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, 336 insertions, 0 deletions

diff --git a/scripts/atomic/gen-atomic-fallback.sh b/scripts/atomic/gen-atomic-fallback.sh
new file mode 100755
index 000000000..f80d69cfe
--- /dev/null
+++ b/scripts/atomic/gen-atomic-fallback.sh
@@ -0,0 +1,336 @@
+#!/bin/sh
+# SPDX-License-Identifier: GPL-2.0
+
+ATOMICDIR=$(dirname $0)
+
+. ${ATOMICDIR}/atomic-tbl.sh
+
+#gen_template_fallback(template, meta, pfx, name, sfx, order, atomic, int, args...)
+gen_template_fallback()
+{
+	local template="$1"; shift
+	local meta="$1"; shift
+	local pfx="$1"; shift
+	local name="$1"; shift
+	local sfx="$1"; shift
+	local order="$1"; shift
+	local atomic="$1"; shift
+	local int="$1"; shift
+
+	local ret="$(gen_ret_type "${meta}" "${int}")"
+	local retstmt="$(gen_ret_stmt "${meta}")"
+	local params="$(gen_params "${int}" "${atomic}" "$@")"
+	local args="$(gen_args "$@")"
+
+	. ${template}
+}
+
+#gen_order_fallback(meta, pfx, name, sfx, order, atomic, int, args...)
+gen_order_fallback()
+{
+	local meta="$1"; shift
+	local pfx="$1"; shift
+	local name="$1"; shift
+	local sfx="$1"; shift
+	local order="$1"; shift
+
+	local tmpl_order=${order#_}
+	local tmpl="${ATOMICDIR}/fallbacks/${tmpl_order:-fence}"
+	gen_template_fallback "${tmpl}" "${meta}" "${pfx}" "${name}" "${sfx}" "${order}" "$@"
+}
+
+#gen_proto_fallback(meta, pfx, name, sfx, order, atomic, int, args...)
+gen_proto_fallback()
+{
+	local meta="$1"; shift
+	local pfx="$1"; shift
+	local name="$1"; shift
+	local sfx="$1"; shift
+	local order="$1"; shift
+
+	local tmpl="$(find_fallback_template "${pfx}" "${name}" "${sfx}" "${order}")"
+	gen_template_fallback "${tmpl}" "${meta}" "${pfx}" "${name}" "${sfx}" "${order}" "$@"
+}
+
+#gen_proto_order_variant(meta, pfx, name, sfx, order, atomic, int, args...)
+gen_proto_order_variant()
+{
+	local meta="$1"; shift
+	local pfx="$1"; shift
+	local name="$1"; shift
+	local sfx="$1"; shift
+	local order="$1"; shift
+	local atomic="$1"; shift
+	local int="$1"; shift
+
+	local atomicname="${atomic}_${pfx}${name}${sfx}${order}"
+	local basename="${atomic}_${pfx}${name}${sfx}"
+
+	local template="$(find_fallback_template "${pfx}" "${name}" "${sfx}" "${order}")"
+
+	local ret="$(gen_ret_type "${meta}" "${int}")"
+	local retstmt="$(gen_ret_stmt "${meta}")"
+	local params="$(gen_params "${int}" "${atomic}" "$@")"
+	local args="$(gen_args "$@")"
+
+	gen_kerneldoc "raw_" "${meta}" "${pfx}" "${name}" "${sfx}" "${order}" "${atomic}" "${int}" "$@"
+
+	printf "static __always_inline ${ret}\n"
+	printf "raw_${atomicname}(${params})\n"
+	printf "{\n"
+
+	# Where there is no possible fallback, this order variant is mandatory
+	# and must be provided by arch code. Add a comment to the header to
+	# make this obvious.
+	#
+	# Ideally we'd error on a missing definition, but arch code might
+	# define this order variant as a C function without a preprocessor
+	# symbol.
+	if [ -z ${template} ] && [ -z "${order}" ] && ! meta_has_relaxed "${meta}"; then
+		printf "\t${retstmt}arch_${atomicname}(${args});\n"
+		printf "}\n\n"
+		return
+	fi
+
+	printf "#if defined(arch_${atomicname})\n"
+	printf "\t${retstmt}arch_${atomicname}(${args});\n"
+
+	# Allow FULL/ACQUIRE/RELEASE ops to be defined in terms of RELAXED ops
+	if [ "${order}" != "_relaxed" ] && meta_has_relaxed "${meta}"; then
+		printf "#elif defined(arch_${basename}_relaxed)\n"
+		gen_order_fallback "${meta}" "${pfx}" "${name}" "${sfx}" "${order}" "${atomic}" "${int}" "$@"
+	fi
+
+	# Allow ACQUIRE/RELEASE/RELAXED ops to be defined in terms of FULL ops
+	if [ ! -z "${order}" ] && ! meta_is_implicitly_relaxed "${meta}"; then
+		printf "#elif defined(arch_${basename})\n"
+		printf "\t${retstmt}arch_${basename}(${args});\n"
+	fi
+
+	printf "#else\n"
+	if [ ! -z "${template}" ]; then
+		gen_proto_fallback "${meta}" "${pfx}" "${name}" "${sfx}" "${order}" "${atomic}" "${int}" "$@"
+	else
+		printf "#error \"Unable to define raw_${atomicname}\"\n"
+	fi
+
+	printf "#endif\n"
+	printf "}\n\n"
+}
+
+
+#gen_proto_order_variants(meta, pfx, name, sfx, atomic, int, args...)
+gen_proto_order_variants()
+{
+	local meta="$1"; shift
+	local pfx="$1"; shift
+	local name="$1"; shift
+	local sfx="$1"; shift
+	local atomic="$1"
+
+	gen_proto_order_variant "${meta}" "${pfx}" "${name}" "${sfx}" "" "$@"
+
+	if meta_has_acquire "${meta}"; then
+		gen_proto_order_variant "${meta}" "${pfx}" "${name}" "${sfx}" "_acquire" "$@"
+	fi
+
+	if meta_has_release "${meta}"; then
+		gen_proto_order_variant "${meta}" "${pfx}" "${name}" "${sfx}" "_release" "$@"
+	fi
+
+	if meta_has_relaxed "${meta}"; then
+		gen_proto_order_variant "${meta}" "${pfx}" "${name}" "${sfx}" "_relaxed" "$@"
+	fi
+}
+
+#gen_basic_fallbacks(basename)
+gen_basic_fallbacks()
+{
+	local basename="$1"; shift
+cat << EOF
+#define raw_${basename}_acquire arch_${basename}
+#define raw_${basename}_release arch_${basename}
+#define raw_${basename}_relaxed arch_${basename}
+EOF
+}
+
+gen_order_fallbacks()
+{
+	local xchg="$1"; shift
+
+cat <<EOF
+
+#define raw_${xchg}_relaxed arch_${xchg}_relaxed
+
+#ifdef arch_${xchg}_acquire
+#define raw_${xchg}_acquire arch_${xchg}_acquire
+#else
+#define raw_${xchg}_acquire(...) \\
+	__atomic_op_acquire(arch_${xchg}, __VA_ARGS__)
+#endif
+
+#ifdef arch_${xchg}_release
+#define raw_${xchg}_release arch_${xchg}_release
+#else
+#define raw_${xchg}_release(...) \\
+	__atomic_op_release(arch_${xchg}, __VA_ARGS__)
+#endif
+
+#ifdef arch_${xchg}
+#define raw_${xchg} arch_${xchg}
+#else
+#define raw_${xchg}(...) \\
+	__atomic_op_fence(arch_${xchg}, __VA_ARGS__)
+#endif
+
+EOF
+}
+
+gen_xchg_order_fallback()
+{
+	local xchg="$1"; shift
+	local order="$1"; shift
+	local forder="${order:-_fence}"
+
+	printf "#if defined(arch_${xchg}${order})\n"
+	printf "#define raw_${xchg}${order} arch_${xchg}${order}\n"
+
+	if [ "${order}" != "_relaxed" ]; then
+		printf "#elif defined(arch_${xchg}_relaxed)\n"
+		printf "#define raw_${xchg}${order}(...) \\\\\n"
+		printf "	__atomic_op${forder}(arch_${xchg}, __VA_ARGS__)\n"
+	fi
+
+	if [ ! -z "${order}" ]; then
+		printf "#elif defined(arch_${xchg})\n"
+		printf "#define raw_${xchg}${order} arch_${xchg}\n"
+	fi
+
+	printf "#else\n"
+	printf "extern void raw_${xchg}${order}_not_implemented(void);\n"
+	printf "#define raw_${xchg}${order}(...) raw_${xchg}${order}_not_implemented()\n"
+	printf "#endif\n\n"
+}
+
+gen_xchg_fallbacks()
+{
+	local xchg="$1"; shift
+
+	for order in "" "_acquire" "_release" "_relaxed"; do
+		gen_xchg_order_fallback "${xchg}" "${order}"
+	done
+}
+
+gen_try_cmpxchg_fallback()
+{
+	local prefix="$1"; shift
+	local cmpxchg="$1"; shift;
+	local suffix="$1"; shift;
+
+cat <<EOF
+#define raw_${prefix}try_${cmpxchg}${suffix}(_ptr, _oldp, _new) \\
+({ \\
+	typeof(*(_ptr)) *___op = (_oldp), ___o = *___op, ___r; \\
+	___r = raw_${prefix}${cmpxchg}${suffix}((_ptr), ___o, (_new)); \\
+	if (unlikely(___r != ___o)) \\
+		*___op = ___r; \\
+	likely(___r == ___o); \\
+})
+EOF
+}
+
+gen_try_cmpxchg_order_fallback()
+{
+	local cmpxchg="$1"; shift
+	local order="$1"; shift
+	local forder="${order:-_fence}"
+
+	printf "#if defined(arch_try_${cmpxchg}${order})\n"
+	printf "#define raw_try_${cmpxchg}${order} arch_try_${cmpxchg}${order}\n"
+
+	if [ "${order}" != "_relaxed" ]; then
+		printf "#elif defined(arch_try_${cmpxchg}_relaxed)\n"
+		printf "#define raw_try_${cmpxchg}${order}(...) \\\\\n"
+		printf "	__atomic_op${forder}(arch_try_${cmpxchg}, __VA_ARGS__)\n"
+	fi
+
+	if [ ! -z "${order}" ]; then
+		printf "#elif defined(arch_try_${cmpxchg})\n"
+		printf "#define raw_try_${cmpxchg}${order} arch_try_${cmpxchg}\n"
+	fi
+
+	printf "#else\n"
+	gen_try_cmpxchg_fallback "" "${cmpxchg}" "${order}"
+	printf "#endif\n\n"
+}
+
+gen_try_cmpxchg_order_fallbacks()
+{
+	local cmpxchg="$1"; shift;
+
+	for order in "" "_acquire" "_release" "_relaxed"; do
+		gen_try_cmpxchg_order_fallback "${cmpxchg}" "${order}"
+	done
+}
+
+gen_def_and_try_cmpxchg_fallback()
+{
+	local prefix="$1"; shift
+	local cmpxchg="$1"; shift
+	local suffix="$1"; shift
+
+	printf "#define raw_${prefix}${cmpxchg}${suffix} arch_${prefix}${cmpxchg}${suffix}\n\n"
+	printf "#ifdef arch_${prefix}try_${cmpxchg}${suffix}\n"
+	printf "#define raw_${prefix}try_${cmpxchg}${suffix} arch_${prefix}try_${cmpxchg}${suffix}\n"
+	printf "#else\n"
+	gen_try_cmpxchg_fallback "${prefix}" "${cmpxchg}" "${suffix}"
+	printf "#endif\n\n"
+}
+
+cat << EOF
+// SPDX-License-Identifier: GPL-2.0
+
+// Generated by $0
+// DO NOT MODIFY THIS FILE DIRECTLY
+
+#ifndef _LINUX_ATOMIC_FALLBACK_H
+#define _LINUX_ATOMIC_FALLBACK_H
+
+#include <linux/compiler.h>
+
+EOF
+
+for xchg in "xchg" "cmpxchg" "cmpxchg64" "cmpxchg128"; do
+	gen_xchg_fallbacks "${xchg}"
+done
+
+for cmpxchg in "cmpxchg" "cmpxchg64" "cmpxchg128"; do
+	gen_try_cmpxchg_order_fallbacks "${cmpxchg}"
+done
+
+for cmpxchg in "cmpxchg" "cmpxchg64" "cmpxchg128"; do
+	gen_def_and_try_cmpxchg_fallback "" "${cmpxchg}" "_local"
+done
+
+for cmpxchg in "cmpxchg"; do
+	gen_def_and_try_cmpxchg_fallback "sync_" "${cmpxchg}" ""
+done
+
+grep '^[a-z]' "$1" | while read name meta args; do
+	gen_proto "${meta}" "${name}" "atomic" "int" ${args}
+done
+
+cat <<EOF
+#ifdef CONFIG_GENERIC_ATOMIC64
+#include <asm-generic/atomic64.h>
+#endif
+
+EOF
+
+grep '^[a-z]' "$1" | while read name meta args; do
+	gen_proto "${meta}" "${name}" "atomic64" "s64" ${args}
+done
+
+cat <<EOF
+#endif /* _LINUX_ATOMIC_FALLBACK_H */
+EOF