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: Björn Roy Baron bjorn3_gh@protonmail.com 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] Signed-off-by: Miguel Ojeda ojeda@kernel.org --- rust/Makefile | 1 + rust/kernel/firmware.rs | 2 +- rust/kernel/kunit.rs | 2 +- rust/kernel/lib.rs | 3 +++ rust/macros/module.rs | 10 +++++----- scripts/Makefile.build | 3 ++- 6 files changed, 13 insertions(+), 8 deletions(-)

diff --git a/rust/Makefile b/rust/Makefile index 27dec7904c3a..115b63b7d1e3 100644 --- a/rust/Makefile +++ b/rust/Makefile @@ -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) \ diff --git a/rust/kernel/firmware.rs b/rust/kernel/firmware.rs index 2494c96e105f..4fe621f35716 100644 --- a/rust/kernel/firmware.rs +++ b/rust/kernel/firmware.rs @@ -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(); }; diff --git a/rust/kernel/kunit.rs b/rust/kernel/kunit.rs index 4b8cdcb21e77..b9e65905e121 100644 --- a/rust/kernel/kunit.rs +++ b/rust/kernel/kunit.rs @@ -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 = diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs index 6b4774b2b1c3..e13d6ed88fa6 100644 --- a/rust/kernel/lib.rs +++ b/rust/kernel/lib.rs @@ -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))] diff --git a/rust/macros/module.rs b/rust/macros/module.rs index 2ddd2eeb2852..75efc6eeeafc 100644 --- a/rust/macros/module.rs +++ b/rust/macros/module.rs @@ -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;

diff --git a/scripts/Makefile.build b/scripts/Makefile.build index a6461ea411f7..ba71b27aa363 100644 --- a/scripts/Makefile.build +++ b/scripts/Makefile.build @@ -312,10 +312,11 @@ $(obj)/%.lst: $(obj)/%.c FORCE # - Stable since Rust 1.82.0: `feature(asm_const)`, `feature(raw_ref_op)`. # - Stable since Rust 1.87.0: `feature(asm_goto)`. # - Expected to become stable: `feature(arbitrary_self_types)`. +# - To be determined: `feature(used_with_arg)`. # # Please see https://github.com/Rust-for-Linux/linux/issues/2 for details on # the unstable features in use. -rust_allowed_features := asm_const,asm_goto,arbitrary_self_types,lint_reasons,raw_ref_op +rust_allowed_features := asm_const,asm_goto,arbitrary_self_types,lint_reasons,raw_ref_op,used_with_arg

# `--out-dir` is required to avoid temporaries being created by `rustc` in the # current working directory, which may be not accessible in the out-of-tree