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, 69 insertions, 0 deletions

diff --git a/tools/testing/memblock/internal.h b/tools/testing/memblock/internal.h
new file mode 100644
index 000000000..b6b1d147f
--- /dev/null
+++ b/tools/testing/memblock/internal.h
@@ -0,0 +1,69 @@
+/* SPDX-License-Identifier: GPL-2.0-or-later */
+#ifndef _MM_INTERNAL_H
+#define _MM_INTERNAL_H
+
+/*
+ * Enable memblock_dbg() messages
+ */
+#ifdef MEMBLOCK_DEBUG
+static int memblock_debug = 1;
+#endif
+
+#define pr_warn_ratelimited(fmt, ...)    printf(fmt, ##__VA_ARGS__)
+
+#define K(x) ((x) << (PAGE_SHIFT-10))
+
+bool mirrored_kernelcore = false;
+
+struct page {};
+static inline void *page_address(struct page *page)
+{
+	BUG();
+	return page;
+}
+
+static inline struct page *virt_to_page(void *virt)
+{
+	BUG();
+	return virt;
+}
+
+void memblock_free_pages(unsigned long pfn, unsigned int order)
+{
+}
+
+static inline void accept_memory(phys_addr_t start, unsigned long size)
+{
+}
+
+unsigned long free_reserved_area(void *start, void *end, int poison, const char *s);
+void free_reserved_page(struct page *page);
+
+static inline bool deferred_pages_enabled(void)
+{
+	return false;
+}
+
+#define for_each_valid_pfn(pfn, start_pfn, end_pfn)			 \
+	for ((pfn) = (start_pfn); (pfn) < (end_pfn); (pfn)++)
+
+static inline void *kasan_reset_tag(const void *addr)
+{
+	return (void *)addr;
+}
+
+static inline bool __is_kernel(unsigned long addr)
+{
+	return false;
+}
+
+#define for_each_valid_pfn(pfn, start_pfn, end_pfn)                     \
+       for ((pfn) = (start_pfn); (pfn) < (end_pfn); (pfn)++)
+
+static inline void init_deferred_page(unsigned long pfn, int nid)
+{
+}
+
+#define __SetPageReserved(p)	((void)(p))
+
+#endif