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, 247 insertions, 0 deletions

diff --git a/arch/arm/mm/cache-fa.S b/arch/arm/mm/cache-fa.S
new file mode 100644
index 000000000..e16417995
--- /dev/null
+++ b/arch/arm/mm/cache-fa.S
@@ -0,0 +1,247 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+/*
+ *  linux/arch/arm/mm/cache-fa.S
+ *
+ *  Copyright (C) 2005 Faraday Corp.
+ *  Copyright (C) 2008-2009 Paulius Zaleckas <paulius.zaleckas@teltonika.lt>
+ *
+ * Based on cache-v4wb.S:
+ *  Copyright (C) 1997-2002 Russell king
+ *
+ *  Processors: FA520 FA526 FA626	
+ */
+#include <linux/linkage.h>
+#include <linux/init.h>
+#include <linux/cfi_types.h>
+#include <asm/assembler.h>
+#include <asm/page.h>
+
+#include "proc-macros.S"
+
+/*
+ * The size of one data cache line.
+ */
+#define CACHE_DLINESIZE	16
+
+/*
+ * The total size of the data cache.
+ */
+#ifdef CONFIG_ARCH_GEMINI
+#define CACHE_DSIZE	8192
+#else
+#define CACHE_DSIZE	16384 
+#endif 
+
+/* FIXME: put optimal value here. Current one is just estimation */
+#define CACHE_DLIMIT	(CACHE_DSIZE * 2)
+
+/*
+ *	flush_icache_all()
+ *
+ *	Unconditionally clean and invalidate the entire icache.
+ */
+SYM_TYPED_FUNC_START(fa_flush_icache_all)
+	mov	r0, #0
+	mcr	p15, 0, r0, c7, c5, 0		@ invalidate I cache
+	ret	lr
+SYM_FUNC_END(fa_flush_icache_all)
+
+/*
+ *	flush_user_cache_all()
+ *
+ *	Clean and invalidate all cache entries in a particular address
+ *	space.
+ */
+SYM_FUNC_ALIAS(fa_flush_user_cache_all, fa_flush_kern_cache_all)
+
+/*
+ *	flush_kern_cache_all()
+ *
+ *	Clean and invalidate the entire cache.
+ */
+SYM_TYPED_FUNC_START(fa_flush_kern_cache_all)
+	mov	ip, #0
+	mov	r2, #VM_EXEC
+__flush_whole_cache:
+	mcr	p15, 0, ip, c7, c14, 0		@ clean/invalidate D cache
+	tst	r2, #VM_EXEC
+	mcrne	p15, 0, ip, c7, c5, 0		@ invalidate I cache
+	mcrne	p15, 0, ip, c7, c5, 6		@ invalidate BTB
+	mcrne	p15, 0, ip, c7, c10, 4		@ drain write buffer
+	mcrne	p15, 0, ip, c7, c5, 4		@ prefetch flush
+	ret	lr
+SYM_FUNC_END(fa_flush_kern_cache_all)
+
+/*
+ *	flush_user_cache_range(start, end, flags)
+ *
+ *	Invalidate a range of cache entries in the specified
+ *	address space.
+ *
+ *	- start - start address (inclusive, page aligned)
+ *	- end	- end address (exclusive, page aligned)
+ *	- flags	- vma_area_struct flags describing address space
+ */
+SYM_TYPED_FUNC_START(fa_flush_user_cache_range)
+	mov	ip, #0
+	sub	r3, r1, r0			@ calculate total size
+	cmp	r3, #CACHE_DLIMIT		@ total size >= limit?
+	bhs	__flush_whole_cache		@ flush whole D cache
+
+1:	tst	r2, #VM_EXEC
+	mcrne	p15, 0, r0, c7, c5, 1		@ invalidate I line
+	mcr	p15, 0, r0, c7, c14, 1		@ clean and invalidate D entry
+	add	r0, r0, #CACHE_DLINESIZE
+	cmp	r0, r1
+	blo	1b
+	tst	r2, #VM_EXEC
+	mcrne	p15, 0, ip, c7, c5, 6		@ invalidate BTB
+	mcrne	p15, 0, ip, c7, c10, 4		@ data write barrier
+	mcrne	p15, 0, ip, c7, c5, 4		@ prefetch flush
+	ret	lr
+SYM_FUNC_END(fa_flush_user_cache_range)
+
+/*
+ *	coherent_kern_range(start, end)
+ *
+ *	Ensure coherency between the Icache and the Dcache in the
+ *	region described by start.  If you have non-snooping
+ *	Harvard caches, you need to implement this function.
+ *
+ *	- start  - virtual start address
+ *	- end	 - virtual end address
+ */
+SYM_TYPED_FUNC_START(fa_coherent_kern_range)
+#ifdef CONFIG_CFI /* Fallthrough if !CFI */
+	b	fa_coherent_user_range
+#endif
+SYM_FUNC_END(fa_coherent_kern_range)
+
+/*
+ *	coherent_user_range(start, end)
+ *
+ *	Ensure coherency between the Icache and the Dcache in the
+ *	region described by start.  If you have non-snooping
+ *	Harvard caches, you need to implement this function.
+ *
+ *	- start  - virtual start address
+ *	- end	 - virtual end address
+ */
+SYM_TYPED_FUNC_START(fa_coherent_user_range)
+	bic	r0, r0, #CACHE_DLINESIZE - 1
+1:	mcr	p15, 0, r0, c7, c14, 1		@ clean and invalidate D entry
+	mcr	p15, 0, r0, c7, c5, 1		@ invalidate I entry
+	add	r0, r0, #CACHE_DLINESIZE
+	cmp	r0, r1
+	blo	1b
+	mov	r0, #0
+	mcr	p15, 0, r0, c7, c5, 6		@ invalidate BTB
+	mcr	p15, 0, r0, c7, c10, 4		@ drain write buffer
+	mcr	p15, 0, r0, c7, c5, 4		@ prefetch flush
+	ret	lr
+SYM_FUNC_END(fa_coherent_user_range)
+
+/*
+ *	flush_kern_dcache_area(void *addr, size_t size)
+ *
+ *	Ensure that the data held in the page kaddr is written back
+ *	to the page in question.
+ *
+ *	- addr	- kernel address
+ *	- size	- size of region
+ */
+SYM_TYPED_FUNC_START(fa_flush_kern_dcache_area)
+	add	r1, r0, r1
+1:	mcr	p15, 0, r0, c7, c14, 1		@ clean & invalidate D line
+	add	r0, r0, #CACHE_DLINESIZE
+	cmp	r0, r1
+	blo	1b
+	mov	r0, #0
+	mcr	p15, 0, r0, c7, c5, 0		@ invalidate I cache
+	mcr	p15, 0, r0, c7, c10, 4		@ drain write buffer
+	ret	lr
+SYM_FUNC_END(fa_flush_kern_dcache_area)
+
+/*
+ *	dma_inv_range(start, end)
+ *
+ *	Invalidate (discard) the specified virtual address range.
+ *	May not write back any entries.  If 'start' or 'end'
+ *	are not cache line aligned, those lines must be written
+ *	back.
+ *
+ *	- start  - virtual start address
+ *	- end	 - virtual end address
+ */
+fa_dma_inv_range:
+	tst	r0, #CACHE_DLINESIZE - 1
+	bic	r0, r0, #CACHE_DLINESIZE - 1
+	mcrne	p15, 0, r0, c7, c14, 1		@ clean & invalidate D entry
+	tst	r1, #CACHE_DLINESIZE - 1
+	bic	r1, r1, #CACHE_DLINESIZE - 1
+	mcrne	p15, 0, r1, c7, c14, 1		@ clean & invalidate D entry
+1:	mcr	p15, 0, r0, c7, c6, 1		@ invalidate D entry
+	add	r0, r0, #CACHE_DLINESIZE
+	cmp	r0, r1
+	blo	1b
+	mov	r0, #0
+	mcr	p15, 0, r0, c7, c10, 4		@ drain write buffer
+	ret	lr
+
+/*
+ *	dma_clean_range(start, end)
+ *
+ *	Clean (write back) the specified virtual address range.
+ *
+ *	- start  - virtual start address
+ *	- end	 - virtual end address
+ */
+fa_dma_clean_range:
+	bic	r0, r0, #CACHE_DLINESIZE - 1
+1:	mcr	p15, 0, r0, c7, c10, 1		@ clean D entry
+	add	r0, r0, #CACHE_DLINESIZE
+	cmp	r0, r1
+	blo	1b
+	mov	r0, #0	
+	mcr	p15, 0, r0, c7, c10, 4		@ drain write buffer
+	ret	lr
+
+/*
+ *	dma_flush_range(start,end)
+ *	- start   - virtual start address of region
+ *	- end     - virtual end address of region
+ */
+SYM_TYPED_FUNC_START(fa_dma_flush_range)
+	bic	r0, r0, #CACHE_DLINESIZE - 1
+1:	mcr	p15, 0, r0, c7, c14, 1		@ clean & invalidate D entry
+	add	r0, r0, #CACHE_DLINESIZE
+	cmp	r0, r1
+	blo	1b
+	mov	r0, #0	
+	mcr	p15, 0, r0, c7, c10, 4		@ drain write buffer
+	ret	lr
+SYM_FUNC_END(fa_dma_flush_range)
+
+/*
+ *	dma_map_area(start, size, dir)
+ *	- start	- kernel virtual start address
+ *	- size	- size of region
+ *	- dir	- DMA direction
+ */
+SYM_TYPED_FUNC_START(fa_dma_map_area)
+	add	r1, r1, r0
+	cmp	r2, #DMA_TO_DEVICE
+	beq	fa_dma_clean_range
+	bcs	fa_dma_inv_range
+	b	fa_dma_flush_range
+SYM_FUNC_END(fa_dma_map_area)
+
+/*
+ *	dma_unmap_area(start, size, dir)
+ *	- start	- kernel virtual start address
+ *	- size	- size of region
+ *	- dir	- DMA direction
+ */
+SYM_TYPED_FUNC_START(fa_dma_unmap_area)
+	ret	lr
+SYM_FUNC_END(fa_dma_unmap_area)