1 file changed, 1 insertion(+)

diff --git a/init/Kconfig b/init/Kconfig
index 6d35728b94b2..9452b4102b99 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -1889,6 +1889,7 @@ config RUST
 	depends on !GCC_PLUGINS
 	depends on !RANDSTRUCT
 	depends on !DEBUG_INFO_BTF || PAHOLE_HAS_LANG_EXCLUDE
+	depends on !RETHUNK
 	select CONSTRUCTORS
 	help
 	  Enables Rust support in the kernel.
-- 
2.42.0