kbuild: rust: keep Rust objects out of Clang LTO with inline helpersgrafted

Under `CONFIG_LTO_CLANG` + `CONFIG_RUST_INLINE_HELPERS`, one may hit
`objtool` errors such as:

    vmlinux.o: error: objtool: _R..._3Gsp4boot+0xd6a:
    can't find jump dest instruction at .text._R..._3Gsp4boot+0x1dfd

The reason is that in such builds, the Clang invocation that compiles
the combined Rust plus helpers bitcode emits LLVM bitcode (again) --
the final code generation happens in the linker's LTO step, which the
`-mllvm` trap options passed to Clang do not reach.

This, in turn, means that unreachable traps are missing, and the
impossible paths do not merely fallthrough to the next symbol, but past
the end of their own section, since LTO builds place each function in
its own section.

Thus filter `CC_FLAGS_LTO` out of the Clang invocation, so that it always
emits machine code directly, with the traps in place.

Assisted-by: LLM
Cc: Gary Guo <gary@garyguo.net>
Cc: Boqun Feng <boqun@kernel.org>
Cc: Alice Ryhl <aliceryhl@google.com>
Cc: Matthew Maurer <mmaurer@google.com>
Cc: Josh Poimboeuf <jpoimboe@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: stable@vger.kernel.org
Fixes: 3a2486cc1da5 ("kbuild: rust: provide an option to inline C helpers into Rust")
Acked-by: Gary Guo <gary@garyguo.net>
Link: https://patch.msgid.link/20260816133233.197500-2-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

1 files changed, 62 insertions, 0 deletions

diff --git a/rust/helpers/task.c b/rust/helpers/task.c
new file mode 100644
index 000000000..c0e1a06ed
--- /dev/null
+++ b/rust/helpers/task.c
@@ -0,0 +1,62 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include <linux/kernel.h>
+#include <linux/sched/task.h>
+
+__rust_helper void rust_helper_might_resched(void)
+{
+	might_resched();
+}
+
+__rust_helper struct task_struct *rust_helper_get_current(void)
+{
+	return current;
+}
+
+__rust_helper void rust_helper_get_task_struct(struct task_struct *t)
+{
+	get_task_struct(t);
+}
+
+__rust_helper void rust_helper_put_task_struct(struct task_struct *t)
+{
+	put_task_struct(t);
+}
+
+__rust_helper kuid_t rust_helper_task_uid(struct task_struct *task)
+{
+	return task_uid(task);
+}
+
+__rust_helper kuid_t rust_helper_task_euid(struct task_struct *task)
+{
+	return task_euid(task);
+}
+
+#ifndef CONFIG_USER_NS
+__rust_helper uid_t rust_helper_from_kuid(struct user_namespace *to, kuid_t uid)
+{
+	return from_kuid(to, uid);
+}
+#endif /* CONFIG_USER_NS */
+
+__rust_helper bool rust_helper_uid_eq(kuid_t left, kuid_t right)
+{
+	return uid_eq(left, right);
+}
+
+__rust_helper kuid_t rust_helper_current_euid(void)
+{
+	return current_euid();
+}
+
+__rust_helper struct user_namespace *rust_helper_current_user_ns(void)
+{
+	return current_user_ns();
+}
+
+__rust_helper pid_t rust_helper_task_tgid_nr_ns(struct task_struct *tsk,
+						struct pid_namespace *ns)
+{
+	return task_tgid_nr_ns(tsk, ns);
+}