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, 99 insertions, 0 deletions

diff --git a/drivers/dax/pmem.c b/drivers/dax/pmem.c
new file mode 100644
index 000000000..bee93066a
--- /dev/null
+++ b/drivers/dax/pmem.c
@@ -0,0 +1,99 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright(c) 2016 - 2018 Intel Corporation. All rights reserved. */
+#include <linux/memremap.h>
+#include <linux/module.h>
+#include "../nvdimm/pfn.h"
+#include "../nvdimm/nd.h"
+#include "bus.h"
+
+static struct dev_dax *__dax_pmem_probe(struct device *dev)
+{
+	struct range range;
+	int rc, id, region_id;
+	resource_size_t offset;
+	struct nd_pfn_sb *pfn_sb;
+	struct dev_dax_data data;
+	struct nd_namespace_io *nsio;
+	struct dax_region *dax_region;
+	struct dev_pagemap pgmap = { };
+	struct nd_namespace_common *ndns;
+	struct nd_dax *nd_dax = to_nd_dax(dev);
+	struct nd_pfn *nd_pfn = &nd_dax->nd_pfn;
+	struct nd_region *nd_region = to_nd_region(dev->parent);
+
+	ndns = nvdimm_namespace_common_probe(dev);
+	if (IS_ERR(ndns))
+		return ERR_CAST(ndns);
+
+	/* parse the 'pfn' info block via ->rw_bytes */
+	rc = devm_namespace_enable(dev, ndns, nd_info_block_reserve());
+	if (rc)
+		return ERR_PTR(rc);
+	rc = nvdimm_setup_pfn(nd_pfn, &pgmap);
+	if (rc)
+		return ERR_PTR(rc);
+	devm_namespace_disable(dev, ndns);
+
+	/* reserve the metadata area, device-dax will reserve the data */
+	pfn_sb = nd_pfn->pfn_sb;
+	offset = le64_to_cpu(pfn_sb->dataoff);
+	nsio = to_nd_namespace_io(&ndns->dev);
+	if (!devm_request_mem_region(dev, nsio->res.start, offset,
+				dev_name(&ndns->dev))) {
+		dev_warn(dev, "could not reserve metadata\n");
+		return ERR_PTR(-EBUSY);
+	}
+
+	rc = sscanf(dev_name(&ndns->dev), "namespace%d.%d", &region_id, &id);
+	if (rc != 2)
+		return ERR_PTR(-EINVAL);
+
+	/* adjust the dax_region range to the start of data */
+	range = pgmap.range;
+	range.start += offset;
+	dax_region = alloc_dax_region(dev, region_id, &range,
+			nd_region->target_node, le32_to_cpu(pfn_sb->align),
+			IORESOURCE_DAX_STATIC);
+	if (!dax_region)
+		return ERR_PTR(-ENOMEM);
+
+	data = (struct dev_dax_data) {
+		.dax_region = dax_region,
+		.id = id,
+		.pgmap = &pgmap,
+		.size = range_len(&range),
+		.memmap_on_memory = false,
+	};
+
+	return devm_create_dev_dax(&data);
+}
+
+static int dax_pmem_probe(struct device *dev)
+{
+	return PTR_ERR_OR_ZERO(__dax_pmem_probe(dev));
+}
+
+static struct nd_device_driver dax_pmem_driver = {
+	.probe = dax_pmem_probe,
+	.drv = {
+		.name = "dax_pmem",
+	},
+	.type = ND_DRIVER_DAX_PMEM,
+};
+
+static int __init dax_pmem_init(void)
+{
+	return nd_driver_register(&dax_pmem_driver);
+}
+module_init(dax_pmem_init);
+
+static void __exit dax_pmem_exit(void)
+{
+	driver_unregister(&dax_pmem_driver.drv);
+}
+module_exit(dax_pmem_exit);
+
+MODULE_DESCRIPTION("PMEM DAX: direct access to persistent memory");
+MODULE_LICENSE("GPL v2");
+MODULE_AUTHOR("Intel Corporation");
+MODULE_ALIAS_ND_DEVICE(ND_DEVICE_DAX_PMEM);