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, 98 insertions, 0 deletions

diff --git a/drivers/power/reset/syscon-poweroff.c b/drivers/power/reset/syscon-poweroff.c
new file mode 100644
index 000000000..203936f4c
--- /dev/null
+++ b/drivers/power/reset/syscon-poweroff.c
@@ -0,0 +1,98 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * Generic Syscon Poweroff Driver
+ *
+ * Copyright (c) 2015, National Instruments Corp.
+ * Author: Moritz Fischer <moritz.fischer@ettus.com>
+ */
+
+#include <linux/delay.h>
+#include <linux/io.h>
+#include <linux/notifier.h>
+#include <linux/mfd/syscon.h>
+#include <linux/of.h>
+#include <linux/platform_device.h>
+#include <linux/pm.h>
+#include <linux/reboot.h>
+#include <linux/regmap.h>
+
+struct syscon_poweroff_data {
+	struct regmap *map;
+	u32 offset;
+	u32 value;
+	u32 mask;
+};
+
+static int syscon_poweroff(struct sys_off_data *off_data)
+{
+	struct syscon_poweroff_data *data = off_data->cb_data;
+
+	/* Issue the poweroff */
+	regmap_update_bits(data->map, data->offset, data->mask, data->value);
+
+	mdelay(1000);
+
+	pr_emerg("Unable to poweroff system\n");
+
+	return NOTIFY_DONE;
+}
+
+static int syscon_poweroff_probe(struct platform_device *pdev)
+{
+	struct device *dev = &pdev->dev;
+	struct syscon_poweroff_data *data;
+	int mask_err, value_err;
+
+	data = devm_kzalloc(dev, sizeof(*data), GFP_KERNEL);
+	if (!data)
+		return -ENOMEM;
+
+	data->map = syscon_regmap_lookup_by_phandle(dev->of_node, "regmap");
+	if (IS_ERR(data->map)) {
+		data->map = syscon_node_to_regmap(dev->parent->of_node);
+		if (IS_ERR(data->map)) {
+			dev_err(dev, "unable to get syscon");
+			return PTR_ERR(data->map);
+		}
+	}
+
+	if (of_property_read_u32(dev->of_node, "offset", &data->offset)) {
+		dev_err(dev, "unable to read 'offset'");
+		return -EINVAL;
+	}
+
+	value_err = of_property_read_u32(dev->of_node, "value", &data->value);
+	mask_err = of_property_read_u32(dev->of_node, "mask", &data->mask);
+	if (value_err && mask_err) {
+		dev_err(dev, "unable to read 'value' and 'mask'");
+		return -EINVAL;
+	}
+
+	if (value_err) {
+		/* support old binding */
+		data->value = data->mask;
+		data->mask = 0xFFFFFFFF;
+	} else if (mask_err) {
+		/* support value without mask*/
+		data->mask = 0xFFFFFFFF;
+	}
+
+	return devm_register_sys_off_handler(&pdev->dev,
+					     SYS_OFF_MODE_POWER_OFF,
+					     SYS_OFF_PRIO_DEFAULT,
+					     syscon_poweroff, data);
+}
+
+static const struct of_device_id syscon_poweroff_of_match[] = {
+	{ .compatible = "syscon-poweroff" },
+	{}
+};
+
+static struct platform_driver syscon_poweroff_driver = {
+	.probe = syscon_poweroff_probe,
+	.driver = {
+		.name = "syscon-poweroff",
+		.of_match_table = syscon_poweroff_of_match,
+	},
+};
+builtin_platform_driver(syscon_poweroff_driver);