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, 333 insertions, 0 deletions

diff --git a/arch/mips/mm/fault.c b/arch/mips/mm/fault.c
new file mode 100644
index 000000000..37fedeaca
--- /dev/null
+++ b/arch/mips/mm/fault.c
@@ -0,0 +1,333 @@
+/*
+ * 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) 1995 - 2000 by Ralf Baechle
+ */
+#include <linux/context_tracking.h>
+#include <linux/signal.h>
+#include <linux/sched.h>
+#include <linux/interrupt.h>
+#include <linux/kernel.h>
+#include <linux/errno.h>
+#include <linux/string.h>
+#include <linux/types.h>
+#include <linux/ptrace.h>
+#include <linux/ratelimit.h>
+#include <linux/mman.h>
+#include <linux/mm.h>
+#include <linux/smp.h>
+#include <linux/kprobes.h>
+#include <linux/perf_event.h>
+#include <linux/uaccess.h>
+
+#include <asm/branch.h>
+#include <asm/mmu_context.h>
+#include <asm/ptrace.h>
+#include <asm/highmem.h>		/* For VMALLOC_END */
+#include <asm/traps.h>
+#include <linux/kdebug.h>
+
+int show_unhandled_signals = 1;
+
+/*
+ * This routine handles page faults.  It determines the address,
+ * and the problem, and then passes it off to one of the appropriate
+ * routines.
+ */
+static void __do_page_fault(struct pt_regs *regs, unsigned long write,
+	unsigned long address)
+{
+	struct vm_area_struct * vma = NULL;
+	struct task_struct *tsk = current;
+	struct mm_struct *mm = tsk->mm;
+	const int field = sizeof(unsigned long) * 2;
+	int si_code;
+	vm_fault_t fault;
+	unsigned int flags = FAULT_FLAG_DEFAULT;
+
+	static DEFINE_RATELIMIT_STATE(ratelimit_state, 5 * HZ, 10);
+
+#if 0
+	printk("Cpu%d[%s:%d:%0*lx:%ld:%0*lx]\n", raw_smp_processor_id(),
+	       current->comm, current->pid, field, address, write,
+	       field, regs->cp0_epc);
+#endif
+
+#ifdef CONFIG_KPROBES
+	/*
+	 * This is to notify the fault handler of the kprobes.
+	 */
+	if (notify_die(DIE_PAGE_FAULT, "page fault", regs, -1,
+		       current->thread.trap_nr, SIGSEGV) == NOTIFY_STOP)
+		return;
+#endif
+
+	si_code = SEGV_MAPERR;
+
+	/*
+	 * We fault-in kernel-space virtual memory on-demand. The
+	 * 'reference' page table is init_mm.pgd.
+	 *
+	 * NOTE! We MUST NOT take any locks for this case. We may
+	 * be in an interrupt or a critical region, and should
+	 * only copy the information from the master page table,
+	 * nothing more.
+	 */
+#ifdef CONFIG_64BIT
+# define VMALLOC_FAULT_TARGET no_context
+#else
+# define VMALLOC_FAULT_TARGET vmalloc_fault
+#endif
+
+	if (unlikely(address >= VMALLOC_START && address <= VMALLOC_END))
+		goto VMALLOC_FAULT_TARGET;
+#ifdef MODULES_VADDR
+	if (unlikely(address >= MODULES_VADDR && address < MODULES_END))
+		goto VMALLOC_FAULT_TARGET;
+#endif
+
+	/*
+	 * If we're in an interrupt or have no user
+	 * context, we must not take the fault..
+	 */
+	if (faulthandler_disabled() || !mm)
+		goto bad_area_nosemaphore;
+
+	if (user_mode(regs))
+		flags |= FAULT_FLAG_USER;
+
+	perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS, 1, regs, address);
+retry:
+	vma = lock_mm_and_find_vma(mm, address, regs);
+	if (!vma)
+		goto bad_area_nosemaphore;
+/*
+ * Ok, we have a good vm_area for this memory access, so
+ * we can handle it..
+ */
+	si_code = SEGV_ACCERR;
+
+	if (write) {
+		if (!(vma->vm_flags & VM_WRITE))
+			goto bad_area;
+		flags |= FAULT_FLAG_WRITE;
+	} else {
+		if (cpu_has_rixi) {
+			if (address == regs->cp0_epc && !(vma->vm_flags & VM_EXEC)) {
+#if 0
+				pr_notice("Cpu%d[%s:%d:%0*lx:%ld:%0*lx] XI violation\n",
+					  raw_smp_processor_id(),
+					  current->comm, current->pid,
+					  field, address, write,
+					  field, regs->cp0_epc);
+#endif
+				goto bad_area;
+			}
+			if (!(vma->vm_flags & VM_READ) &&
+			    exception_epc(regs) != address) {
+#if 0
+				pr_notice("Cpu%d[%s:%d:%0*lx:%ld:%0*lx] RI violation\n",
+					  raw_smp_processor_id(),
+					  current->comm, current->pid,
+					  field, address, write,
+					  field, regs->cp0_epc);
+#endif
+				goto bad_area;
+			}
+		} else {
+			if (unlikely(!vma_is_accessible(vma)))
+				goto bad_area;
+		}
+	}
+
+	/*
+	 * If for any reason at all we couldn't handle the fault,
+	 * make sure we exit gracefully rather than endlessly redo
+	 * the fault.
+	 */
+	fault = handle_mm_fault(vma, address, flags, regs);
+
+	if (fault_signal_pending(fault, regs)) {
+		if (!user_mode(regs))
+			goto no_context;
+		return;
+	}
+
+	/* The fault is fully completed (including releasing mmap lock) */
+	if (fault & VM_FAULT_COMPLETED)
+		return;
+
+	if (unlikely(fault & VM_FAULT_ERROR)) {
+		if (fault & VM_FAULT_OOM)
+			goto out_of_memory;
+		else if (fault & VM_FAULT_SIGSEGV)
+			goto bad_area;
+		else if (fault & VM_FAULT_SIGBUS)
+			goto do_sigbus;
+		BUG();
+	}
+
+	if (fault & VM_FAULT_RETRY) {
+		flags |= FAULT_FLAG_TRIED;
+
+		/*
+		 * No need to mmap_read_unlock(mm) as we would
+		 * have already released it in __lock_page_or_retry
+		 * in mm/filemap.c.
+		 */
+
+		goto retry;
+	}
+
+	mmap_read_unlock(mm);
+	return;
+
+/*
+ * Something tried to access memory that isn't in our memory map..
+ * Fix it, but check if it's kernel or user first..
+ */
+bad_area:
+	mmap_read_unlock(mm);
+
+bad_area_nosemaphore:
+	/* User mode accesses just cause a SIGSEGV */
+	if (user_mode(regs)) {
+		tsk->thread.cp0_badvaddr = address;
+		tsk->thread.error_code = write;
+		if (show_unhandled_signals &&
+		    unhandled_signal(tsk, SIGSEGV) &&
+		    __ratelimit(&ratelimit_state)) {
+			pr_info("do_page_fault(): sending SIGSEGV to %s for invalid %s %0*lx\n",
+				tsk->comm,
+				write ? "write access to" : "read access from",
+				field, address);
+			pr_info("epc = %0*lx in", field,
+				(unsigned long) regs->cp0_epc);
+			print_vma_addr(KERN_CONT " ", regs->cp0_epc);
+			pr_cont("\n");
+			pr_info("ra  = %0*lx in", field,
+				(unsigned long) regs->regs[31]);
+			print_vma_addr(KERN_CONT " ", regs->regs[31]);
+			pr_cont("\n");
+		}
+		current->thread.trap_nr = (regs->cp0_cause >> 2) & 0x1f;
+		force_sig_fault(SIGSEGV, si_code, (void __user *)address);
+		return;
+	}
+
+no_context:
+	/* Are we prepared to handle this kernel fault?	 */
+	if (fixup_exception(regs)) {
+		current->thread.cp0_baduaddr = address;
+		return;
+	}
+
+	/*
+	 * Oops. The kernel tried to access some bad page. We'll have to
+	 * terminate things with extreme prejudice.
+	 */
+	bust_spinlocks(1);
+
+	printk(KERN_ALERT "CPU %d Unable to handle kernel paging request at "
+	       "virtual address %0*lx, epc == %0*lx, ra == %0*lx\n",
+	       raw_smp_processor_id(), field, address, field, regs->cp0_epc,
+	       field,  regs->regs[31]);
+	die("Oops", regs);
+
+out_of_memory:
+	/*
+	 * We ran out of memory, call the OOM killer, and return the userspace
+	 * (which will retry the fault, or kill us if we got oom-killed).
+	 */
+	mmap_read_unlock(mm);
+	if (!user_mode(regs))
+		goto no_context;
+	pagefault_out_of_memory();
+	return;
+
+do_sigbus:
+	mmap_read_unlock(mm);
+
+	/* Kernel mode? Handle exceptions or die */
+	if (!user_mode(regs))
+		goto no_context;
+
+	/*
+	 * Send a sigbus, regardless of whether we were in kernel
+	 * or user mode.
+	 */
+#if 0
+	printk("do_page_fault() #3: sending SIGBUS to %s for "
+	       "invalid %s\n%0*lx (epc == %0*lx, ra == %0*lx)\n",
+	       tsk->comm,
+	       write ? "write access to" : "read access from",
+	       field, address,
+	       field, (unsigned long) regs->cp0_epc,
+	       field, (unsigned long) regs->regs[31]);
+#endif
+	current->thread.trap_nr = (regs->cp0_cause >> 2) & 0x1f;
+	tsk->thread.cp0_badvaddr = address;
+	force_sig_fault(SIGBUS, BUS_ADRERR, (void __user *)address);
+
+	return;
+#ifndef CONFIG_64BIT
+vmalloc_fault:
+	{
+		/*
+		 * Synchronize this task's top level page-table
+		 * with the 'reference' page table.
+		 *
+		 * Do _not_ use "tsk" here. We might be inside
+		 * an interrupt in the middle of a task switch..
+		 */
+		int offset = pgd_index(address);
+		pgd_t *pgd, *pgd_k;
+		p4d_t *p4d, *p4d_k;
+		pud_t *pud, *pud_k;
+		pmd_t *pmd, *pmd_k;
+		pte_t *pte_k;
+
+		pgd = (pgd_t *) pgd_current[raw_smp_processor_id()] + offset;
+		pgd_k = init_mm.pgd + offset;
+
+		if (!pgd_present(*pgd_k))
+			goto no_context;
+		set_pgd(pgd, *pgd_k);
+
+		p4d = p4d_offset(pgd, address);
+		p4d_k = p4d_offset(pgd_k, address);
+		if (!p4d_present(*p4d_k))
+			goto no_context;
+
+		pud = pud_offset(p4d, address);
+		pud_k = pud_offset(p4d_k, address);
+		if (!pud_present(*pud_k))
+			goto no_context;
+
+		pmd = pmd_offset(pud, address);
+		pmd_k = pmd_offset(pud_k, address);
+		if (!pmd_present(*pmd_k))
+			goto no_context;
+		set_pmd(pmd, *pmd_k);
+
+		pte_k = pte_offset_kernel(pmd_k, address);
+		if (!pte_present(*pte_k))
+			goto no_context;
+		return;
+	}
+#endif
+}
+NOKPROBE_SYMBOL(__do_page_fault);
+
+asmlinkage void do_page_fault(struct pt_regs *regs,
+	unsigned long write, unsigned long address)
+{
+	enum ctx_state prev_state;
+
+	prev_state = exception_enter();
+	__do_page_fault(regs, write, address);
+	exception_exit(prev_state);
+}
+NOKPROBE_SYMBOL(do_page_fault);