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: Alice Ryhl aliceryhl@google.com Acked-by: Björn Roy Baron bjorn3_gh@protonmail.com Link: https://lore.kernel.org/r/20250712160103.1244945-3-ojeda@kernel.org Signed-off-by: Miguel Ojeda ojeda@kernel.org (cherry picked from commit 7498159226772d66f150dd406be462d75964a366) Signed-off-by: Miguel Ojeda ojeda@kernel.org --- rust/Makefile | 1 + rust/kernel/lib.rs | 1 + rust/macros/module.rs | 10 +++++----- scripts/Makefile.build | 2 +- 4 files changed, 8 insertions(+), 6 deletions(-)
diff --git a/rust/Makefile b/rust/Makefile index b8b7f817c48e..17491d8229a4 100644 --- a/rust/Makefile +++ b/rust/Makefile @@ -157,6 +157,7 @@ quiet_cmd_rustdoc_test = RUSTDOC T $< cmd_rustdoc_test = \ OBJTREE=$(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/lib.rs b/rust/kernel/lib.rs index 904d241604db..889ddcb1a2dd 100644 --- a/rust/kernel/lib.rs +++ b/rust/kernel/lib.rs @@ -18,6 +18,7 @@ #![feature(inline_const)] #![feature(lint_reasons)] #![feature(unsize)] +#![feature(used_with_arg)]
// Ensure conditional compilation based on the kernel configuration works; // otherwise we may silently break things like initcall handling. diff --git a/rust/macros/module.rs b/rust/macros/module.rs index a5ea5850e307..edb23b28f446 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)] #[link_section = ".modinfo"] - #[used] + #[used(compiler)] pub static __{module}_{counter}: [u8; {length}] = *{string}; ", cfg = if builtin { @@ -230,7 +230,7 @@ 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: Option<{type_}> = None; @@ -253,7 +253,7 @@ 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;
@@ -273,7 +273,7 @@ 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;
@@ -283,7 +283,7 @@ mod __module_init {{ #[cfg(not(CONFIG_HAVE_ARCH_PREL32_RELOCATIONS))] #[doc(hidden)] #[link_section = "{initcall_section}"] - #[used] + #[used(compiler)] pub static __{name}_initcall: extern "C" fn() -> kernel::ffi::c_int = __{name}_init;
#[cfg(not(MODULE))] diff --git a/scripts/Makefile.build b/scripts/Makefile.build index 2bba59e790b8..2c5c1a214f3b 100644 --- a/scripts/Makefile.build +++ b/scripts/Makefile.build @@ -248,7 +248,7 @@ $(obj)/%.lst: $(obj)/%.c FORCE # Compile Rust sources (.rs) # ---------------------------------------------------------------------------
-rust_allowed_features := arbitrary_self_types,lint_reasons +rust_allowed_features := arbitrary_self_types,lint_reasons,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