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, 214 insertions, 0 deletions

diff --git a/arch/alpha/boot/bootp.c b/arch/alpha/boot/bootp.c
new file mode 100644
index 000000000..842e85776
--- /dev/null
+++ b/arch/alpha/boot/bootp.c
@@ -0,0 +1,214 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * arch/alpha/boot/bootp.c
+ *
+ * Copyright (C) 1997 Jay Estabrook
+ *
+ * This file is used for creating a bootp file for the Linux/AXP kernel
+ *
+ * based significantly on the arch/alpha/boot/main.c of Linus Torvalds
+ */
+#include <linux/kernel.h>
+#include <linux/slab.h>
+#include <linux/string.h>
+#include <generated/utsrelease.h>
+#include <linux/mm.h>
+
+#include <asm/console.h>
+#include <asm/hwrpb.h>
+#include <asm/io.h>
+
+#include <linux/stdarg.h>
+
+#include "ksize.h"
+
+extern unsigned long switch_to_osf_pal(unsigned long nr,
+	struct pcb_struct *pcb_va, struct pcb_struct *pcb_pa,
+	unsigned long *vptb);
+
+extern void move_stack(unsigned long new_stack);
+
+struct hwrpb_struct *hwrpb = INIT_HWRPB;
+static struct pcb_struct pcb_va[1];
+
+/*
+ * Find a physical address of a virtual object..
+ *
+ * This is easy using the virtual page table address.
+ */
+
+static inline void *
+find_pa(unsigned long *vptb, void *ptr)
+{
+	unsigned long address = (unsigned long) ptr;
+	unsigned long result;
+
+	result = vptb[address >> 13];
+	result >>= 32;
+	result <<= 13;
+	result |= address & 0x1fff;
+	return (void *) result;
+}	
+
+/*
+ * This function moves into OSF/1 pal-code, and has a temporary
+ * PCB for that. The kernel proper should replace this PCB with
+ * the real one as soon as possible.
+ *
+ * The page table muckery in here depends on the fact that the boot
+ * code has the L1 page table identity-map itself in the second PTE
+ * in the L1 page table. Thus the L1-page is virtually addressable
+ * itself (through three levels) at virtual address 0x200802000.
+ */
+
+#define VPTB	((unsigned long *) 0x200000000)
+#define L1	((unsigned long *) 0x200802000)
+
+void
+pal_init(void)
+{
+	unsigned long i, rev;
+	struct percpu_struct * percpu;
+	struct pcb_struct * pcb_pa;
+
+	/* Create the dummy PCB.  */
+	pcb_va->ksp = 0;
+	pcb_va->usp = 0;
+	pcb_va->ptbr = L1[1] >> 32;
+	pcb_va->asn = 0;
+	pcb_va->pcc = 0;
+	pcb_va->unique = 0;
+	pcb_va->flags = 1;
+	pcb_va->res1 = 0;
+	pcb_va->res2 = 0;
+	pcb_pa = find_pa(VPTB, pcb_va);
+
+	/*
+	 * a0 = 2 (OSF)
+	 * a1 = return address, but we give the asm the vaddr of the PCB
+	 * a2 = physical addr of PCB
+	 * a3 = new virtual page table pointer
+	 * a4 = KSP (but the asm sets it)
+	 */
+	srm_printk("Switching to OSF PAL-code .. ");
+
+	i = switch_to_osf_pal(2, pcb_va, pcb_pa, VPTB);
+	if (i) {
+		srm_printk("failed, code %ld\n", i);
+		__halt();
+	}
+
+	percpu = (struct percpu_struct *)
+		(INIT_HWRPB->processor_offset + (unsigned long) INIT_HWRPB);
+	rev = percpu->pal_revision = percpu->palcode_avail[2];
+
+	srm_printk("Ok (rev %lx)\n", rev);
+
+	tbia(); /* do it directly in case we are SMP */
+}
+
+static inline void
+load(unsigned long dst, unsigned long src, unsigned long count)
+{
+	memcpy((void *)dst, (void *)src, count);
+}
+
+/*
+ * Start the kernel.
+ */
+static inline void
+runkernel(void)
+{
+	__asm__ __volatile__(
+		"bis %0,%0,$27\n\t"
+		"jmp ($27)"
+		: /* no outputs: it doesn't even return */
+		: "r" (START_ADDR));
+}
+
+extern char _end;
+#define KERNEL_ORIGIN \
+	((((unsigned long)&_end) + 511) & ~511)
+
+void
+start_kernel(void)
+{
+	/*
+	 * Note that this crufty stuff with static and envval
+	 * and envbuf is because:
+	 *
+	 * 1. Frequently, the stack is short, and we don't want to overrun;
+	 * 2. Frequently the stack is where we are going to copy the kernel to;
+	 * 3. A certain SRM console required the GET_ENV output to stack.
+	 *    ??? A comment in the aboot sources indicates that the GET_ENV
+	 *    destination must be quadword aligned.  Might this explain the
+	 *    behaviour, rather than requiring output to the stack, which
+	 *    seems rather far-fetched.
+	 */
+	static long nbytes;
+	static char envval[256] __attribute__((aligned(8)));
+	static unsigned long initrd_start;
+
+	srm_printk("Linux/AXP bootp loader for Linux " UTS_RELEASE "\n");
+	if (INIT_HWRPB->pagesize != 8192) {
+		srm_printk("Expected 8kB pages, got %ldkB\n",
+		           INIT_HWRPB->pagesize >> 10);
+		return;
+	}
+	if (INIT_HWRPB->vptb != (unsigned long) VPTB) {
+		srm_printk("Expected vptb at %p, got %p\n",
+			   VPTB, (void *)INIT_HWRPB->vptb);
+		return;
+	}
+	pal_init();
+
+	/* The initrd must be page-aligned.  See below for the 
+	   cause of the magic number 5.  */
+	initrd_start = ((START_ADDR + 5*KERNEL_SIZE + PAGE_SIZE) |
+			(PAGE_SIZE-1)) + 1;
+#ifdef INITRD_IMAGE_SIZE
+	srm_printk("Initrd positioned at %#lx\n", initrd_start);
+#endif
+
+	/*
+	 * Move the stack to a safe place to ensure it won't be
+	 * overwritten by kernel image.
+	 */
+	move_stack(initrd_start - PAGE_SIZE);
+
+	nbytes = callback_getenv(ENV_BOOTED_OSFLAGS, envval, sizeof(envval));
+	if (nbytes < 0 || nbytes >= sizeof(envval)) {
+		nbytes = 0;
+	}
+	envval[nbytes] = '\0';
+	srm_printk("Loading the kernel...'%s'\n", envval);
+
+	/* NOTE: *no* callbacks or printouts from here on out!!! */
+
+	/* This is a hack, as some consoles seem to get virtual 20000000 (ie
+	 * where the SRM console puts the kernel bootp image) memory
+	 * overlapping physical memory where the kernel wants to be put,
+	 * which causes real problems when attempting to copy the former to
+	 * the latter... :-(
+	 *
+	 * So, we first move the kernel virtual-to-physical way above where
+	 * we physically want the kernel to end up, then copy it from there
+	 * to its final resting place... ;-}
+	 *
+	 * Sigh...  */
+
+#ifdef INITRD_IMAGE_SIZE
+	load(initrd_start, KERNEL_ORIGIN+KERNEL_SIZE, INITRD_IMAGE_SIZE);
+#endif
+        load(START_ADDR+(4*KERNEL_SIZE), KERNEL_ORIGIN, KERNEL_SIZE);
+        load(START_ADDR, START_ADDR+(4*KERNEL_SIZE), KERNEL_SIZE);
+
+	memset((char*)ZERO_PGE, 0, PAGE_SIZE);
+	strcpy((char*)ZERO_PGE, envval);
+#ifdef INITRD_IMAGE_SIZE
+	((long *)(ZERO_PGE+256))[0] = initrd_start;
+	((long *)(ZERO_PGE+256))[1] = INITRD_IMAGE_SIZE;
+#endif
+
+	runkernel();
+}