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, 279 insertions, 0 deletions

diff --git a/drivers/base/soc.c b/drivers/base/soc.c
new file mode 100644
index 000000000..65ce72d49
--- /dev/null
+++ b/drivers/base/soc.c
@@ -0,0 +1,279 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Copyright (C) ST-Ericsson SA 2011
+ *
+ * Author: Lee Jones <lee.jones@linaro.org> for ST-Ericsson.
+ */
+
+#include <linux/err.h>
+#include <linux/glob.h>
+#include <linux/idr.h>
+#include <linux/init.h>
+#include <linux/of.h>
+#include <linux/slab.h>
+#include <linux/spinlock.h>
+#include <linux/stat.h>
+#include <linux/sysfs.h>
+#include <linux/sys_soc.h>
+
+static DEFINE_IDA(soc_ida);
+
+/* Prototype to allow declarations of DEVICE_ATTR(<foo>) before soc_info_show */
+static ssize_t soc_info_show(struct device *dev, struct device_attribute *attr,
+			     char *buf);
+
+struct soc_device {
+	struct device dev;
+	struct soc_device_attribute *attr;
+	int soc_dev_num;
+};
+
+static const struct bus_type soc_bus_type = {
+	.name  = "soc",
+};
+static bool soc_bus_registered;
+
+static DEVICE_ATTR(machine,		0444, soc_info_show,  NULL);
+static DEVICE_ATTR(family,		0444, soc_info_show,  NULL);
+static DEVICE_ATTR(serial_number,	0444, soc_info_show,  NULL);
+static DEVICE_ATTR(soc_id,		0444, soc_info_show,  NULL);
+static DEVICE_ATTR(revision,		0444, soc_info_show,  NULL);
+
+struct device *soc_device_to_device(struct soc_device *soc_dev)
+{
+	return &soc_dev->dev;
+}
+
+static umode_t soc_attribute_mode(struct kobject *kobj,
+				struct attribute *attr,
+				int index)
+{
+	struct device *dev = kobj_to_dev(kobj);
+	struct soc_device *soc_dev = container_of(dev, struct soc_device, dev);
+
+	if ((attr == &dev_attr_machine.attr) && soc_dev->attr->machine)
+		return attr->mode;
+	if ((attr == &dev_attr_family.attr) && soc_dev->attr->family)
+		return attr->mode;
+	if ((attr == &dev_attr_revision.attr) && soc_dev->attr->revision)
+		return attr->mode;
+	if ((attr == &dev_attr_serial_number.attr) && soc_dev->attr->serial_number)
+		return attr->mode;
+	if ((attr == &dev_attr_soc_id.attr) && soc_dev->attr->soc_id)
+		return attr->mode;
+
+	/* Unknown or unfilled attribute */
+	return 0;
+}
+
+static ssize_t soc_info_show(struct device *dev, struct device_attribute *attr,
+			     char *buf)
+{
+	struct soc_device *soc_dev = container_of(dev, struct soc_device, dev);
+	const char *output;
+
+	if (attr == &dev_attr_machine)
+		output = soc_dev->attr->machine;
+	else if (attr == &dev_attr_family)
+		output = soc_dev->attr->family;
+	else if (attr == &dev_attr_revision)
+		output = soc_dev->attr->revision;
+	else if (attr == &dev_attr_serial_number)
+		output = soc_dev->attr->serial_number;
+	else if (attr == &dev_attr_soc_id)
+		output = soc_dev->attr->soc_id;
+	else
+		return -EINVAL;
+
+	return sysfs_emit(buf, "%s\n", output);
+}
+
+static struct attribute *soc_attr[] = {
+	&dev_attr_machine.attr,
+	&dev_attr_family.attr,
+	&dev_attr_serial_number.attr,
+	&dev_attr_soc_id.attr,
+	&dev_attr_revision.attr,
+	NULL,
+};
+
+static const struct attribute_group soc_attr_group = {
+	.attrs = soc_attr,
+	.is_visible = soc_attribute_mode,
+};
+
+static void soc_release(struct device *dev)
+{
+	struct soc_device *soc_dev = container_of(dev, struct soc_device, dev);
+
+	ida_free(&soc_ida, soc_dev->soc_dev_num);
+	kfree(soc_dev->dev.groups);
+	kfree(soc_dev);
+}
+
+int soc_attr_read_machine(struct soc_device_attribute *soc_dev_attr)
+{
+	if (soc_dev_attr->machine)
+		return -EBUSY;
+
+	return of_machine_read_model(&soc_dev_attr->machine);
+}
+EXPORT_SYMBOL_GPL(soc_attr_read_machine);
+
+static struct soc_device_attribute *early_soc_dev_attr;
+
+struct soc_device *soc_device_register(struct soc_device_attribute *soc_dev_attr)
+{
+	struct soc_device *soc_dev;
+	const struct attribute_group **soc_attr_groups;
+	int ret;
+
+	soc_attr_read_machine(soc_dev_attr);
+
+	if (!soc_bus_registered) {
+		if (early_soc_dev_attr)
+			return ERR_PTR(-EBUSY);
+		early_soc_dev_attr = soc_dev_attr;
+		return NULL;
+	}
+
+	soc_dev = kzalloc_obj(*soc_dev);
+	if (!soc_dev) {
+		ret = -ENOMEM;
+		goto out1;
+	}
+
+	soc_attr_groups = kzalloc_objs(*soc_attr_groups, 3);
+	if (!soc_attr_groups) {
+		ret = -ENOMEM;
+		goto out2;
+	}
+	soc_attr_groups[0] = &soc_attr_group;
+	soc_attr_groups[1] = soc_dev_attr->custom_attr_group;
+
+	/* Fetch a unique (reclaimable) SOC ID. */
+	ret = ida_alloc(&soc_ida, GFP_KERNEL);
+	if (ret < 0)
+		goto out3;
+	soc_dev->soc_dev_num = ret;
+
+	soc_dev->attr = soc_dev_attr;
+	soc_dev->dev.bus = &soc_bus_type;
+	soc_dev->dev.groups = soc_attr_groups;
+	soc_dev->dev.release = soc_release;
+
+	dev_set_name(&soc_dev->dev, "soc%d", soc_dev->soc_dev_num);
+
+	ret = device_register(&soc_dev->dev);
+	if (ret) {
+		put_device(&soc_dev->dev);
+		return ERR_PTR(ret);
+	}
+
+	return soc_dev;
+
+out3:
+	kfree(soc_attr_groups);
+out2:
+	kfree(soc_dev);
+out1:
+	return ERR_PTR(ret);
+}
+EXPORT_SYMBOL_GPL(soc_device_register);
+
+/* Ensure soc_dev->attr is freed after calling soc_device_unregister. */
+void soc_device_unregister(struct soc_device *soc_dev)
+{
+	device_unregister(&soc_dev->dev);
+	early_soc_dev_attr = NULL;
+}
+EXPORT_SYMBOL_GPL(soc_device_unregister);
+
+static int __init soc_bus_register(void)
+{
+	int ret;
+
+	ret = bus_register(&soc_bus_type);
+	if (ret)
+		return ret;
+	soc_bus_registered = true;
+
+	if (early_soc_dev_attr)
+		return PTR_ERR(soc_device_register(early_soc_dev_attr));
+
+	return 0;
+}
+core_initcall(soc_bus_register);
+
+static int soc_device_match_attr(const struct soc_device_attribute *attr,
+				 const struct soc_device_attribute *match)
+{
+	if (match->machine &&
+	    (!attr->machine || !glob_match(match->machine, attr->machine)))
+		return 0;
+
+	if (match->family &&
+	    (!attr->family || !glob_match(match->family, attr->family)))
+		return 0;
+
+	if (match->revision &&
+	    (!attr->revision || !glob_match(match->revision, attr->revision)))
+		return 0;
+
+	if (match->soc_id &&
+	    (!attr->soc_id || !glob_match(match->soc_id, attr->soc_id)))
+		return 0;
+
+	return 1;
+}
+
+static int soc_device_match_one(struct device *dev, void *arg)
+{
+	struct soc_device *soc_dev = container_of(dev, struct soc_device, dev);
+
+	return soc_device_match_attr(soc_dev->attr, arg);
+}
+
+/*
+ * soc_device_match - identify the SoC in the machine
+ * @matches: zero-terminated array of possible matches
+ *
+ * returns the first matching entry of the argument array, or NULL
+ * if none of them match.
+ *
+ * This function is meant as a helper in place of of_match_node()
+ * in cases where either no device tree is available or the information
+ * in a device node is insufficient to identify a particular variant
+ * by its compatible strings or other properties. For new devices,
+ * the DT binding should always provide unique compatible strings
+ * that allow the use of of_match_node() instead.
+ *
+ * The calling function can use the .data entry of the
+ * soc_device_attribute to pass a structure or function pointer for
+ * each entry.
+ */
+const struct soc_device_attribute *soc_device_match(
+	const struct soc_device_attribute *matches)
+{
+	int ret;
+
+	if (!matches)
+		return NULL;
+
+	while (matches->machine || matches->family || matches->revision ||
+	       matches->soc_id) {
+		ret = bus_for_each_dev(&soc_bus_type, NULL, (void *)matches,
+				       soc_device_match_one);
+		if (ret < 0 && early_soc_dev_attr)
+			ret = soc_device_match_attr(early_soc_dev_attr,
+						    matches);
+		if (ret < 0)
+			return NULL;
+		if (ret)
+			return matches;
+
+		matches++;
+	}
+	return NULL;
+}
+EXPORT_SYMBOL_GPL(soc_device_match);