Re: [PATCH v2 1/3] kasan, arm64: use ARCH_SLAB_MINALIGN instead of manual aligning

From: Andrew Morton <akpm@linux-foundation.org>
To: Andrey Konovalov <andreyknvl@google.com>
Cc: Andrey Ryabinin <aryabinin@virtuozzo.com>,
	Alexander Potapenko <glider@google.com>,
	Dmitry Vyukov <dvyukov@google.com>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will.deacon@arm.com>,
	Christoph Lameter <cl@linux.com>,
	Mark Rutland <mark.rutland@arm.com>,
	Nick Desaulniers <ndesaulniers@google.com>,
	Marc Zyngier <marc.zyngier@arm.com>,
	Dave Martin <dave.martin@arm.com>,
	Ard Biesheuvel <ard.biesheuvel@linaro.org>,
	"Eric W . Biederman" <ebiederm@xmission.com>,
	Ingo Molnar <mingo@kernel.org>,
	Paul Lawrence <paullawrence@google.com>,
	Geert Uytterhoeven <geert@linux-m68k.org>,
	Arnd Bergmann <arnd@arndb.de>,
	"Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Kate Stewart <kstewart@linuxfoundation.org>,
	Mike Rapoport <rppt@linux.vnet.ibm.com>,
	Vincenzo Frascino <vincenzo.frascino@arm.com>,
	kasan-dev@googlegroups.com, linux-doc@vger.kernel.org,
	linux-kernel@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org,
	linux-sparse@vger.kernel.org, linux-mm@kvack.org,
	linux-kbuild@vger.kernel.org, Kostya Serebryany <kcc@google.com>,
	Evgeniy Stepanov <eugenis@google.com>,
	Lee Smith <Lee.Smith@arm.com>,
	Ramana Radhakrishnan <Ramana.Radhakrishnan@arm.com>,
	Jacob Bramley <Jacob.Bramley@arm.com>,
	Ruben Ayrapetyan <Ruben.Ayrapetyan@arm.com>,
	Jann Horn <jannh@google.com>, Mark Brand <markbrand@google.com>,
	Chintan Pandya <cpandya@codeaurora.org>,
	Vishwath Mohan <vishwath@google.com>
Subject: Re: [PATCH v2 1/3] kasan, arm64: use ARCH_SLAB_MINALIGN instead of manual aligning
Date: Wed, 2 Jan 2019 12:14:36 -0800	[thread overview]
Message-ID: <20190102121436.5c2b72d1b0ec49affadc9692@linux-foundation.org> (raw)
In-Reply-To: <b16c90197bb2c06c780e6e981c40345e03fda465.1546450432.git.andreyknvl@google.com>

On Wed,  2 Jan 2019 18:36:06 +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);
> diff --git a/include/linux/slab.h b/include/linux/slab.h
> index 11b45f7ae405..d87f913ab4e8 100644
> --- a/include/linux/slab.h
> +++ b/include/linux/slab.h
> @@ -16,6 +16,7 @@
>  #include <linux/overflow.h>
>  #include <linux/types.h>
>  #include <linux/workqueue.h>
> +#include <linux/kasan.h>
>  

This still seems unadvisable.  Like other architectures, arm defines
ARCH_SLAB_MINALIGN in arch/arm/include/asm/cache.h. 
arch/arm/include/asm64/cache.h doesn't define ARCH_SLAB_MINALIGN
afaict.

If arch/arm/include/asm64/cache.h later gets a definition of
ARCH_SLAB_MINALIGN then we again face the risk that different .c files
will see different values of ARCH_SLAB_MINALIGN depending on which
headers they include.

So what to say about this?  The architecture's ARCH_SLAB_MINALIGN
should be defined in the architecture's cache.h, end of story.  Not in
slab.h, not in kasan.h.