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, 218 insertions, 0 deletions

diff --git a/arch/mips/mm/cache.c b/arch/mips/mm/cache.c
new file mode 100644
index 000000000..ad9b0430a
--- /dev/null
+++ b/arch/mips/mm/cache.c
@@ -0,0 +1,218 @@
+/*
+ * This file is subject to the terms and conditions of the GNU General Public
+ * License.  See the file "COPYING" in the main directory of this archive
+ * for more details.
+ *
+ * Copyright (C) 1994 - 2003, 06, 07 by Ralf Baechle (ralf@linux-mips.org)
+ * Copyright (C) 2007 MIPS Technologies, Inc.
+ */
+#include <linux/fs.h>
+#include <linux/fcntl.h>
+#include <linux/kernel.h>
+#include <linux/linkage.h>
+#include <linux/export.h>
+#include <linux/sched.h>
+#include <linux/syscalls.h>
+#include <linux/mm.h>
+#include <linux/highmem.h>
+#include <linux/pagemap.h>
+
+#include <asm/bcache.h>
+#include <asm/cacheflush.h>
+#include <asm/processor.h>
+#include <asm/cpu.h>
+#include <asm/cpu-features.h>
+#include <asm/setup.h>
+#include <asm/pgtable.h>
+
+/* Cache operations. */
+void (*flush_cache_all)(void);
+void (*__flush_cache_all)(void);
+EXPORT_SYMBOL_GPL(__flush_cache_all);
+void (*flush_cache_mm)(struct mm_struct *mm);
+void (*flush_cache_range)(struct vm_area_struct *vma, unsigned long start,
+	unsigned long end);
+void (*flush_cache_page)(struct vm_area_struct *vma, unsigned long page,
+	unsigned long pfn);
+void (*flush_icache_range)(unsigned long start, unsigned long end);
+EXPORT_SYMBOL_GPL(flush_icache_range);
+void (*local_flush_icache_range)(unsigned long start, unsigned long end);
+EXPORT_SYMBOL_GPL(local_flush_icache_range);
+void (*__flush_icache_user_range)(unsigned long start, unsigned long end);
+void (*__local_flush_icache_user_range)(unsigned long start, unsigned long end);
+EXPORT_SYMBOL_GPL(__local_flush_icache_user_range);
+
+void (*__flush_cache_vmap)(void);
+void (*__flush_cache_vunmap)(void);
+
+void (*__flush_kernel_vmap_range)(unsigned long vaddr, int size);
+EXPORT_SYMBOL_GPL(__flush_kernel_vmap_range);
+
+/* MIPS specific cache operations */
+void (*flush_data_cache_page)(unsigned long addr);
+void (*flush_icache_all)(void);
+
+EXPORT_SYMBOL(flush_data_cache_page);
+EXPORT_SYMBOL(flush_icache_all);
+
+/*
+ * Dummy cache handling routine
+ */
+
+void cache_noop(void) {}
+
+#ifdef CONFIG_BOARD_SCACHE
+
+static struct bcache_ops no_sc_ops = {
+	.bc_enable = (void *)cache_noop,
+	.bc_disable = (void *)cache_noop,
+	.bc_wback_inv = (void *)cache_noop,
+	.bc_inv = (void *)cache_noop
+};
+
+struct bcache_ops *bcops = &no_sc_ops;
+#endif
+
+#ifdef CONFIG_DMA_NONCOHERENT
+
+/* DMA cache operations. */
+void (*_dma_cache_wback_inv)(unsigned long start, unsigned long size);
+void (*_dma_cache_wback)(unsigned long start, unsigned long size);
+void (*_dma_cache_inv)(unsigned long start, unsigned long size);
+
+#endif /* CONFIG_DMA_NONCOHERENT */
+
+/*
+ * We could optimize the case where the cache argument is not BCACHE but
+ * that seems very atypical use ...
+ */
+SYSCALL_DEFINE3(cacheflush, unsigned long, addr, unsigned long, bytes,
+	unsigned int, cache)
+{
+	if (bytes == 0)
+		return 0;
+	if (!access_ok((void __user *) addr, bytes))
+		return -EFAULT;
+
+	__flush_icache_user_range(addr, addr + bytes);
+
+	return 0;
+}
+
+void __flush_dcache_folio_pages(struct folio *folio, struct page *page,
+		unsigned int nr)
+{
+	struct address_space *mapping = folio_flush_mapping(folio);
+	unsigned long addr;
+	unsigned int i;
+
+	if (mapping && !mapping_mapped(mapping)) {
+		folio_set_dcache_dirty(folio);
+		return;
+	}
+
+	/*
+	 * We could delay the flush for the !folio_mapping case too.  But that
+	 * case is for exec env/arg pages and those are %99 certainly going to
+	 * get faulted into the tlb (and thus flushed) anyways.
+	 */
+	for (i = 0; i < nr; i++) {
+		addr = (unsigned long)kmap_local_page(page + i);
+		flush_data_cache_page(addr);
+		kunmap_local((void *)addr);
+	}
+}
+EXPORT_SYMBOL(__flush_dcache_folio_pages);
+
+void __flush_anon_page(struct page *page, unsigned long vmaddr)
+{
+	unsigned long addr = (unsigned long) page_address(page);
+	struct folio *folio = page_folio(page);
+
+	if (pages_do_alias(addr, vmaddr)) {
+		if (folio_mapped(folio) && !folio_test_dcache_dirty(folio)) {
+			void *kaddr;
+
+			kaddr = kmap_coherent(page, vmaddr);
+			flush_data_cache_page((unsigned long)kaddr);
+			kunmap_coherent();
+		} else
+			flush_data_cache_page(addr);
+	}
+}
+
+EXPORT_SYMBOL(__flush_anon_page);
+
+void __update_cache(unsigned long address, pte_t pte)
+{
+	struct folio *folio;
+	unsigned long pfn, addr;
+	int exec = !pte_no_exec(pte) && !cpu_has_ic_fills_f_dc;
+	unsigned int i;
+
+	pfn = pte_pfn(pte);
+	if (unlikely(!pfn_valid(pfn)))
+		return;
+
+	folio = page_folio(pfn_to_page(pfn));
+	address &= PAGE_MASK;
+	address -= offset_in_folio(folio, pfn << PAGE_SHIFT);
+
+	if (folio_test_dcache_dirty(folio)) {
+		for (i = 0; i < folio_nr_pages(folio); i++) {
+			addr = (unsigned long)kmap_local_folio(folio, i);
+
+			if (exec || pages_do_alias(addr, address))
+				flush_data_cache_page(addr);
+			kunmap_local((void *)addr);
+			address += PAGE_SIZE;
+		}
+		folio_clear_dcache_dirty(folio);
+	}
+}
+
+unsigned long _page_cachable_default;
+EXPORT_SYMBOL(_page_cachable_default);
+
+#define PM(p)	__pgprot(_page_cachable_default | (p))
+
+static pgprot_t protection_map[16] __ro_after_init;
+DECLARE_VM_GET_PAGE_PROT
+
+static inline void setup_protection_map(void)
+{
+	protection_map[0]  = PM(_PAGE_PRESENT | _PAGE_NO_EXEC | _PAGE_NO_READ);
+	protection_map[1]  = PM(_PAGE_PRESENT | _PAGE_NO_EXEC);
+	protection_map[2]  = PM(_PAGE_PRESENT | _PAGE_NO_EXEC | _PAGE_NO_READ);
+	protection_map[3]  = PM(_PAGE_PRESENT | _PAGE_NO_EXEC);
+	protection_map[4]  = PM(_PAGE_PRESENT);
+	protection_map[5]  = PM(_PAGE_PRESENT);
+	protection_map[6]  = PM(_PAGE_PRESENT);
+	protection_map[7]  = PM(_PAGE_PRESENT);
+
+	protection_map[8]  = PM(_PAGE_PRESENT | _PAGE_NO_EXEC | _PAGE_NO_READ);
+	protection_map[9]  = PM(_PAGE_PRESENT | _PAGE_NO_EXEC);
+	protection_map[10] = PM(_PAGE_PRESENT | _PAGE_NO_EXEC | _PAGE_WRITE |
+				_PAGE_NO_READ);
+	protection_map[11] = PM(_PAGE_PRESENT | _PAGE_NO_EXEC | _PAGE_WRITE);
+	protection_map[12] = PM(_PAGE_PRESENT);
+	protection_map[13] = PM(_PAGE_PRESENT);
+	protection_map[14] = PM(_PAGE_PRESENT | _PAGE_WRITE);
+	protection_map[15] = PM(_PAGE_PRESENT | _PAGE_WRITE);
+}
+
+#undef PM
+
+void cpu_cache_init(void)
+{
+	if (IS_ENABLED(CONFIG_CPU_R3000) && cpu_has_3k_cache)
+		r3k_cache_init();
+	if ((IS_ENABLED(CONFIG_CPU_R4K_CACHE_TLB) ||
+	     IS_ENABLED(CONFIG_CPU_SB1)) && cpu_has_4k_cache)
+		r4k_cache_init();
+
+	if (IS_ENABLED(CONFIG_CPU_CAVIUM_OCTEON) && cpu_has_octeon_cache)
+		octeon_cache_init();
+
+	setup_protection_map();
+}