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, 131 insertions, 0 deletions

diff --git a/arch/arm/lib/backtrace.S b/arch/arm/lib/backtrace.S
new file mode 100644
index 000000000..293a2716b
--- /dev/null
+++ b/arch/arm/lib/backtrace.S
@@ -0,0 +1,131 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+/*
+ *  linux/arch/arm/lib/backtrace.S
+ *
+ *  Copyright (C) 1995, 1996 Russell King
+ *
+ * 27/03/03 Ian Molton Clean up CONFIG_CPU
+ */
+#include <linux/kern_levels.h>
+#include <linux/linkage.h>
+#include <asm/assembler.h>
+		.text
+
+@ fp is 0 or stack frame
+
+#define frame	r4
+#define sv_fp	r5
+#define sv_pc	r6
+#define mask	r7
+#define offset	r8
+#define loglvl	r9
+
+ENTRY(c_backtrace)
+
+#if !defined(CONFIG_FRAME_POINTER) || !defined(CONFIG_PRINTK)
+		ret	lr
+ENDPROC(c_backtrace)
+#else
+		stmfd	sp!, {r4 - r9, lr}	@ Save an extra register so we have a location...
+		movs	frame, r0		@ if frame pointer is zero
+		beq	no_frame		@ we have no stack frames
+		mov	loglvl, r2
+
+		tst	r1, #0x10		@ 26 or 32-bit mode?
+ ARM(		moveq	mask, #0xfc000003	)
+ THUMB(		moveq	mask, #0xfc000000	)
+ THUMB(		orreq	mask, #0x03		)
+		movne	mask, #0		@ mask for 32-bit
+
+1:		stmfd	sp!, {pc}		@ calculate offset of PC stored
+		ldr	r0, [sp], #4		@ by stmfd for this CPU
+		adr	r1, 1b
+		sub	offset, r0, r1
+
+/*
+ * Stack frame layout:
+ *             optionally saved caller registers (r4 - r10)
+ *             saved fp
+ *             saved sp
+ *             saved lr
+ *    frame => saved pc
+ *             optionally saved arguments (r0 - r3)
+ * saved sp => <next word>
+ *
+ * Functions start with the following code sequence:
+ *                  mov   ip, sp
+ *                  stmfd sp!, {r0 - r3} (optional)
+ * corrected pc =>  stmfd sp!, {..., fp, ip, lr, pc}
+ */
+for_each_frame:	tst	frame, mask		@ Check for address exceptions
+		bne	no_frame
+
+1001:		ldr	sv_pc, [frame, #0]	@ get saved pc
+1002:		ldr	sv_fp, [frame, #-12]	@ get saved fp
+
+		sub	sv_pc, sv_pc, offset	@ Correct PC for prefetching
+		bic	sv_pc, sv_pc, mask	@ mask PC/LR for the mode
+
+1003:		ldr	r2, [sv_pc, #-4]	@ if stmfd sp!, {args} exists,
+		ldr	r3, .Ldsi+4		@ adjust saved 'pc' back one
+		teq	r3, r2, lsr #11		@ instruction
+		subne	r0, sv_pc, #4		@ allow for mov
+		subeq	r0, sv_pc, #8		@ allow for mov + stmia
+
+		ldr	r1, [frame, #-4]	@ get saved lr
+		mov	r2, frame
+		bic	r1, r1, mask		@ mask PC/LR for the mode
+		mov	r3, loglvl
+		bl	dump_backtrace_entry
+
+		ldr	r1, [sv_pc, #-4]	@ if stmfd sp!, {args} exists,
+		ldr	r3, .Ldsi+4
+		teq	r3, r1, lsr #11
+		ldreq	r0, [frame, #-8]	@ get sp
+		subeq	r0, r0, #4		@ point at the last arg
+		mov	r2, loglvl
+		bleq	dump_backtrace_stm	@ dump saved registers
+
+1004:		ldr	r1, [sv_pc, #0]		@ if stmfd sp!, {..., fp, ip, lr, pc}
+		ldr	r3, .Ldsi		@ instruction exists,
+		teq	r3, r1, lsr #11
+		subeq	r0, frame, #16
+		mov	r2, loglvl
+		bleq	dump_backtrace_stm	@ dump saved registers
+
+		teq	sv_fp, #0		@ zero saved fp means
+		beq	no_frame		@ no further frames
+
+		cmp	sv_fp, frame		@ next frame must be
+		mov	frame, sv_fp		@ above the current frame
+#ifdef CONFIG_IRQSTACKS
+		@
+		@ Kernel stacks may be discontiguous in memory. If the next
+		@ frame is below the previous frame, accept it as long as it
+		@ lives in kernel memory.
+		@
+		cmpls	sv_fp, #PAGE_OFFSET
+#endif
+		bhi	for_each_frame
+
+1006:		adr	r0, .Lbad
+		mov	r1, loglvl
+		mov	r2, frame
+		bl	_printk
+no_frame:	ldmfd	sp!, {r4 - r9, pc}
+ENDPROC(c_backtrace)
+		
+		.pushsection __ex_table,"a"
+		.align	3
+		.long	1001b, 1006b
+		.long	1002b, 1006b
+		.long	1003b, 1006b
+		.long	1004b, 1006b
+		.popsection
+
+.Lbad:		.asciz	"%sBacktrace aborted due to bad frame pointer <%p>\n"
+		.align
+.Ldsi:		.word	0xe92dd800 >> 11	@ stmfd sp!, {... fp, ip, lr, pc}
+		.word	0xe92d0000 >> 11	@ stmfd sp!, {}
+
+#endif