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, 241 insertions, 0 deletions

diff --git a/samples/ftrace/ftrace-direct-multi.c b/samples/ftrace/ftrace-direct-multi.c
new file mode 100644
index 000000000..3fe6ddaf0
--- /dev/null
+++ b/samples/ftrace/ftrace-direct-multi.c
@@ -0,0 +1,241 @@
+// SPDX-License-Identifier: GPL-2.0-only
+#include <linux/module.h>
+
+#include <linux/mm.h> /* for handle_mm_fault() */
+#include <linux/ftrace.h>
+#include <linux/sched/stat.h>
+#if !defined(CONFIG_ARM64) && !defined(CONFIG_PPC32)
+#include <asm/asm-offsets.h>
+#endif
+
+extern void my_direct_func(unsigned long ip);
+
+void my_direct_func(unsigned long ip)
+{
+	trace_printk("ip %lx\n", ip);
+}
+
+extern void my_tramp(void *);
+
+#ifdef CONFIG_RISCV
+#include <asm/asm.h>
+
+asm (
+"       .pushsection    .text, \"ax\", @progbits\n"
+"       .type           my_tramp, @function\n"
+"       .globl          my_tramp\n"
+"   my_tramp:\n"
+"       addi	sp,sp,-3*"SZREG"\n"
+"       "REG_S"	a0,0*"SZREG"(sp)\n"
+"       "REG_S"	t0,1*"SZREG"(sp)\n"
+"       "REG_S"	ra,2*"SZREG"(sp)\n"
+"       mv	a0,t0\n"
+"       call	my_direct_func\n"
+"       "REG_L"	a0,0*"SZREG"(sp)\n"
+"       "REG_L"	t0,1*"SZREG"(sp)\n"
+"       "REG_L"	ra,2*"SZREG"(sp)\n"
+"       addi	sp,sp,3*"SZREG"\n"
+"       jr	t0\n"
+"       .size           my_tramp, .-my_tramp\n"
+"       .popsection\n"
+);
+
+#endif /* CONFIG_RISCV */
+
+#ifdef CONFIG_X86_64
+
+#include <asm/ibt.h>
+#include <asm/nospec-branch.h>
+
+asm (
+"	.pushsection    .text, \"ax\", @progbits\n"
+"	.type		my_tramp, @function\n"
+"	.globl		my_tramp\n"
+"   my_tramp:"
+	ASM_ENDBR
+"	pushq %rbp\n"
+"	movq %rsp, %rbp\n"
+	CALL_DEPTH_ACCOUNT
+"	pushq %rdi\n"
+"	movq 8(%rbp), %rdi\n"
+"	call my_direct_func\n"
+"	popq %rdi\n"
+"	leave\n"
+	ASM_RET
+"	.size		my_tramp, .-my_tramp\n"
+"	.popsection\n"
+);
+
+#endif /* CONFIG_X86_64 */
+
+#ifdef CONFIG_S390
+
+asm (
+"	.pushsection	.text, \"ax\", @progbits\n"
+"	.type		my_tramp, @function\n"
+"	.globl		my_tramp\n"
+"   my_tramp:"
+"	lgr		%r1,%r15\n"
+"	stmg		%r0,%r5,"__stringify(__SF_GPRS)"(%r15)\n"
+"	stg		%r14,"__stringify(__SF_GPRS+8*8)"(%r15)\n"
+"	aghi		%r15,"__stringify(-STACK_FRAME_OVERHEAD)"\n"
+"	stg		%r1,"__stringify(__SF_BACKCHAIN)"(%r15)\n"
+"	lgr		%r2,%r0\n"
+"	brasl		%r14,my_direct_func\n"
+"	aghi		%r15,"__stringify(STACK_FRAME_OVERHEAD)"\n"
+"	lmg		%r0,%r5,"__stringify(__SF_GPRS)"(%r15)\n"
+"	lg		%r14,"__stringify(__SF_GPRS+8*8)"(%r15)\n"
+"	lgr		%r1,%r0\n"
+"	br		%r1\n"
+"	.size		my_tramp, .-my_tramp\n"
+"	.popsection\n"
+);
+
+#endif /* CONFIG_S390 */
+
+#ifdef CONFIG_ARM64
+
+asm (
+"	.pushsection	.text, \"ax\", @progbits\n"
+"	.type		my_tramp, @function\n"
+"	.globl		my_tramp\n"
+"   my_tramp:"
+"	hint	34\n" // bti	c
+"	sub	sp, sp, #32\n"
+"	stp	x9, x30, [sp]\n"
+"	str	x0, [sp, #16]\n"
+"	mov	x0, x30\n"
+"	bl	my_direct_func\n"
+"	ldp	x30, x9, [sp]\n"
+"	ldr	x0, [sp, #16]\n"
+"	add	sp, sp, #32\n"
+"	ret	x9\n"
+"	.size		my_tramp, .-my_tramp\n"
+"	.popsection\n"
+);
+
+#endif /* CONFIG_ARM64 */
+
+#ifdef CONFIG_LOONGARCH
+
+#include <asm/asm.h>
+asm (
+"	.pushsection	.text, \"ax\", @progbits\n"
+"	.type		my_tramp, @function\n"
+"	.globl		my_tramp\n"
+"   my_tramp:\n"
+"	addi.d	$sp, $sp, -32\n"
+"	st.d	$a0, $sp, 0\n"
+"	st.d	$t0, $sp, 8\n"
+"	st.d	$ra, $sp, 16\n"
+"	move	$a0, $t0\n"
+"	bl	my_direct_func\n"
+"	ld.d	$a0, $sp, 0\n"
+"	ld.d	$ra, $sp, 8\n"
+"	ld.d	$t0, $sp, 16\n"
+"	addi.d	$sp, $sp, 32\n"
+"	jr	$t0\n"
+"	.size		my_tramp, .-my_tramp\n"
+"	.popsection\n"
+);
+
+#endif /* CONFIG_LOONGARCH */
+
+#ifdef CONFIG_PPC
+#include <asm/ppc_asm.h>
+
+#ifdef CONFIG_PPC64
+#define STACK_FRAME_SIZE 48
+#else
+#define STACK_FRAME_SIZE 24
+#endif
+
+#if defined(CONFIG_PPC64_ELF_ABI_V2) && !defined(CONFIG_PPC_KERNEL_PCREL)
+#define PPC64_TOC_SAVE_AND_UPDATE			\
+"	std		2, 24(1)\n"			\
+"	bcl		20, 31, 1f\n"			\
+"   1:	mflr		12\n"				\
+"	ld		2, (99f - 1b)(12)\n"
+#define PPC64_TOC_RESTORE				\
+"	ld		2, 24(1)\n"
+#define PPC64_TOC					\
+"   99:	.quad		.TOC.@tocbase\n"
+#else
+#define PPC64_TOC_SAVE_AND_UPDATE ""
+#define PPC64_TOC_RESTORE ""
+#define PPC64_TOC ""
+#endif
+
+#ifdef CONFIG_PPC_FTRACE_OUT_OF_LINE
+#define PPC_FTRACE_RESTORE_LR				\
+	PPC_LL"		0, "__stringify(PPC_LR_STKOFF)"(1)\n"	\
+"	mtlr		0\n"
+#define PPC_FTRACE_RET					\
+"	blr\n"
+#define PPC_FTRACE_RECOVER_IP				\
+"	lwz		8, 4(3)\n"			\
+"	li		9, 6\n"				\
+"	slw		8, 8, 9\n"			\
+"	sraw		8, 8, 9\n"			\
+"	add		3, 3, 8\n"			\
+"	addi		3, 3, 4\n"
+#else
+#define PPC_FTRACE_RESTORE_LR				\
+	PPC_LL"		0, "__stringify(PPC_LR_STKOFF)"(1)\n"	\
+"	mtctr		0\n"
+#define PPC_FTRACE_RET					\
+"	mtlr		0\n"				\
+"	bctr\n"
+#define PPC_FTRACE_RECOVER_IP ""
+#endif
+
+asm (
+"	.pushsection	.text, \"ax\", @progbits\n"
+"	.type		my_tramp, @function\n"
+"	.globl		my_tramp\n"
+"   my_tramp:\n"
+	PPC_STL"	0, "__stringify(PPC_LR_STKOFF)"(1)\n"
+	PPC_STLU"	1, -"__stringify(STACK_FRAME_MIN_SIZE)"(1)\n"
+"	mflr		0\n"
+	PPC_STL"	0, "__stringify(PPC_LR_STKOFF)"(1)\n"
+	PPC_STLU"	1, -"__stringify(STACK_FRAME_SIZE)"(1)\n"
+	PPC64_TOC_SAVE_AND_UPDATE
+	PPC_STL"	3, "__stringify(STACK_FRAME_MIN_SIZE)"(1)\n"
+"	mr		3, 0\n"
+	PPC_FTRACE_RECOVER_IP
+"	bl		my_direct_func\n"
+	PPC_LL"		3, "__stringify(STACK_FRAME_MIN_SIZE)"(1)\n"
+	PPC64_TOC_RESTORE
+"	addi		1, 1, "__stringify(STACK_FRAME_SIZE)"\n"
+	PPC_FTRACE_RESTORE_LR
+"	addi		1, 1, "__stringify(STACK_FRAME_MIN_SIZE)"\n"
+	PPC_LL"		0, "__stringify(PPC_LR_STKOFF)"(1)\n"
+	PPC_FTRACE_RET
+	PPC64_TOC
+"	.size		my_tramp, .-my_tramp\n"
+"	.popsection\n"
+);
+
+#endif /* CONFIG_PPC */
+
+static struct ftrace_ops direct;
+
+static int __init ftrace_direct_multi_init(void)
+{
+	ftrace_set_filter_ip(&direct, (unsigned long) wake_up_process, 0, 0);
+	ftrace_set_filter_ip(&direct, (unsigned long) schedule, 0, 0);
+
+	return register_ftrace_direct(&direct, (unsigned long) my_tramp);
+}
+
+static void __exit ftrace_direct_multi_exit(void)
+{
+	unregister_ftrace_direct(&direct, (unsigned long) my_tramp, true);
+}
+
+module_init(ftrace_direct_multi_init);
+module_exit(ftrace_direct_multi_exit);
+
+MODULE_AUTHOR("Jiri Olsa");
+MODULE_DESCRIPTION("Example use case of using register_ftrace_direct_multi()");
+MODULE_LICENSE("GPL");