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, 299 insertions, 0 deletions

diff --git a/drivers/misc/ds1682.c b/drivers/misc/ds1682.c
new file mode 100644
index 000000000..cb09e0565
--- /dev/null
+++ b/drivers/misc/ds1682.c
@@ -0,0 +1,299 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Dallas Semiconductor DS1682 Elapsed Time Recorder device driver
+ *
+ * Written by: Grant Likely <grant.likely@secretlab.ca>
+ *
+ * Copyright (C) 2007 Secret Lab Technologies Ltd.
+ */
+
+/*
+ * The DS1682 elapsed timer recorder is a simple device that implements
+ * one elapsed time counter, one event counter, an alarm signal and 10
+ * bytes of general purpose EEPROM.
+ *
+ * This driver provides access to the DS1682 counters and user data via
+ * the sysfs.  The following attributes are added to the device node:
+ *     elapsed_time (u32): Total elapsed event time in ms resolution
+ *     alarm_time (u32): When elapsed time exceeds the value in alarm_time,
+ *                       then the alarm pin is asserted.
+ *     event_count (u16): number of times the event pin has gone low.
+ *     eeprom (u8[10]): general purpose EEPROM
+ *
+ * Counter registers and user data are both read/write unless the device
+ * has been write protected.  This driver does not support turning off write
+ * protection.  Once write protection is turned on, it is impossible to
+ * turn it off again, so I have left the feature out of this driver to avoid
+ * accidental enabling, but it is trivial to add write protect support.
+ *
+ */
+
+#include <linux/module.h>
+#include <linux/i2c.h>
+#include <linux/string.h>
+#include <linux/list.h>
+#include <linux/nvmem-provider.h>
+#include <linux/sysfs.h>
+#include <linux/ctype.h>
+#include <linux/hwmon-sysfs.h>
+
+/* Device registers */
+#define DS1682_REG_CONFIG		0x00
+#define DS1682_REG_ALARM		0x01
+#define DS1682_REG_ELAPSED		0x05
+#define DS1682_REG_EVT_CNTR		0x09
+#define DS1682_REG_EEPROM		0x0b
+#define DS1682_REG_RESET		0x1d
+#define DS1682_REG_WRITE_DISABLE	0x1e
+#define DS1682_REG_WRITE_MEM_DISABLE	0x1f
+
+#define DS1682_EEPROM_SIZE		10
+
+/*
+ * Generic counter attributes
+ */
+static ssize_t ds1682_show(struct device *dev, struct device_attribute *attr,
+			   char *buf)
+{
+	struct sensor_device_attribute_2 *sattr = to_sensor_dev_attr_2(attr);
+	struct i2c_client *client = to_i2c_client(dev);
+	unsigned long long val, check;
+	__le32 val_le = 0;
+	int rc;
+
+	dev_dbg(dev, "ds1682_show() called on %s\n", attr->attr.name);
+
+	/* Read the register */
+	rc = i2c_smbus_read_i2c_block_data(client, sattr->index, sattr->nr,
+					   (u8 *)&val_le);
+	if (rc < 0)
+		return -EIO;
+
+	val = le32_to_cpu(val_le);
+
+	if (sattr->index == DS1682_REG_ELAPSED) {
+		int retries = 5;
+
+		/* Detect and retry when a tick occurs mid-read */
+		do {
+			rc = i2c_smbus_read_i2c_block_data(client, sattr->index,
+							   sattr->nr,
+							   (u8 *)&val_le);
+			if (rc < 0 || retries <= 0)
+				return -EIO;
+
+			check = val;
+			val = le32_to_cpu(val_le);
+			retries--;
+		} while (val != check && val != (check + 1));
+	}
+
+	/* Format the output string and return # of bytes
+	 * Special case: the 32 bit regs are time values with 1/4s
+	 * resolution, scale them up to milliseconds
+	 */
+	return sprintf(buf, "%llu\n", (sattr->nr == 4) ? (val * 250) : val);
+}
+
+static ssize_t ds1682_store(struct device *dev, struct device_attribute *attr,
+			    const char *buf, size_t count)
+{
+	struct sensor_device_attribute_2 *sattr = to_sensor_dev_attr_2(attr);
+	struct i2c_client *client = to_i2c_client(dev);
+	u64 val;
+	__le32 val_le;
+	int rc;
+
+	dev_dbg(dev, "ds1682_store() called on %s\n", attr->attr.name);
+
+	/* Decode input */
+	rc = kstrtoull(buf, 0, &val);
+	if (rc < 0) {
+		dev_dbg(dev, "input string not a number\n");
+		return -EINVAL;
+	}
+
+	/* Special case: the 32 bit regs are time values with 1/4s
+	 * resolution, scale input down to quarter-seconds */
+	if (sattr->nr == 4)
+		do_div(val, 250);
+
+	/* write out the value */
+	val_le = cpu_to_le32(val);
+	rc = i2c_smbus_write_i2c_block_data(client, sattr->index, sattr->nr,
+					    (u8 *) & val_le);
+	if (rc < 0) {
+		dev_err(dev, "register write failed; reg=0x%x, size=%i\n",
+			sattr->index, sattr->nr);
+		return -EIO;
+	}
+
+	return count;
+}
+
+/*
+ * Simple register attributes
+ */
+static SENSOR_DEVICE_ATTR_2(elapsed_time, S_IRUGO | S_IWUSR, ds1682_show,
+			    ds1682_store, 4, DS1682_REG_ELAPSED);
+static SENSOR_DEVICE_ATTR_2(alarm_time, S_IRUGO | S_IWUSR, ds1682_show,
+			    ds1682_store, 4, DS1682_REG_ALARM);
+static SENSOR_DEVICE_ATTR_2(event_count, S_IRUGO | S_IWUSR, ds1682_show,
+			    ds1682_store, 2, DS1682_REG_EVT_CNTR);
+
+static const struct attribute_group ds1682_group = {
+	.attrs = (struct attribute *[]) {
+		&sensor_dev_attr_elapsed_time.dev_attr.attr,
+		&sensor_dev_attr_alarm_time.dev_attr.attr,
+		&sensor_dev_attr_event_count.dev_attr.attr,
+		NULL,
+	},
+};
+
+/*
+ * User data attribute
+ */
+static ssize_t ds1682_eeprom_read(struct file *filp, struct kobject *kobj,
+				  const struct bin_attribute *attr,
+				  char *buf, loff_t off, size_t count)
+{
+	struct i2c_client *client = kobj_to_i2c_client(kobj);
+	int rc;
+
+	dev_dbg(&client->dev, "ds1682_eeprom_read(p=%p, off=%lli, c=%zi)\n",
+		buf, off, count);
+
+	rc = i2c_smbus_read_i2c_block_data(client, DS1682_REG_EEPROM + off,
+					   count, buf);
+	if (rc < 0)
+		return -EIO;
+
+	return count;
+}
+
+static ssize_t ds1682_eeprom_write(struct file *filp, struct kobject *kobj,
+				   const struct bin_attribute *attr,
+				   char *buf, loff_t off, size_t count)
+{
+	struct i2c_client *client = kobj_to_i2c_client(kobj);
+
+	dev_dbg(&client->dev, "ds1682_eeprom_write(p=%p, off=%lli, c=%zi)\n",
+		buf, off, count);
+
+	/* Write out to the device */
+	if (i2c_smbus_write_i2c_block_data(client, DS1682_REG_EEPROM + off,
+					   count, buf) < 0)
+		return -EIO;
+
+	return count;
+}
+
+static const struct bin_attribute ds1682_eeprom_attr = {
+	.attr = {
+		.name = "eeprom",
+		.mode = S_IRUGO | S_IWUSR,
+	},
+	.size = DS1682_EEPROM_SIZE,
+	.read = ds1682_eeprom_read,
+	.write = ds1682_eeprom_write,
+};
+
+static int ds1682_nvmem_read(void *priv, unsigned int offset, void *val,
+			     size_t bytes)
+{
+	struct i2c_client *client = priv;
+	int ret;
+
+	ret = i2c_smbus_read_i2c_block_data(client, DS1682_REG_EEPROM + offset,
+					    bytes, val);
+	return ret < 0 ? ret : 0;
+}
+
+static int ds1682_nvmem_write(void *priv, unsigned int offset, void *val,
+			      size_t bytes)
+{
+	struct i2c_client *client = priv;
+	int ret;
+
+	ret = i2c_smbus_write_i2c_block_data(client, DS1682_REG_EEPROM + offset,
+					     bytes, val);
+	return ret < 0 ? ret : 0;
+}
+
+/*
+ * Called when a ds1682 device is matched with this driver
+ */
+static int ds1682_probe(struct i2c_client *client)
+{
+	struct nvmem_config config = {
+		.dev = &client->dev,
+		.owner = THIS_MODULE,
+		.type = NVMEM_TYPE_EEPROM,
+		.reg_read = ds1682_nvmem_read,
+		.reg_write = ds1682_nvmem_write,
+		.size = DS1682_EEPROM_SIZE,
+		.priv = client,
+	};
+	struct nvmem_device *nvmem;
+	int rc;
+
+	if (!i2c_check_functionality(client->adapter,
+				     I2C_FUNC_SMBUS_I2C_BLOCK)) {
+		dev_err(&client->dev, "i2c bus does not support the ds1682\n");
+		rc = -ENODEV;
+		goto exit;
+	}
+
+	nvmem = devm_nvmem_register(&client->dev, &config);
+	if (IS_ENABLED(CONFIG_NVMEM) && IS_ERR(nvmem))
+		return PTR_ERR(nvmem);
+
+	rc = sysfs_create_group(&client->dev.kobj, &ds1682_group);
+	if (rc)
+		goto exit;
+
+	rc = sysfs_create_bin_file(&client->dev.kobj, &ds1682_eeprom_attr);
+	if (rc)
+		goto exit_bin_attr;
+
+	return 0;
+
+ exit_bin_attr:
+	sysfs_remove_group(&client->dev.kobj, &ds1682_group);
+ exit:
+	return rc;
+}
+
+static void ds1682_remove(struct i2c_client *client)
+{
+	sysfs_remove_bin_file(&client->dev.kobj, &ds1682_eeprom_attr);
+	sysfs_remove_group(&client->dev.kobj, &ds1682_group);
+}
+
+static const struct i2c_device_id ds1682_id[] = {
+	{ "ds1682" },
+	{ }
+};
+MODULE_DEVICE_TABLE(i2c, ds1682_id);
+
+static const struct of_device_id ds1682_of_match[] = {
+	{ .compatible = "dallas,ds1682", },
+	{}
+};
+MODULE_DEVICE_TABLE(of, ds1682_of_match);
+
+static struct i2c_driver ds1682_driver = {
+	.driver = {
+		.name = "ds1682",
+		.of_match_table = ds1682_of_match,
+	},
+	.probe = ds1682_probe,
+	.remove = ds1682_remove,
+	.id_table = ds1682_id,
+};
+
+module_i2c_driver(ds1682_driver);
+
+MODULE_AUTHOR("Grant Likely <grant.likely@secretlab.ca>");
+MODULE_DESCRIPTION("DS1682 Elapsed Time Indicator driver");
+MODULE_LICENSE("GPL");