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x86/rust: depend on !RETHUNK
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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: rust-lang/rust#116853 [1]
Link: rust-lang/rust#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@mail.gmail.com/ [4]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
Acked-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>
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ojeda authored and Fabo committed Dec 16, 2023
1 parent 774b034 commit c32a2ca
Showing 1 changed file with 1 addition and 0 deletions.
1 change: 1 addition & 0 deletions init/Kconfig
Original file line number Diff line number Diff line change
Expand Up @@ -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.
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