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, 365 insertions, 0 deletions

diff --git a/lib/test_kho.c b/lib/test_kho.c
new file mode 100644
index 000000000..aa6a0956b
--- /dev/null
+++ b/lib/test_kho.c
@@ -0,0 +1,365 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * Test module for KHO
+ * Copyright (c) 2025 Microsoft Corporation.
+ *
+ * Authors:
+ *   Saurabh Sengar <ssengar@microsoft.com>
+ *   Mike Rapoport <rppt@kernel.org>
+ */
+
+#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
+
+#include <linux/mm.h>
+#include <linux/gfp.h>
+#include <linux/slab.h>
+#include <linux/kexec.h>
+#include <linux/libfdt.h>
+#include <linux/module.h>
+#include <linux/printk.h>
+#include <linux/vmalloc.h>
+#include <linux/kexec_handover.h>
+#include <linux/kho/abi/kexec_handover.h>
+
+#include <net/checksum.h>
+
+#define KHO_TEST_MAGIC	0x4b484f21	/* KHO! */
+#define KHO_TEST_FDT	"kho_test"
+#define KHO_TEST_COMPAT "kho-test-v1"
+
+static long max_mem = (PAGE_SIZE << MAX_PAGE_ORDER) * 2;
+module_param(max_mem, long, 0644);
+
+struct kho_test_state {
+	unsigned int nr_folios;
+	struct folio **folios;
+	phys_addr_t *folios_info;
+	struct kho_vmalloc folios_info_phys;
+	int nr_folios_preserved;
+	struct folio *fdt;
+	__wsum csum;
+};
+
+static struct kho_test_state kho_test_state;
+
+static void kho_test_unpreserve_data(struct kho_test_state *state)
+{
+	for (int i = 0; i < state->nr_folios_preserved; i++)
+		kho_unpreserve_folio(state->folios[i]);
+
+	kho_unpreserve_vmalloc(&state->folios_info_phys);
+	vfree(state->folios_info);
+}
+
+static int kho_test_preserve_data(struct kho_test_state *state)
+{
+	struct kho_vmalloc folios_info_phys;
+	phys_addr_t *folios_info;
+	int err;
+
+	folios_info = vmalloc_array(state->nr_folios, sizeof(*folios_info));
+	if (!folios_info)
+		return -ENOMEM;
+
+	err = kho_preserve_vmalloc(folios_info, &folios_info_phys);
+	if (err)
+		goto err_free_info;
+
+	state->folios_info_phys = folios_info_phys;
+	state->folios_info = folios_info;
+
+	for (int i = 0; i < state->nr_folios; i++) {
+		struct folio *folio = state->folios[i];
+		unsigned int order = folio_order(folio);
+
+		folios_info[i] = virt_to_phys(folio_address(folio)) | order;
+		err = kho_preserve_folio(folio);
+		if (err)
+			goto err_unpreserve;
+		state->nr_folios_preserved++;
+	}
+
+	return 0;
+
+err_unpreserve:
+	/*
+	 * kho_test_unpreserve_data frees folio_info, bail out immediately to
+	 * avoid double free
+	 */
+	kho_test_unpreserve_data(state);
+	return err;
+
+err_free_info:
+	vfree(folios_info);
+	return err;
+}
+
+static int kho_test_prepare_fdt(struct kho_test_state *state, ssize_t fdt_size)
+{
+	const char compatible[] = KHO_TEST_COMPAT;
+	unsigned int magic = KHO_TEST_MAGIC;
+	void *fdt = folio_address(state->fdt);
+	int err;
+
+	err = fdt_create(fdt, fdt_size);
+	err |= fdt_finish_reservemap(fdt);
+	err |= fdt_begin_node(fdt, "");
+	err |= fdt_property(fdt, "compatible", compatible, sizeof(compatible));
+	err |= fdt_property(fdt, "magic", &magic, sizeof(magic));
+
+	err |= fdt_begin_node(fdt, "data");
+	err |= fdt_property(fdt, "nr_folios", &state->nr_folios,
+			    sizeof(state->nr_folios));
+	err |= fdt_property(fdt, "folios_info", &state->folios_info_phys,
+			    sizeof(state->folios_info_phys));
+	err |= fdt_property(fdt, "csum", &state->csum, sizeof(state->csum));
+	err |= fdt_end_node(fdt);
+
+	err |= fdt_end_node(fdt);
+	err |= fdt_finish(fdt);
+
+	return err;
+}
+
+static int kho_test_preserve(struct kho_test_state *state)
+{
+	ssize_t fdt_size;
+	int err;
+
+	fdt_size = state->nr_folios * sizeof(phys_addr_t) + PAGE_SIZE;
+	state->fdt = folio_alloc(GFP_KERNEL, get_order(fdt_size));
+	if (!state->fdt)
+		return -ENOMEM;
+
+	err = kho_preserve_folio(state->fdt);
+	if (err)
+		goto err_free_fdt;
+
+	err = kho_test_preserve_data(state);
+	if (err)
+		goto err_unpreserve_fdt;
+
+	err = kho_test_prepare_fdt(state, fdt_size);
+	if (err)
+		goto err_unpreserve_data;
+
+	err = kho_add_subtree(KHO_TEST_FDT, folio_address(state->fdt),
+			      fdt_totalsize(folio_address(state->fdt)));
+	if (err)
+		goto err_unpreserve_data;
+
+	return 0;
+
+err_unpreserve_data:
+	kho_test_unpreserve_data(state);
+err_unpreserve_fdt:
+	kho_unpreserve_folio(state->fdt);
+err_free_fdt:
+	folio_put(state->fdt);
+	return err;
+}
+
+static int kho_test_generate_data(struct kho_test_state *state)
+{
+	size_t alloc_size = 0;
+	__wsum csum = 0;
+
+	while (alloc_size < max_mem) {
+		int order = get_random_u32() % NR_PAGE_ORDERS;
+		struct folio *folio;
+		unsigned int size;
+		void *addr;
+
+		/*
+		 * Since get_order() rounds up, make sure that actual
+		 * allocation is smaller so that we won't exceed max_mem
+		 */
+		if (alloc_size + (PAGE_SIZE << order) > max_mem) {
+			order = get_order(max_mem - alloc_size);
+			if (order)
+				order--;
+		}
+		size = PAGE_SIZE << order;
+
+		folio = folio_alloc(GFP_KERNEL | __GFP_NORETRY, order);
+		if (!folio)
+			goto err_free_folios;
+
+		state->folios[state->nr_folios++] = folio;
+		addr = folio_address(folio);
+		get_random_bytes(addr, size);
+		csum = csum_partial(addr, size, csum);
+		alloc_size += size;
+	}
+
+	state->csum = csum;
+	return 0;
+
+err_free_folios:
+	for (int i = 0; i < state->nr_folios; i++)
+		folio_put(state->folios[i]);
+	state->nr_folios = 0;
+	return -ENOMEM;
+}
+
+static int kho_test_save(void)
+{
+	struct kho_test_state *state = &kho_test_state;
+	struct folio **folios;
+	unsigned long max_nr;
+	int err;
+
+	max_mem = PAGE_ALIGN(max_mem);
+	max_nr = max_mem >> PAGE_SHIFT;
+
+	folios = kvmalloc_objs(*state->folios, max_nr);
+	if (!folios)
+		return -ENOMEM;
+	state->folios = folios;
+
+	err = kho_test_generate_data(state);
+	if (err)
+		goto err_free_folios;
+
+	err = kho_test_preserve(state);
+	if (err)
+		goto err_free_folios;
+
+	return 0;
+
+err_free_folios:
+	kvfree(folios);
+	return err;
+}
+
+static int kho_test_restore_data(const void *fdt, int node)
+{
+	const struct kho_vmalloc *folios_info_phys;
+	const unsigned int *nr_folios;
+	phys_addr_t *folios_info;
+	const __wsum *old_csum;
+	__wsum csum = 0;
+	int len;
+
+	node = fdt_path_offset(fdt, "/data");
+
+	nr_folios = fdt_getprop(fdt, node, "nr_folios", &len);
+	if (!nr_folios || len != sizeof(*nr_folios))
+		return -EINVAL;
+
+	old_csum = fdt_getprop(fdt, node, "csum", &len);
+	if (!old_csum || len != sizeof(*old_csum))
+		return -EINVAL;
+
+	folios_info_phys = fdt_getprop(fdt, node, "folios_info", &len);
+	if (!folios_info_phys || len != sizeof(*folios_info_phys))
+		return -EINVAL;
+
+	folios_info = kho_restore_vmalloc(folios_info_phys);
+	if (!folios_info)
+		return -EINVAL;
+
+	for (int i = 0; i < *nr_folios; i++) {
+		unsigned int order = folios_info[i] & ~PAGE_MASK;
+		phys_addr_t phys = folios_info[i] & PAGE_MASK;
+		unsigned int size = PAGE_SIZE << order;
+		struct folio *folio;
+
+		folio = kho_restore_folio(phys);
+		if (!folio)
+			break;
+
+		if (folio_order(folio) != order)
+			break;
+
+		csum = csum_partial(folio_address(folio), size, csum);
+		folio_put(folio);
+	}
+
+	vfree(folios_info);
+
+	if (csum != *old_csum)
+		return -EINVAL;
+
+	return 0;
+}
+
+static int kho_test_restore(phys_addr_t fdt_phys)
+{
+	void *fdt = phys_to_virt(fdt_phys);
+	const unsigned int *magic;
+	int node, len, err;
+
+	node = fdt_path_offset(fdt, "/");
+	if (node < 0)
+		return -EINVAL;
+
+	if (fdt_node_check_compatible(fdt, node, KHO_TEST_COMPAT))
+		return -EINVAL;
+
+	magic = fdt_getprop(fdt, node, "magic", &len);
+	if (!magic || len != sizeof(*magic))
+		return -EINVAL;
+
+	if (*magic != KHO_TEST_MAGIC)
+		return -EINVAL;
+
+	err = kho_test_restore_data(fdt, node);
+	if (err)
+		return err;
+
+	return 0;
+}
+
+static int __init kho_test_init(void)
+{
+	phys_addr_t fdt_phys;
+	int err;
+
+	if (!kho_is_enabled())
+		return 0;
+
+	err = kho_retrieve_subtree(KHO_TEST_FDT, &fdt_phys, NULL);
+	if (!err) {
+		err = kho_test_restore(fdt_phys);
+		if (err)
+			pr_err("KHO restore failed\n");
+		else
+			pr_info("KHO restore succeeded\n");
+
+		return err;
+	}
+
+	if (err != -ENOENT) {
+		pr_warn("failed to retrieve %s FDT: %d\n", KHO_TEST_FDT, err);
+		return err;
+	}
+
+	return kho_test_save();
+}
+module_init(kho_test_init);
+
+static void kho_test_cleanup(void)
+{
+	/* unpreserve and free the data stored in folios */
+	kho_test_unpreserve_data(&kho_test_state);
+	for (int i = 0; i < kho_test_state.nr_folios; i++)
+		folio_put(kho_test_state.folios[i]);
+
+	kvfree(kho_test_state.folios);
+
+	/* Unpreserve and release the FDT folio */
+	kho_unpreserve_folio(kho_test_state.fdt);
+	folio_put(kho_test_state.fdt);
+}
+
+static void __exit kho_test_exit(void)
+{
+	kho_remove_subtree(folio_address(kho_test_state.fdt));
+	kho_test_cleanup();
+}
+module_exit(kho_test_exit);
+
+MODULE_AUTHOR("Mike Rapoport <rppt@kernel.org>");
+MODULE_DESCRIPTION("KHO test module");
+MODULE_LICENSE("GPL");