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>

2 files changed, 236 insertions, 0 deletions

diff --git a/tools/testing/selftests/cpu-hotplug/Makefile b/tools/testing/selftests/cpu-hotplug/Makefile
new file mode 100644
index 000000000..8b66c4738
--- /dev/null
+++ b/tools/testing/selftests/cpu-hotplug/Makefile
@@ -0,0 +1,11 @@
+# SPDX-License-Identifier: GPL-2.0
+all:
+
+TEST_PROGS := cpu-on-off-test.sh
+
+include ../lib.mk
+
+run_full_test:
+	@/bin/bash ./cpu-on-off-test.sh -a && echo "cpu-hotplug selftests: [PASS]" || echo "cpu-hotplug selftests: [FAIL]"
+
+clean:
diff --git a/tools/testing/selftests/cpu-hotplug/cpu-on-off-test.sh b/tools/testing/selftests/cpu-hotplug/cpu-on-off-test.sh
new file mode 100755
index 000000000..287cfd580
--- /dev/null
+++ b/tools/testing/selftests/cpu-hotplug/cpu-on-off-test.sh
@@ -0,0 +1,225 @@
+#!/bin/bash
+# SPDX-License-Identifier: GPL-2.0
+
+SYSFS=
+# Kselftest framework requirement - SKIP code is 4.
+ksft_skip=4
+retval=0
+
+prerequisite()
+{
+	msg="skip all tests:"
+
+	if [ $UID != 0 ]; then
+		echo $msg must be run as root >&2
+		exit $ksft_skip
+	fi
+
+	taskset -p 01 $$
+
+	SYSFS=`mount -t sysfs | head -1 | awk '{ print $3 }'`
+
+	if [ ! -d "$SYSFS" ]; then
+		echo $msg sysfs is not mounted >&2
+		exit $ksft_skip
+	fi
+
+	if ! ls $SYSFS/devices/system/cpu/cpu*/online > /dev/null 2>&1; then
+		echo $msg cpu hotplug is not supported >&2
+		exit $ksft_skip
+	fi
+
+	echo "CPU online/offline summary:"
+	online_cpus=`cat $SYSFS/devices/system/cpu/online`
+	online_max=${online_cpus##*-}
+
+	if [[ "$online_cpus" = "$online_max" ]]; then
+		echo "$msg: since there is only one cpu: $online_cpus"
+		exit $ksft_skip
+	fi
+
+	present_cpus=`cat $SYSFS/devices/system/cpu/present`
+	present_max=${present_cpus##*-}
+	echo "present_cpus = $present_cpus present_max = $present_max"
+
+	echo -e "\t Cpus in online state: $online_cpus"
+
+	offline_cpus=`cat $SYSFS/devices/system/cpu/offline`
+	if [[ "a$offline_cpus" = "a" ]]; then
+		offline_cpus=0
+	else
+		offline_max=${offline_cpus##*-}
+	fi
+	echo -e "\t Cpus in offline state: $offline_cpus"
+}
+
+#
+# list all hot-pluggable CPUs
+#
+hotpluggable_cpus()
+{
+	local state=${1:-.\*}
+
+	for cpu in $SYSFS/devices/system/cpu/cpu*; do
+		if [ -f $cpu/online ] && grep -q $state $cpu/online; then
+			echo ${cpu##/*/cpu}
+		fi
+	done
+}
+
+hotpluggable_offline_cpus()
+{
+	hotpluggable_cpus 0
+}
+
+hotpluggable_online_cpus()
+{
+	hotpluggable_cpus 1
+}
+
+cpu_is_online()
+{
+	grep -q 1 $SYSFS/devices/system/cpu/cpu$1/online
+}
+
+cpu_is_offline()
+{
+	grep -q 0 $SYSFS/devices/system/cpu/cpu$1/online
+}
+
+online_cpu()
+{
+	echo 1 > $SYSFS/devices/system/cpu/cpu$1/online
+}
+
+offline_cpu()
+{
+	echo 0 > $SYSFS/devices/system/cpu/cpu$1/online
+}
+
+online_cpu_expect_success()
+{
+	local cpu=$1
+
+	if ! online_cpu $cpu; then
+		echo $FUNCNAME $cpu: unexpected fail >&2
+		retval=1
+	elif ! cpu_is_online $cpu; then
+		echo $FUNCNAME $cpu: unexpected offline >&2
+		retval=1
+	fi
+}
+
+online_cpu_expect_fail()
+{
+	local cpu=$1
+
+	if online_cpu $cpu 2> /dev/null; then
+		echo $FUNCNAME $cpu: unexpected success >&2
+		retval=1
+	elif ! cpu_is_offline $cpu; then
+		echo $FUNCNAME $cpu: unexpected online >&2
+		retval=1
+	fi
+}
+
+offline_cpu_expect_success()
+{
+	local cpu=$1
+
+	if ! offline_cpu $cpu; then
+		echo $FUNCNAME $cpu: unexpected fail >&2
+		retval=1
+	elif ! cpu_is_offline $cpu; then
+		echo $FUNCNAME $cpu: unexpected offline >&2
+		retval=1
+	fi
+}
+
+offline_cpu_expect_fail()
+{
+	local cpu=$1
+
+	if offline_cpu $cpu 2> /dev/null; then
+		echo $FUNCNAME $cpu: unexpected success >&2
+		retval=1
+	elif ! cpu_is_online $cpu; then
+		echo $FUNCNAME $cpu: unexpected offline >&2
+		retval=1
+	fi
+}
+
+online_all_hot_pluggable_cpus()
+{
+	for cpu in `hotpluggable_offline_cpus`; do
+		online_cpu_expect_success $cpu
+	done
+}
+
+offline_all_hot_pluggable_cpus()
+{
+	local reserve_cpu=$online_max
+	for cpu in `hotpluggable_online_cpus`; do
+		# Reserve one cpu oneline at least.
+		if [ $cpu -eq $reserve_cpu ];then
+			continue
+		fi
+		offline_cpu_expect_success $cpu
+	done
+}
+
+allcpus=0
+online_cpus=0
+online_max=0
+offline_cpus=0
+offline_max=0
+present_cpus=0
+present_max=0
+
+while getopts ah opt; do
+	case $opt in
+	a)
+		allcpus=1
+		;;
+	h)
+		echo "Usage $0 [ -a ]"
+		echo -e "\t default offline one cpu"
+		echo -e "\t run with -a option to offline all cpus"
+		exit
+		;;
+	esac
+done
+
+prerequisite
+
+#
+# Safe test (default) - offline and online one cpu
+#
+if [ $allcpus -eq 0 ]; then
+	echo "Limited scope test: one hotplug cpu"
+	echo -e "\t (leaves cpu in the original state):"
+	echo -e "\t online to offline to online: cpu $online_max"
+	offline_cpu_expect_success $online_max
+	online_cpu_expect_success $online_max
+
+	if [[ $offline_cpus -gt 0 ]]; then
+		echo -e "\t online to offline to online: cpu $present_max"
+		online_cpu_expect_success $present_max
+		offline_cpu_expect_success $present_max
+		online_cpu $present_max
+	fi
+	exit $retval
+else
+	echo "Full scope test: all hotplug cpus"
+	echo -e "\t online all offline cpus"
+	echo -e "\t offline all online cpus"
+	echo -e "\t online all offline cpus"
+fi
+
+online_all_hot_pluggable_cpus
+
+offline_all_hot_pluggable_cpus
+
+online_all_hot_pluggable_cpus
+
+exit $retval