Re: [PATCH mm] kasan, arm64: use ARCH_SLAB_MINALIGN instead of manual aligning

On Tue, Dec 18, 2018 at 9:55 PM Andrew Morton [off-list ref] wrote:

On Tue, 18 Dec 2018 14:30:33 +0100 Andrey Konovalov [off-list ref] wrote:

quoted

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.

KASAN code doesn't use this macro directly, so I don't think it needs
to get it's definition.

What do you think about adding #include <linux/kasan.h> into
linux/slab.h? Perhaps with a comment that this is needed to get
definition of ARCH_SLAB_MINALIGN?

_______________________________________________
linux-arm-kernel mailing list
linux-arm-kernel@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-arm-kernel