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

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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>
This commit is contained in:
Miguel Ojeda 2025-07-12 18:01:03 +02:00
parent aa7b65c2a2
commit 7498159226
6 changed files with 13 additions and 8 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

1
rust/Makefile
View File

@ -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) \

2
rust/kernel/firmware.rs
View File

@ -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();
};

2
rust/kernel/kunit.rs
View File

@ -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 =

3
rust/kernel/lib.rs
View File

@ -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))]

10
rust/macros/module.rs
View File

@ -57,7 +57,7 @@ impl<'a> ModInfoBuilder<'a> {
{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 @@ pub(crate) fn module(ts: TokenStream) -> TokenStream {
// 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 @@ pub(crate) fn module(ts: TokenStream) -> TokenStream {
#[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 @@ pub(crate) fn module(ts: TokenStream) -> TokenStream {
#[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 @@ pub(crate) fn module(ts: TokenStream) -> TokenStream {
#[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;

3
scripts/Makefile.build
View File

@ -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