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, 143 insertions, 0 deletions

diff --git a/drivers/misc/hmc6352.c b/drivers/misc/hmc6352.c
new file mode 100644
index 000000000..ff92c6edf
--- /dev/null
+++ b/drivers/misc/hmc6352.c
@@ -0,0 +1,143 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * hmc6352.c - Honeywell Compass Driver
+ *
+ * Copyright (C) 2009 Intel Corp
+ *
+ *  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ *
+ * ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ */
+
+#include <linux/module.h>
+#include <linux/slab.h>
+#include <linux/i2c.h>
+#include <linux/err.h>
+#include <linux/delay.h>
+#include <linux/sysfs.h>
+#include <linux/nospec.h>
+
+static DEFINE_MUTEX(compass_mutex);
+
+static int compass_command(struct i2c_client *c, u8 cmd)
+{
+	int ret = i2c_master_send(c, &cmd, 1);
+	if (ret < 0)
+		dev_warn(&c->dev, "command '%c' failed.\n", cmd);
+	return ret;
+}
+
+static int compass_store(struct device *dev, const char *buf, size_t count,
+			const char *map)
+{
+	struct i2c_client *c = to_i2c_client(dev);
+	int ret;
+	unsigned long val;
+
+	ret = kstrtoul(buf, 10, &val);
+	if (ret)
+		return ret;
+	if (val >= strlen(map))
+		return -EINVAL;
+	val = array_index_nospec(val, strlen(map));
+	mutex_lock(&compass_mutex);
+	ret = compass_command(c, map[val]);
+	mutex_unlock(&compass_mutex);
+	if (ret < 0)
+		return ret;
+	return count;
+}
+
+static ssize_t compass_calibration_store(struct device *dev,
+		struct device_attribute *attr, const char *buf, size_t count)
+{
+	return compass_store(dev, buf, count, "EC");
+}
+
+static ssize_t compass_power_mode_store(struct device *dev,
+		struct device_attribute *attr, const  char *buf, size_t count)
+{
+	return compass_store(dev, buf, count, "SW");
+}
+
+static ssize_t compass_heading_data_show(struct device *dev,
+			struct device_attribute *attr, char *buf)
+{
+	struct i2c_client *client = to_i2c_client(dev);
+	unsigned char i2c_data[2];
+	int ret;
+
+	mutex_lock(&compass_mutex);
+	ret = compass_command(client, 'A');
+	if (ret != 1) {
+		mutex_unlock(&compass_mutex);
+		return ret;
+	}
+	msleep(10); /* sending 'A' cmd we need to wait for 7-10 millisecs */
+	ret = i2c_master_recv(client, i2c_data, 2);
+	mutex_unlock(&compass_mutex);
+	if (ret < 0) {
+		dev_warn(dev, "i2c read data cmd failed\n");
+		return ret;
+	}
+	ret = (i2c_data[0] << 8) | i2c_data[1];
+	return sprintf(buf, "%d.%d\n", ret/10, ret%10);
+}
+
+
+static DEVICE_ATTR(heading0_input, S_IRUGO, compass_heading_data_show, NULL);
+static DEVICE_ATTR(calibration, S_IWUSR, NULL, compass_calibration_store);
+static DEVICE_ATTR(power_state, S_IWUSR, NULL, compass_power_mode_store);
+
+static struct attribute *mid_att_compass[] = {
+	&dev_attr_heading0_input.attr,
+	&dev_attr_calibration.attr,
+	&dev_attr_power_state.attr,
+	NULL
+};
+
+static const struct attribute_group m_compass_gr = {
+	.name = "hmc6352",
+	.attrs = mid_att_compass
+};
+
+static int hmc6352_probe(struct i2c_client *client)
+{
+	int res;
+
+	res = sysfs_create_group(&client->dev.kobj, &m_compass_gr);
+	if (res) {
+		dev_err(&client->dev, "device_create_file failed\n");
+		return res;
+	}
+	dev_info(&client->dev, "%s HMC6352 compass chip found\n",
+							client->name);
+	return 0;
+}
+
+static void hmc6352_remove(struct i2c_client *client)
+{
+	sysfs_remove_group(&client->dev.kobj, &m_compass_gr);
+}
+
+static const struct i2c_device_id hmc6352_id[] = {
+	{ "hmc6352" },
+	{ }
+};
+
+MODULE_DEVICE_TABLE(i2c, hmc6352_id);
+
+static struct i2c_driver hmc6352_driver = {
+	.driver = {
+		.name = "hmc6352",
+	},
+	.probe = hmc6352_probe,
+	.remove = hmc6352_remove,
+	.id_table = hmc6352_id,
+};
+
+module_i2c_driver(hmc6352_driver);
+
+MODULE_AUTHOR("Kalhan Trisal <kalhan.trisal@intel.com");
+MODULE_DESCRIPTION("hmc6352 Compass Driver");
+MODULE_LICENSE("GPL v2");