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, 101 insertions, 0 deletions

diff --git a/arch/arm/common/vlock.S b/arch/arm/common/vlock.S
new file mode 100644
index 000000000..c5eaed5a7
--- /dev/null
+++ b/arch/arm/common/vlock.S
@@ -0,0 +1,101 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+/*
+ * vlock.S - simple voting lock implementation for ARM
+ *
+ * Created by:	Dave Martin, 2012-08-16
+ * Copyright:	(C) 2012-2013  Linaro Limited
+ *
+ * This algorithm is described in more detail in
+ * Documentation/arch/arm/vlocks.rst.
+ */
+
+#include <linux/linkage.h>
+#include "vlock.h"
+
+.arch armv7-a
+
+/* Select different code if voting flags  can fit in a single word. */
+#if VLOCK_VOTING_SIZE > 4
+#define FEW(x...)
+#define MANY(x...) x
+#else
+#define FEW(x...) x
+#define MANY(x...)
+#endif
+
+@ voting lock for first-man coordination
+
+.macro voting_begin rbase:req, rcpu:req, rscratch:req
+	mov	\rscratch, #1
+	strb	\rscratch, [\rbase, \rcpu]
+	dmb
+.endm
+
+.macro voting_end rbase:req, rcpu:req, rscratch:req
+	dmb
+	mov	\rscratch, #0
+	strb	\rscratch, [\rbase, \rcpu]
+	dsb	st
+	sev
+.endm
+
+/*
+ * The vlock structure must reside in Strongly-Ordered or Device memory.
+ * This implementation deliberately eliminates most of the barriers which
+ * would be required for other memory types, and assumes that independent
+ * writes to neighbouring locations within a cacheline do not interfere
+ * with one another.
+ */
+
+@ r0: lock structure base
+@ r1: CPU ID (0-based index within cluster)
+ENTRY(vlock_trylock)
+	add	r1, r1, #VLOCK_VOTING_OFFSET
+
+	voting_begin	r0, r1, r2
+
+	ldrb	r2, [r0, #VLOCK_OWNER_OFFSET]	@ check whether lock is held
+	cmp	r2, #VLOCK_OWNER_NONE
+	bne	trylock_fail			@ fail if so
+
+	@ Control dependency implies strb not observable before previous ldrb.
+
+	strb	r1, [r0, #VLOCK_OWNER_OFFSET]	@ submit my vote
+
+	voting_end	r0, r1, r2		@ implies DMB
+
+	@ Wait for the current round of voting to finish:
+
+ MANY(	mov	r3, #VLOCK_VOTING_OFFSET			)
+0:
+ MANY(	ldr	r2, [r0, r3]					)
+ FEW(	ldr	r2, [r0, #VLOCK_VOTING_OFFSET]			)
+	cmp	r2, #0
+	wfene
+	bne	0b
+ MANY(	add	r3, r3, #4					)
+ MANY(	cmp	r3, #VLOCK_VOTING_OFFSET + VLOCK_VOTING_SIZE	)
+ MANY(	bne	0b						)
+
+	@ Check who won:
+
+	dmb
+	ldrb	r2, [r0, #VLOCK_OWNER_OFFSET]
+	eor	r0, r1, r2			@ zero if I won, else nonzero
+	bx	lr
+
+trylock_fail:
+	voting_end	r0, r1, r2
+	mov	r0, #1				@ nonzero indicates that I lost
+	bx	lr
+ENDPROC(vlock_trylock)
+
+@ r0: lock structure base
+ENTRY(vlock_unlock)
+	dmb
+	mov	r1, #VLOCK_OWNER_NONE
+	strb	r1, [r0, #VLOCK_OWNER_OFFSET]
+	dsb	st
+	sev
+	bx	lr
+ENDPROC(vlock_unlock)