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, 319 insertions, 0 deletions

diff --git a/drivers/dax/kmem.c b/drivers/dax/kmem.c
new file mode 100644
index 000000000..a18e2b968
--- /dev/null
+++ b/drivers/dax/kmem.c
@@ -0,0 +1,319 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright(c) 2016-2019 Intel Corporation. All rights reserved. */
+#include <linux/memremap.h>
+#include <linux/pagemap.h>
+#include <linux/memory.h>
+#include <linux/module.h>
+#include <linux/device.h>
+#include <linux/slab.h>
+#include <linux/dax.h>
+#include <linux/fs.h>
+#include <linux/mm.h>
+#include <linux/mman.h>
+#include <linux/memory-tiers.h>
+#include <linux/memory_hotplug.h>
+#include <linux/string_helpers.h>
+#include "dax-private.h"
+#include "bus.h"
+
+/*
+ * Default abstract distance assigned to the NUMA node onlined
+ * by DAX/kmem if the low level platform driver didn't initialize
+ * one for this NUMA node.
+ */
+#define MEMTIER_DEFAULT_DAX_ADISTANCE	(MEMTIER_ADISTANCE_DRAM * 5)
+
+/* Memory resource name used for add_memory_driver_managed(). */
+static const char *kmem_name;
+/* Set if any memory will remain added when the driver will be unloaded. */
+static bool any_hotremove_failed;
+
+static int dax_kmem_range(struct dev_dax *dev_dax, int i, struct range *r)
+{
+	struct dev_dax_range *dax_range = &dev_dax->ranges[i];
+	struct range *range = &dax_range->range;
+
+	/* memory-block align the hotplug range */
+	r->start = ALIGN(range->start, memory_block_size_bytes());
+	r->end = ALIGN_DOWN(range->end + 1, memory_block_size_bytes()) - 1;
+	if (r->start >= r->end) {
+		r->start = range->start;
+		r->end = range->end;
+		return -ENOSPC;
+	}
+	return 0;
+}
+
+struct dax_kmem_data {
+	const char *res_name;
+	int mgid;
+	struct resource *res[];
+};
+
+static DEFINE_MUTEX(kmem_memory_type_lock);
+static LIST_HEAD(kmem_memory_types);
+
+static struct memory_dev_type *kmem_find_alloc_memory_type(int adist)
+{
+	guard(mutex)(&kmem_memory_type_lock);
+	return mt_find_alloc_memory_type(adist, &kmem_memory_types);
+}
+
+static void kmem_put_memory_types(void)
+{
+	guard(mutex)(&kmem_memory_type_lock);
+	mt_put_memory_types(&kmem_memory_types);
+}
+
+static int dev_dax_kmem_probe(struct dev_dax *dev_dax)
+{
+	struct device *dev = &dev_dax->dev;
+	unsigned long total_len = 0, orig_len = 0;
+	struct dax_kmem_data *data;
+	struct memory_dev_type *mtype;
+	int i, rc, mapped = 0;
+	mhp_t mhp_flags;
+	int numa_node;
+	int adist = MEMTIER_DEFAULT_DAX_ADISTANCE;
+
+	/*
+	 * Ensure good NUMA information for the persistent memory.
+	 * Without this check, there is a risk that slow memory
+	 * could be mixed in a node with faster memory, causing
+	 * unavoidable performance issues.
+	 */
+	numa_node = dev_dax->target_node;
+	if (numa_node < 0) {
+		dev_warn(dev, "rejecting DAX region with invalid node: %d\n",
+				numa_node);
+		return -EINVAL;
+	}
+
+	mt_calc_adistance(numa_node, &adist);
+	mtype = kmem_find_alloc_memory_type(adist);
+	if (IS_ERR(mtype))
+		return PTR_ERR(mtype);
+
+	for (i = 0; i < dev_dax->nr_range; i++) {
+		struct range range;
+
+		orig_len += range_len(&dev_dax->ranges[i].range);
+		rc = dax_kmem_range(dev_dax, i, &range);
+		if (rc) {
+			dev_info(dev, "mapping%d: %#llx-%#llx too small after alignment\n",
+					i, range.start, range.end);
+			continue;
+		}
+		total_len += range_len(&range);
+	}
+
+	if (!total_len) {
+		dev_warn(dev, "rejecting DAX region without any memory after alignment\n");
+		return -EINVAL;
+	} else if (total_len != orig_len) {
+		char buf[16];
+
+		string_get_size(orig_len - total_len, 1, STRING_UNITS_2,
+				buf, sizeof(buf));
+		dev_warn(dev, "DAX region truncated by %s due to alignment\n", buf);
+	}
+
+	init_node_memory_type(numa_node, mtype);
+
+	rc = -ENOMEM;
+	data = kzalloc_flex(*data, res, dev_dax->nr_range);
+	if (!data)
+		goto err_dax_kmem_data;
+
+	data->res_name = kstrdup(dev_name(dev), GFP_KERNEL);
+	if (!data->res_name)
+		goto err_res_name;
+
+	rc = memory_group_register_static(numa_node, PFN_UP(total_len));
+	if (rc < 0)
+		goto err_reg_mgid;
+	data->mgid = rc;
+
+	for (i = 0; i < dev_dax->nr_range; i++) {
+		struct resource *res;
+		struct range range;
+
+		rc = dax_kmem_range(dev_dax, i, &range);
+		if (rc)
+			continue;
+
+		/* Region is permanently reserved if hotremove fails. */
+		res = request_mem_region(range.start, range_len(&range), data->res_name);
+		if (!res) {
+			dev_warn(dev, "mapping%d: %#llx-%#llx could not reserve region\n",
+					i, range.start, range.end);
+			/*
+			 * Once some memory has been onlined we can't
+			 * assume that it can be un-onlined safely.
+			 */
+			if (mapped)
+				continue;
+			rc = -EBUSY;
+			goto err_request_mem;
+		}
+		data->res[i] = res;
+
+		/*
+		 * Set flags appropriate for System RAM.  Leave ..._BUSY clear
+		 * so that add_memory() can add a child resource.  Do not
+		 * inherit flags from the parent since it may set new flags
+		 * unknown to us that will break add_memory() below.
+		 */
+		res->flags = IORESOURCE_SYSTEM_RAM;
+
+		mhp_flags = MHP_NID_IS_MGID;
+		if (dev_dax->memmap_on_memory)
+			mhp_flags |= MHP_MEMMAP_ON_MEMORY;
+
+		/*
+		 * Ensure that future kexec'd kernels will not treat
+		 * this as RAM automatically.
+		 */
+		rc = add_memory_driver_managed(data->mgid, range.start,
+				range_len(&range), kmem_name, mhp_flags);
+
+		if (rc) {
+			dev_warn(dev, "mapping%d: %#llx-%#llx memory add failed\n",
+					i, range.start, range.end);
+			remove_resource(res);
+			kfree(res);
+			data->res[i] = NULL;
+			if (mapped)
+				continue;
+			goto err_request_mem;
+		}
+		mapped++;
+	}
+
+	dev_set_drvdata(dev, data);
+
+	return 0;
+
+err_request_mem:
+	memory_group_unregister(data->mgid);
+err_reg_mgid:
+	kfree(data->res_name);
+err_res_name:
+	kfree(data);
+err_dax_kmem_data:
+	clear_node_memory_type(numa_node, mtype);
+	return rc;
+}
+
+#ifdef CONFIG_MEMORY_HOTREMOVE
+static void dev_dax_kmem_remove(struct dev_dax *dev_dax)
+{
+	int i, success = 0;
+	int node = dev_dax->target_node;
+	struct device *dev = &dev_dax->dev;
+	struct dax_kmem_data *data = dev_get_drvdata(dev);
+
+	/*
+	 * We have one shot for removing memory, if some memory blocks were not
+	 * offline prior to calling this function remove_memory() will fail, and
+	 * there is no way to hotremove this memory until reboot because device
+	 * unbind will succeed even if we return failure.
+	 */
+	for (i = 0; i < dev_dax->nr_range; i++) {
+		struct range range;
+		int rc;
+
+		rc = dax_kmem_range(dev_dax, i, &range);
+		if (rc)
+			continue;
+
+		/* range was never added during probe */
+		if (!data->res[i]) {
+			success++;
+			continue;
+		}
+
+		rc = remove_memory(range.start, range_len(&range));
+		if (rc == 0) {
+			remove_resource(data->res[i]);
+			kfree(data->res[i]);
+			data->res[i] = NULL;
+			success++;
+			continue;
+		}
+		any_hotremove_failed = true;
+		dev_err(dev,
+			"mapping%d: %#llx-%#llx cannot be hotremoved until the next reboot\n",
+				i, range.start, range.end);
+	}
+
+	if (success >= dev_dax->nr_range) {
+		memory_group_unregister(data->mgid);
+		kfree(data->res_name);
+		kfree(data);
+		dev_set_drvdata(dev, NULL);
+		/*
+		 * Clear the memtype association on successful unplug.
+		 * If not, we have memory blocks left which can be
+		 * offlined/onlined later. We need to keep memory_dev_type
+		 * for that. This implies this reference will be around
+		 * till next reboot.
+		 */
+		clear_node_memory_type(node, NULL);
+	}
+}
+#else
+static void dev_dax_kmem_remove(struct dev_dax *dev_dax)
+{
+	/*
+	 * Without hotremove purposely leak the request_mem_region() for the
+	 * device-dax range and return '0' to ->remove() attempts. The removal
+	 * of the device from the driver always succeeds, but the region is
+	 * permanently pinned as reserved by the unreleased
+	 * request_mem_region().
+	 */
+	any_hotremove_failed = true;
+}
+#endif /* CONFIG_MEMORY_HOTREMOVE */
+
+static struct dax_device_driver device_dax_kmem_driver = {
+	.probe = dev_dax_kmem_probe,
+	.remove = dev_dax_kmem_remove,
+	.type = DAXDRV_KMEM_TYPE,
+};
+
+static int __init dax_kmem_init(void)
+{
+	int rc;
+
+	/* Resource name is permanently allocated if any hotremove fails. */
+	kmem_name = kstrdup_const("System RAM (kmem)", GFP_KERNEL);
+	if (!kmem_name)
+		return -ENOMEM;
+
+	rc = dax_driver_register(&device_dax_kmem_driver);
+	if (rc)
+		goto error_dax_driver;
+
+	return rc;
+
+error_dax_driver:
+	kmem_put_memory_types();
+	kfree_const(kmem_name);
+	return rc;
+}
+
+static void __exit dax_kmem_exit(void)
+{
+	dax_driver_unregister(&device_dax_kmem_driver);
+	if (!any_hotremove_failed)
+		kfree_const(kmem_name);
+	kmem_put_memory_types();
+}
+
+MODULE_AUTHOR("Intel Corporation");
+MODULE_DESCRIPTION("KMEM DAX: map dax-devices as System-RAM");
+MODULE_LICENSE("GPL v2");
+module_init(dax_kmem_init);
+module_exit(dax_kmem_exit);
+MODULE_ALIAS_DAX_DEVICE(0);