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, 267 insertions, 0 deletions

diff --git a/arch/alpha/mm/init.c b/arch/alpha/mm/init.c
new file mode 100644
index 000000000..9531cbc76
--- /dev/null
+++ b/arch/alpha/mm/init.c
@@ -0,0 +1,267 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ *  linux/arch/alpha/mm/init.c
+ *
+ *  Copyright (C) 1995  Linus Torvalds
+ */
+
+/* 2.3.x zone allocator, 1999 Andrea Arcangeli <andrea@suse.de> */
+
+#include <linux/pagemap.h>
+#include <linux/signal.h>
+#include <linux/sched.h>
+#include <linux/kernel.h>
+#include <linux/errno.h>
+#include <linux/string.h>
+#include <linux/types.h>
+#include <linux/ptrace.h>
+#include <linux/mman.h>
+#include <linux/mm.h>
+#include <linux/swap.h>
+#include <linux/init.h>
+#include <linux/memblock.h> /* max_low_pfn */
+#include <linux/vmalloc.h>
+#include <linux/gfp.h>
+
+#include <linux/uaccess.h>
+#include <asm/pgalloc.h>
+#include <asm/hwrpb.h>
+#include <asm/dma.h>
+#include <asm/mmu_context.h>
+#include <asm/console.h>
+#include <asm/tlb.h>
+#include <asm/setup.h>
+#include <asm/sections.h>
+
+#include "../kernel/proto.h"
+
+static struct pcb_struct original_pcb;
+
+pgd_t *
+pgd_alloc(struct mm_struct *mm)
+{
+	pgd_t *ret, *init;
+
+	ret = __pgd_alloc(mm, 0);
+	init = pgd_offset(&init_mm, 0UL);
+	if (ret) {
+#ifdef CONFIG_ALPHA_LARGE_VMALLOC
+		memcpy (ret + USER_PTRS_PER_PGD, init + USER_PTRS_PER_PGD,
+			(PTRS_PER_PGD - USER_PTRS_PER_PGD - 1)*sizeof(pgd_t));
+#else
+		pgd_val(ret[PTRS_PER_PGD-2]) = pgd_val(init[PTRS_PER_PGD-2]);
+#endif
+
+		/* The last PGD entry is the VPTB self-map.  */
+		pgd_val(ret[PTRS_PER_PGD-1])
+		  = pte_val(mk_pte(virt_to_page(ret), PAGE_KERNEL));
+	}
+	return ret;
+}
+
+
+static inline unsigned long
+load_PCB(struct pcb_struct *pcb)
+{
+	register unsigned long sp __asm__("$30");
+	pcb->ksp = sp;
+	return __reload_thread(pcb);
+}
+
+/* Set up initial PCB, VPTB, and other such nicities.  */
+
+static inline void
+switch_to_system_map(void)
+{
+	unsigned long newptbr;
+	unsigned long original_pcb_ptr;
+
+	/* Initialize the kernel's page tables.  Linux puts the vptb in
+	   the last slot of the L1 page table.  */
+	memset(swapper_pg_dir, 0, PAGE_SIZE);
+	newptbr = ((unsigned long) swapper_pg_dir - PAGE_OFFSET) >> PAGE_SHIFT;
+	pgd_val(swapper_pg_dir[1023]) =
+		(newptbr << 32) | pgprot_val(PAGE_KERNEL);
+
+	/* Set the vptb.  This is often done by the bootloader, but 
+	   shouldn't be required.  */
+	if (hwrpb->vptb != 0xfffffffe00000000UL) {
+		wrvptptr(0xfffffffe00000000UL);
+		hwrpb->vptb = 0xfffffffe00000000UL;
+		hwrpb_update_checksum(hwrpb);
+	}
+
+	/* Also set up the real kernel PCB while we're at it.  */
+	init_thread_info.pcb.ptbr = newptbr;
+	init_thread_info.pcb.flags = 1;	/* set FEN, clear everything else */
+	original_pcb_ptr = load_PCB(&init_thread_info.pcb);
+	tbia();
+
+	/* Save off the contents of the original PCB so that we can
+	   restore the original console's page tables for a clean reboot.
+
+	   Note that the PCB is supposed to be a physical address, but
+	   since KSEG values also happen to work, folks get confused.
+	   Check this here.  */
+
+	if (original_pcb_ptr < PAGE_OFFSET) {
+		original_pcb_ptr = (unsigned long)
+			phys_to_virt(original_pcb_ptr);
+	}
+	original_pcb = *(struct pcb_struct *) original_pcb_ptr;
+}
+
+int callback_init_done;
+
+void * __init
+callback_init(void * kernel_end)
+{
+	struct crb_struct * crb;
+	pgd_t *pgd;
+	p4d_t *p4d;
+	pud_t *pud;
+	pmd_t *pmd;
+	void *two_pages;
+
+	/* Starting at the HWRPB, locate the CRB. */
+	crb = (struct crb_struct *)((char *)hwrpb + hwrpb->crb_offset);
+
+	if (alpha_using_srm) {
+		/* Tell the console whither it is to be remapped. */
+		if (srm_fixup(VMALLOC_START, (unsigned long)hwrpb))
+			__halt();		/* "We're boned."  --Bender */
+
+		/* Edit the procedure descriptors for DISPATCH and FIXUP. */
+		crb->dispatch_va = (struct procdesc_struct *)
+			(VMALLOC_START + (unsigned long)crb->dispatch_va
+			 - crb->map[0].va);
+		crb->fixup_va = (struct procdesc_struct *)
+			(VMALLOC_START + (unsigned long)crb->fixup_va
+			 - crb->map[0].va);
+	}
+
+	switch_to_system_map();
+
+	/* Allocate one PGD and one PMD.  In the case of SRM, we'll need
+	   these to actually remap the console.  There is an assumption
+	   here that only one of each is needed, and this allows for 8MB.
+	   On systems with larger consoles, additional pages will be
+	   allocated as needed during the mapping process.
+
+	   In the case of not SRM, but not CONFIG_ALPHA_LARGE_VMALLOC,
+	   we need to allocate the PGD we use for vmalloc before we start
+	   forking other tasks.  */
+
+	two_pages = (void *)
+	  (((unsigned long)kernel_end + ~PAGE_MASK) & PAGE_MASK);
+	kernel_end = two_pages + 2*PAGE_SIZE;
+	memset(two_pages, 0, 2*PAGE_SIZE);
+
+	pgd = pgd_offset_k(VMALLOC_START);
+	p4d = p4d_offset(pgd, VMALLOC_START);
+	pud = pud_offset(p4d, VMALLOC_START);
+	pud_set(pud, (pmd_t *)two_pages);
+	pmd = pmd_offset(pud, VMALLOC_START);
+	pmd_set(pmd, (pte_t *)(two_pages + PAGE_SIZE));
+
+	if (alpha_using_srm) {
+		static struct vm_struct console_remap_vm;
+		unsigned long nr_pages = 0;
+		unsigned long vaddr;
+		unsigned long i, j;
+
+		/* calculate needed size */
+		for (i = 0; i < crb->map_entries; ++i)
+			nr_pages += crb->map[i].count;
+
+		/* register the vm area */
+		console_remap_vm.flags = VM_ALLOC;
+		console_remap_vm.size = nr_pages << PAGE_SHIFT;
+		vm_area_register_early(&console_remap_vm, PAGE_SIZE);
+
+		vaddr = (unsigned long)console_remap_vm.addr;
+
+		/* Set up the third level PTEs and update the virtual
+		   addresses of the CRB entries.  */
+		for (i = 0; i < crb->map_entries; ++i) {
+			unsigned long pfn = crb->map[i].pa >> PAGE_SHIFT;
+			crb->map[i].va = vaddr;
+			for (j = 0; j < crb->map[i].count; ++j) {
+				/* Newer consoles (especially on larger
+				   systems) may require more pages of
+				   PTEs. Grab additional pages as needed. */
+				if (pmd != pmd_offset(pud, vaddr)) {
+					memset(kernel_end, 0, PAGE_SIZE);
+					pmd = pmd_offset(pud, vaddr);
+					pmd_set(pmd, (pte_t *)kernel_end);
+					kernel_end += PAGE_SIZE;
+				}
+				set_pte(pte_offset_kernel(pmd, vaddr),
+					pfn_pte(pfn, PAGE_KERNEL));
+				pfn++;
+				vaddr += PAGE_SIZE;
+			}
+		}
+	}
+
+	callback_init_done = 1;
+	return kernel_end;
+}
+
+void __init arch_zone_limits_init(unsigned long *max_zone_pfn)
+{
+	unsigned long dma_pfn;
+
+	dma_pfn = virt_to_phys((char *)MAX_DMA_ADDRESS) >> PAGE_SHIFT;
+	max_pfn = max_low_pfn;
+
+	max_zone_pfn[ZONE_DMA] = dma_pfn;
+	max_zone_pfn[ZONE_NORMAL] = max_pfn;
+}
+
+/*
+ * paging_init() initializes the kernel's ZERO_PGE.
+ */
+void __init paging_init(void)
+{
+	memset(absolute_pointer(ZERO_PGE), 0, PAGE_SIZE);
+}
+
+#if defined(CONFIG_ALPHA_GENERIC) || defined(CONFIG_ALPHA_SRM)
+void
+srm_paging_stop (void)
+{
+	/* Move the vptb back to where the SRM console expects it.  */
+	swapper_pg_dir[1] = swapper_pg_dir[1023];
+	tbia();
+	wrvptptr(0x200000000UL);
+	hwrpb->vptb = 0x200000000UL;
+	hwrpb_update_checksum(hwrpb);
+
+	/* Reload the page tables that the console had in use.  */
+	load_PCB(&original_pcb);
+	tbia();
+}
+#endif
+
+static const pgprot_t protection_map[16] = {
+	[VM_NONE]					= _PAGE_P(_PAGE_FOE | _PAGE_FOW |
+								  _PAGE_FOR),
+	[VM_READ]					= _PAGE_P(_PAGE_FOE | _PAGE_FOW),
+	[VM_WRITE]					= _PAGE_P(_PAGE_FOE),
+	[VM_WRITE | VM_READ]				= _PAGE_P(_PAGE_FOE),
+	[VM_EXEC]					= _PAGE_P(_PAGE_FOW | _PAGE_FOR),
+	[VM_EXEC | VM_READ]				= _PAGE_P(_PAGE_FOW),
+	[VM_EXEC | VM_WRITE]				= _PAGE_P(0),
+	[VM_EXEC | VM_WRITE | VM_READ]			= _PAGE_P(0),
+	[VM_SHARED]					= _PAGE_S(_PAGE_FOE | _PAGE_FOW |
+								  _PAGE_FOR),
+	[VM_SHARED | VM_READ]				= _PAGE_S(_PAGE_FOE | _PAGE_FOW),
+	[VM_SHARED | VM_WRITE]				= _PAGE_S(_PAGE_FOE),
+	[VM_SHARED | VM_WRITE | VM_READ]		= _PAGE_S(_PAGE_FOE),
+	[VM_SHARED | VM_EXEC]				= _PAGE_S(_PAGE_FOW | _PAGE_FOR),
+	[VM_SHARED | VM_EXEC | VM_READ]			= _PAGE_S(_PAGE_FOW),
+	[VM_SHARED | VM_EXEC | VM_WRITE]		= _PAGE_S(0),
+	[VM_SHARED | VM_EXEC | VM_WRITE | VM_READ]	= _PAGE_S(0)
+};
+DECLARE_VM_GET_PAGE_PROT