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, 173 insertions, 0 deletions

diff --git a/drivers/clocksource/timer-mp-csky.c b/drivers/clocksource/timer-mp-csky.c
new file mode 100644
index 000000000..183a99551
--- /dev/null
+++ b/drivers/clocksource/timer-mp-csky.c
@@ -0,0 +1,173 @@
+// SPDX-License-Identifier: GPL-2.0
+// Copyright (C) 2018 Hangzhou C-SKY Microsystems co.,ltd.
+
+#include <linux/init.h>
+#include <linux/interrupt.h>
+#include <linux/sched_clock.h>
+#include <linux/cpu.h>
+#include <linux/of_irq.h>
+#include <asm/reg_ops.h>
+
+#include "timer-of.h"
+
+#define PTIM_CCVR	"cr<3, 14>"
+#define PTIM_CTLR	"cr<0, 14>"
+#define PTIM_LVR	"cr<6, 14>"
+#define PTIM_TSR	"cr<1, 14>"
+
+static int csky_mptimer_irq;
+
+static int csky_mptimer_set_next_event(unsigned long delta,
+				       struct clock_event_device *ce)
+{
+	mtcr(PTIM_LVR, delta);
+
+	return 0;
+}
+
+static int csky_mptimer_shutdown(struct clock_event_device *ce)
+{
+	mtcr(PTIM_CTLR, 0);
+
+	return 0;
+}
+
+static int csky_mptimer_oneshot(struct clock_event_device *ce)
+{
+	mtcr(PTIM_CTLR, 1);
+
+	return 0;
+}
+
+static int csky_mptimer_oneshot_stopped(struct clock_event_device *ce)
+{
+	mtcr(PTIM_CTLR, 0);
+
+	return 0;
+}
+
+static DEFINE_PER_CPU(struct timer_of, csky_to) = {
+	.flags					= TIMER_OF_CLOCK,
+	.clkevt = {
+		.rating				= 300,
+		.features			= CLOCK_EVT_FEAT_PERCPU |
+						  CLOCK_EVT_FEAT_ONESHOT,
+		.set_state_shutdown		= csky_mptimer_shutdown,
+		.set_state_oneshot		= csky_mptimer_oneshot,
+		.set_state_oneshot_stopped	= csky_mptimer_oneshot_stopped,
+		.set_next_event			= csky_mptimer_set_next_event,
+	},
+};
+
+static irqreturn_t csky_timer_interrupt(int irq, void *dev)
+{
+	struct timer_of *to = this_cpu_ptr(&csky_to);
+
+	mtcr(PTIM_TSR, 0);
+
+	to->clkevt.event_handler(&to->clkevt);
+
+	return IRQ_HANDLED;
+}
+
+/*
+ * clock event for percpu
+ */
+static int csky_mptimer_starting_cpu(unsigned int cpu)
+{
+	struct timer_of *to = per_cpu_ptr(&csky_to, cpu);
+
+	to->clkevt.cpumask = cpumask_of(cpu);
+
+	enable_percpu_irq(csky_mptimer_irq, 0);
+
+	clockevents_config_and_register(&to->clkevt, timer_of_rate(to),
+					2, ULONG_MAX);
+
+	return 0;
+}
+
+static int csky_mptimer_dying_cpu(unsigned int cpu)
+{
+	disable_percpu_irq(csky_mptimer_irq);
+
+	return 0;
+}
+
+/*
+ * clock source
+ */
+static u64 notrace sched_clock_read(void)
+{
+	return (u64)mfcr(PTIM_CCVR);
+}
+
+static u64 clksrc_read(struct clocksource *c)
+{
+	return (u64)mfcr(PTIM_CCVR);
+}
+
+struct clocksource csky_clocksource = {
+	.name	= "csky",
+	.rating	= 400,
+	.mask	= CLOCKSOURCE_MASK(32),
+	.flags	= CLOCK_SOURCE_IS_CONTINUOUS,
+	.read	= clksrc_read,
+};
+
+static int __init csky_mptimer_init(struct device_node *np)
+{
+	int ret, cpu, cpu_rollback;
+	struct timer_of *to = NULL;
+
+	/*
+	 * Csky_mptimer is designed for C-SKY SMP multi-processors and
+	 * every core has it's own private irq and regs for clkevt and
+	 * clksrc.
+	 *
+	 * The regs is accessed by cpu instruction: mfcr/mtcr instead of
+	 * mmio map style. So we needn't mmio-address in dts, but we still
+	 * need to give clk and irq number.
+	 *
+	 * We use private irq for the mptimer and irq number is the same
+	 * for every core. So we use request_percpu_irq() in timer_of_init.
+	 */
+	csky_mptimer_irq = irq_of_parse_and_map(np, 0);
+	if (csky_mptimer_irq <= 0)
+		return -EINVAL;
+
+	ret = request_percpu_irq(csky_mptimer_irq, csky_timer_interrupt,
+				 "csky_mp_timer", &csky_to);
+	if (ret)
+		return -EINVAL;
+
+	for_each_possible_cpu(cpu) {
+		to = per_cpu_ptr(&csky_to, cpu);
+		ret = timer_of_init(np, to);
+		if (ret)
+			goto rollback;
+	}
+
+	clocksource_register_hz(&csky_clocksource, timer_of_rate(to));
+	sched_clock_register(sched_clock_read, 32, timer_of_rate(to));
+
+	ret = cpuhp_setup_state(CPUHP_AP_CSKY_TIMER_STARTING,
+				"clockevents/csky/timer:starting",
+				csky_mptimer_starting_cpu,
+				csky_mptimer_dying_cpu);
+	if (ret)
+		return -EINVAL;
+
+	return 0;
+
+rollback:
+	for_each_possible_cpu(cpu_rollback) {
+		if (cpu_rollback == cpu)
+			break;
+
+		to = per_cpu_ptr(&csky_to, cpu_rollback);
+		timer_of_cleanup(to);
+	}
+	return -EINVAL;
+}
+TIMER_OF_DECLARE(csky_mptimer, "csky,mptimer", csky_mptimer_init);