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SubjectRe: [PATCH mm] kasan, arm64: use ARCH_SLAB_MINALIGN instead of manual aligning
On Tue, 18 Dec 2018 14:30:33 +0100 Andrey Konovalov <andreyknvl@google.com> wrote:

> Instead of changing cache->align to be aligned to KASAN_SHADOW_SCALE_SIZE
> in kasan_cache_create() we can reuse the ARCH_SLAB_MINALIGN macro.
>
> ...
>
> --- a/arch/arm64/include/asm/kasan.h
> +++ b/arch/arm64/include/asm/kasan.h
> @@ -36,6 +36,10 @@
> #define KASAN_SHADOW_OFFSET (KASAN_SHADOW_END - (1ULL << \
> (64 - KASAN_SHADOW_SCALE_SHIFT)))
>
> +#ifdef CONFIG_KASAN_SW_TAGS
> +#define ARCH_SLAB_MINALIGN (1ULL << KASAN_SHADOW_SCALE_SHIFT)
> +#endif
> +
> void kasan_init(void);
> void kasan_copy_shadow(pgd_t *pgdir);
> asmlinkage void kasan_early_init(void);

This looks unreliable. include/linux/slab.h has

/*
* Setting ARCH_SLAB_MINALIGN in arch headers allows a different alignment.
* Intended for arches that get misalignment faults even for 64 bit integer
* aligned buffers.
*/
#ifndef ARCH_SLAB_MINALIGN
#define ARCH_SLAB_MINALIGN __alignof__(unsigned long long)
#endif

so if a .c file includes arch/arm64/include/asm/kasan.h after
include/linux/slab.h, it can get a macro-redefined warning. If the .c
file includes those headers in the other order, ARCH_SLAB_MINALIGN will
get a different value compared to other .c files.

Or something like that.

Different architectures define ARCH_SLAB_MINALIGN in different place:

./arch/microblaze/include/asm/page.h:#define ARCH_SLAB_MINALIGN L1_CACHE_BYTES
./arch/arm/include/asm/cache.h:#define ARCH_SLAB_MINALIGN 8
./arch/sh/include/asm/page.h:#define ARCH_SLAB_MINALIGN 8
./arch/c6x/include/asm/cache.h:#define ARCH_SLAB_MINALIGN L1_CACHE_BYTES
./arch/sparc/include/asm/cache.h:#define ARCH_SLAB_MINALIGN __alignof__(unsigned long long)
./arch/xtensa/include/asm/processor.h:#define ARCH_SLAB_MINALIGN STACK_ALIGN

which is rather bad of us.

But still. I think your definition should occur in an arch header file
which is reliably included from slab.h. And kasan code should get its
definition of ARCH_SLAB_MINALIGN by including slab.h.

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