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, 133 insertions, 0 deletions

diff --git a/include/asm-generic/atomic.h b/include/asm-generic/atomic.h
new file mode 100644
index 000000000..22142c71d
--- /dev/null
+++ b/include/asm-generic/atomic.h
@@ -0,0 +1,133 @@
+/* SPDX-License-Identifier: GPL-2.0-or-later */
+/*
+ * Generic C implementation of atomic counter operations. Do not include in
+ * machine independent code.
+ *
+ * Copyright (C) 2007 Red Hat, Inc. All Rights Reserved.
+ * Written by David Howells (dhowells@redhat.com)
+ */
+#ifndef __ASM_GENERIC_ATOMIC_H
+#define __ASM_GENERIC_ATOMIC_H
+
+#include <asm/cmpxchg.h>
+#include <asm/barrier.h>
+
+#ifdef CONFIG_SMP
+
+/* we can build all atomic primitives from cmpxchg */
+
+#define ATOMIC_OP(op, c_op)						\
+static inline void generic_atomic_##op(int i, atomic_t *v)		\
+{									\
+	int c, old;							\
+									\
+	c = v->counter;							\
+	while ((old = arch_cmpxchg(&v->counter, c, c c_op i)) != c)	\
+		c = old;						\
+}
+
+#define ATOMIC_OP_RETURN(op, c_op)					\
+static inline int generic_atomic_##op##_return(int i, atomic_t *v)	\
+{									\
+	int c, old;							\
+									\
+	c = v->counter;							\
+	while ((old = arch_cmpxchg(&v->counter, c, c c_op i)) != c)	\
+		c = old;						\
+									\
+	return c c_op i;						\
+}
+
+#define ATOMIC_FETCH_OP(op, c_op)					\
+static inline int generic_atomic_fetch_##op(int i, atomic_t *v)		\
+{									\
+	int c, old;							\
+									\
+	c = v->counter;							\
+	while ((old = arch_cmpxchg(&v->counter, c, c c_op i)) != c)	\
+		c = old;						\
+									\
+	return c;							\
+}
+
+#else
+
+#include <linux/irqflags.h>
+
+#define ATOMIC_OP(op, c_op)						\
+static inline void generic_atomic_##op(int i, atomic_t *v)		\
+{									\
+	unsigned long flags;						\
+									\
+	raw_local_irq_save(flags);					\
+	v->counter = v->counter c_op i;					\
+	raw_local_irq_restore(flags);					\
+}
+
+#define ATOMIC_OP_RETURN(op, c_op)					\
+static inline int generic_atomic_##op##_return(int i, atomic_t *v)	\
+{									\
+	unsigned long flags;						\
+	int ret;							\
+									\
+	raw_local_irq_save(flags);					\
+	ret = (v->counter = v->counter c_op i);				\
+	raw_local_irq_restore(flags);					\
+									\
+	return ret;							\
+}
+
+#define ATOMIC_FETCH_OP(op, c_op)					\
+static inline int generic_atomic_fetch_##op(int i, atomic_t *v)		\
+{									\
+	unsigned long flags;						\
+	int ret;							\
+									\
+	raw_local_irq_save(flags);					\
+	ret = v->counter;						\
+	v->counter = v->counter c_op i;					\
+	raw_local_irq_restore(flags);					\
+									\
+	return ret;							\
+}
+
+#endif /* CONFIG_SMP */
+
+ATOMIC_OP_RETURN(add, +)
+ATOMIC_OP_RETURN(sub, -)
+
+ATOMIC_FETCH_OP(add, +)
+ATOMIC_FETCH_OP(sub, -)
+ATOMIC_FETCH_OP(and, &)
+ATOMIC_FETCH_OP(or, |)
+ATOMIC_FETCH_OP(xor, ^)
+
+ATOMIC_OP(add, +)
+ATOMIC_OP(sub, -)
+ATOMIC_OP(and, &)
+ATOMIC_OP(or, |)
+ATOMIC_OP(xor, ^)
+
+#undef ATOMIC_FETCH_OP
+#undef ATOMIC_OP_RETURN
+#undef ATOMIC_OP
+
+#define arch_atomic_add_return			generic_atomic_add_return
+#define arch_atomic_sub_return			generic_atomic_sub_return
+
+#define arch_atomic_fetch_add			generic_atomic_fetch_add
+#define arch_atomic_fetch_sub			generic_atomic_fetch_sub
+#define arch_atomic_fetch_and			generic_atomic_fetch_and
+#define arch_atomic_fetch_or			generic_atomic_fetch_or
+#define arch_atomic_fetch_xor			generic_atomic_fetch_xor
+
+#define arch_atomic_add				generic_atomic_add
+#define arch_atomic_sub				generic_atomic_sub
+#define arch_atomic_and				generic_atomic_and
+#define arch_atomic_or				generic_atomic_or
+#define arch_atomic_xor				generic_atomic_xor
+
+#define arch_atomic_read(v)			READ_ONCE((v)->counter)
+#define arch_atomic_set(v, i)			WRITE_ONCE(((v)->counter), (i))
+
+#endif /* __ASM_GENERIC_ATOMIC_H */