Commit 74981592 authored by Miguel Ojeda's avatar Miguel Ojeda
Browse files

rust: use `#[used(compiler)]` to fix build and `modpost` with Rust >= 1.89.0 

Starting with Rust 1.89.0 (expected 2025-08-07), the Rust compiler fails
to build the `rusttest` target due to undefined references such as:

    kernel...-cgu.0:(.text....+0x116): undefined reference to
    `rust_helper_kunit_get_current_test'

Moreover, tooling like `modpost` gets confused:

    WARNING: modpost: missing MODULE_DESCRIPTION() in drivers/gpu/drm/nova/nova.o
    ERROR: modpost: missing MODULE_LICENSE() in drivers/gpu/nova-core/nova_core.o

The reason behind both issues is that the Rust compiler will now [1]
treat `#[used]` as `#[used(linker)]` instead of `#[used(compiler)]`
for our targets. This means that the retain section flag (`R`,
`SHF_GNU_RETAIN`) will be used and that they will be marked as `unique`
too, with different IDs. In turn, that means we end up with undefined
references that did not get discarded in `rusttest` and that multiple
`.modinfo` sections are generated, which confuse tooling like `modpost`
because they only expect one.

Thus start using `#[used(compiler)]` to keep the previous behavior
and to be explicit about what we want. Sadly, it is an unstable feature
(`used_with_arg`) [2] -- we will talk to upstream Rust about it. The good
news is that it has been available for a long time (Rust >= 1.60) [3].

The changes should also be fine for previous Rust versions, since they
behave the same way as before [4].

Alternatively, we could use `#[no_mangle]` or `#[export_name = ...]`
since those still behave like `#[used(compiler)]`, but of course it is
not really what we want to express, and it requires other changes to
avoid symbol conflicts.

Cc: David Wood <david@davidtw.co>
Cc: Wesley Wiser <wwiser@gmail.com>
Cc: stable@vger.kernel.org # Needed in 6.12.y and later (Rust is pinned in older LTSs).
Link: https://github.com/rust-lang/rust/pull/140872 [1]
Link: https://github.com/rust-lang/rust/issues/93798 [2]
Link: https://github.com/rust-lang/rust/pull/91504 [3]
Link: https://godbolt.org/z/sxzWTMfzW

 [4]
Reviewed-by: default avatarAlice Ryhl <aliceryhl@google.com>
Acked-by: default avatarBjörn Roy Baron <bjorn3_gh@protonmail.com>
Link: https://lore.kernel.org/r/20250712160103.1244945-3-ojeda@kernel.org


Signed-off-by: default avatarMiguel Ojeda <ojeda@kernel.org>
parent aa7b65c2

rust/Makefile

+1 −0
Original line number Diff line number Diff line
@@ -194,6 +194,7 @@ quiet_cmd_rustdoc_test = RUSTDOC T $< 
	RUST_MODFILE=test.rs \

	OBJTREE=$(abspath $(objtree)) \

	$(RUSTDOC) --test $(rust_common_flags) \

		-Zcrate-attr='feature(used_with_arg)' \

		@$(objtree)/include/generated/rustc_cfg \

		$(rustc_target_flags) $(rustdoc_test_target_flags) \

		$(rustdoc_test_quiet) \

rust/kernel/firmware.rs

+1 −1
Original line number Diff line number Diff line
@@ -202,7 +202,7 @@ macro_rules! module_firmware { 
            };



            #[link_section = ".modinfo"]

            #[used]

            #[used(compiler)]

            static __MODULE_FIRMWARE: [u8; $($builder)*::create(__MODULE_FIRMWARE_PREFIX)

                .build_length()] = $($builder)*::create(__MODULE_FIRMWARE_PREFIX).build();

        };

rust/kernel/kunit.rs

+1 −1
Original line number Diff line number Diff line
@@ -302,7 +302,7 @@ macro_rules! kunit_unsafe_test_suite { 
                    is_init: false,

                };



            #[used]

            #[used(compiler)]

            #[allow(unused_unsafe)]

            #[cfg_attr(not(target_os = "macos"), link_section = ".kunit_test_suites")]

            static mut KUNIT_TEST_SUITE_ENTRY: *const ::kernel::bindings::kunit_suite =

rust/kernel/lib.rs

+3 −0
Original line number Diff line number Diff line
@@ -34,6 +34,9 @@ 
// Expected to become stable.

#![feature(arbitrary_self_types)]

//

// To be determined.

#![feature(used_with_arg)]

//

// `feature(derive_coerce_pointee)` is expected to become stable. Before Rust

// 1.84.0, it did not exist, so enable the predecessor features.

#![cfg_attr(CONFIG_RUSTC_HAS_COERCE_POINTEE, feature(derive_coerce_pointee))]

rust/macros/module.rs

+5 −5
Original line number Diff line number Diff line
@@ -57,7 +57,7 @@ fn emit_base(&mut self, field: &str, content: &str, builtin: bool) { 
                {cfg}

                #[doc(hidden)]

                #[cfg_attr(not(target_os = \"macos\"), link_section = \".modinfo\")]

                #[used]

                #[used(compiler)]

                pub static __{module}_{counter}: [u8; {length}] = *{string};

            ",

            cfg = if builtin {
@@ -249,7 +249,7 @@ mod __module_init {{ 
                    // key or a new section. For the moment, keep it simple.

                    #[cfg(MODULE)]

                    #[doc(hidden)]

                    #[used]

                    #[used(compiler)]

                    static __IS_RUST_MODULE: () = ();



                    static mut __MOD: ::core::mem::MaybeUninit<{type_}> =
@@ -273,7 +273,7 @@ mod __module_init {{ 


                    #[cfg(MODULE)]

                    #[doc(hidden)]

                    #[used]

                    #[used(compiler)]

                    #[link_section = \".init.data\"]

                    static __UNIQUE_ID___addressable_init_module: unsafe extern \"C\" fn() -> i32 = init_module;
@@ -293,7 +293,7 @@ mod __module_init {{ 


                    #[cfg(MODULE)]

                    #[doc(hidden)]

                    #[used]

                    #[used(compiler)]

                    #[link_section = \".exit.data\"]

                    static __UNIQUE_ID___addressable_cleanup_module: extern \"C\" fn() = cleanup_module;
@@ -303,7 +303,7 @@ mod __module_init {{ 
                    #[cfg(not(CONFIG_HAVE_ARCH_PREL32_RELOCATIONS))]

                    #[doc(hidden)]

                    #[link_section = \"{initcall_section}\"]

                    #[used]

                    #[used(compiler)]

                    pub static __{ident}_initcall: extern \"C\" fn() ->

                        ::kernel::ffi::c_int = __{ident}_init;