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, 190 insertions, 0 deletions

diff --git a/arch/arm64/lib/strcmp.S b/arch/arm64/lib/strcmp.S
new file mode 100644
index 000000000..9b89b4533
--- /dev/null
+++ b/arch/arm64/lib/strcmp.S
@@ -0,0 +1,190 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+/*
+ * Copyright (c) 2012-2022, Arm Limited.
+ *
+ * Adapted from the original at:
+ * https://github.com/ARM-software/optimized-routines/blob/189dfefe37d54c5b/string/aarch64/strcmp.S
+ */
+
+#include <linux/linkage.h>
+#include <asm/assembler.h>
+
+/* Assumptions:
+ *
+ * ARMv8-a, AArch64.
+ * MTE compatible.
+ */
+
+#define L(label) .L ## label
+
+#define REP8_01 0x0101010101010101
+#define REP8_7f 0x7f7f7f7f7f7f7f7f
+
+#define src1		x0
+#define src2		x1
+#define result		x0
+
+#define data1		x2
+#define data1w		w2
+#define data2		x3
+#define data2w		w3
+#define has_nul		x4
+#define diff		x5
+#define off1		x5
+#define syndrome	x6
+#define tmp		x6
+#define data3		x7
+#define zeroones	x8
+#define shift		x9
+#define off2		x10
+
+/* On big-endian early bytes are at MSB and on little-endian LSB.
+   LS_FW means shifting towards early bytes.  */
+#ifdef __AARCH64EB__
+# define LS_FW lsl
+#else
+# define LS_FW lsr
+#endif
+
+/* NUL detection works on the principle that (X - 1) & (~X) & 0x80
+   (=> (X - 1) & ~(X | 0x7f)) is non-zero iff a byte is zero, and
+   can be done in parallel across the entire word.
+   Since carry propagation makes 0x1 bytes before a NUL byte appear
+   NUL too in big-endian, byte-reverse the data before the NUL check.  */
+
+
+SYM_FUNC_START(__pi_strcmp)
+	sub	off2, src2, src1
+	mov	zeroones, REP8_01
+	and	tmp, src1, 7
+	tst	off2, 7
+	b.ne	L(misaligned8)
+	cbnz	tmp, L(mutual_align)
+
+	.p2align 4
+
+L(loop_aligned):
+	ldr	data2, [src1, off2]
+	ldr	data1, [src1], 8
+L(start_realigned):
+#ifdef __AARCH64EB__
+	rev	tmp, data1
+	sub	has_nul, tmp, zeroones
+	orr	tmp, tmp, REP8_7f
+#else
+	sub	has_nul, data1, zeroones
+	orr	tmp, data1, REP8_7f
+#endif
+	bics	has_nul, has_nul, tmp	/* Non-zero if NUL terminator.  */
+	ccmp	data1, data2, 0, eq
+	b.eq	L(loop_aligned)
+#ifdef __AARCH64EB__
+	rev	has_nul, has_nul
+#endif
+	eor	diff, data1, data2
+	orr	syndrome, diff, has_nul
+L(end):
+#ifndef __AARCH64EB__
+	rev	syndrome, syndrome
+	rev	data1, data1
+	rev	data2, data2
+#endif
+	clz	shift, syndrome
+	/* The most-significant-non-zero bit of the syndrome marks either the
+	   first bit that is different, or the top bit of the first zero byte.
+	   Shifting left now will bring the critical information into the
+	   top bits.  */
+	lsl	data1, data1, shift
+	lsl	data2, data2, shift
+	/* But we need to zero-extend (char is unsigned) the value and then
+	   perform a signed 32-bit subtraction.  */
+	lsr	data1, data1, 56
+	sub	result, data1, data2, lsr 56
+	ret
+
+	.p2align 4
+
+L(mutual_align):
+	/* Sources are mutually aligned, but are not currently at an
+	   alignment boundary.  Round down the addresses and then mask off
+	   the bytes that precede the start point.  */
+	bic	src1, src1, 7
+	ldr	data2, [src1, off2]
+	ldr	data1, [src1], 8
+	neg	shift, src2, lsl 3	/* Bits to alignment -64.  */
+	mov	tmp, -1
+	LS_FW	tmp, tmp, shift
+	orr	data1, data1, tmp
+	orr	data2, data2, tmp
+	b	L(start_realigned)
+
+L(misaligned8):
+	/* Align SRC1 to 8 bytes and then compare 8 bytes at a time, always
+	   checking to make sure that we don't access beyond the end of SRC2.  */
+	cbz	tmp, L(src1_aligned)
+L(do_misaligned):
+	ldrb	data1w, [src1], 1
+	ldrb	data2w, [src2], 1
+	cmp	data1w, 0
+	ccmp	data1w, data2w, 0, ne	/* NZCV = 0b0000.  */
+	b.ne	L(done)
+	tst	src1, 7
+	b.ne	L(do_misaligned)
+
+L(src1_aligned):
+	neg	shift, src2, lsl 3
+	bic	src2, src2, 7
+	ldr	data3, [src2], 8
+#ifdef __AARCH64EB__
+	rev	data3, data3
+#endif
+	lsr	tmp, zeroones, shift
+	orr	data3, data3, tmp
+	sub	has_nul, data3, zeroones
+	orr	tmp, data3, REP8_7f
+	bics	has_nul, has_nul, tmp
+	b.ne	L(tail)
+
+	sub	off1, src2, src1
+
+	.p2align 4
+
+L(loop_unaligned):
+	ldr	data3, [src1, off1]
+	ldr	data2, [src1, off2]
+#ifdef __AARCH64EB__
+	rev	data3, data3
+#endif
+	sub	has_nul, data3, zeroones
+	orr	tmp, data3, REP8_7f
+	ldr	data1, [src1], 8
+	bics	has_nul, has_nul, tmp
+	ccmp	data1, data2, 0, eq
+	b.eq	L(loop_unaligned)
+
+	lsl	tmp, has_nul, shift
+#ifdef __AARCH64EB__
+	rev	tmp, tmp
+#endif
+	eor	diff, data1, data2
+	orr	syndrome, diff, tmp
+	cbnz	syndrome, L(end)
+L(tail):
+	ldr	data1, [src1]
+	neg	shift, shift
+	lsr	data2, data3, shift
+	lsr	has_nul, has_nul, shift
+#ifdef __AARCH64EB__
+	rev     data2, data2
+	rev	has_nul, has_nul
+#endif
+	eor	diff, data1, data2
+	orr	syndrome, diff, has_nul
+	b	L(end)
+
+L(done):
+	sub	result, data1, data2
+	ret
+SYM_FUNC_END(__pi_strcmp)
+SYM_FUNC_ALIAS_WEAK(strcmp, __pi_strcmp)
+EXPORT_SYMBOL_NOKASAN(strcmp)