[PATCH v4 12/18] x86: Minimal SLAB alignment

[Maciej Wieczor-Retman]

8 byte minimal SLAB alignment interferes with KASAN's granularity of 16
bytes. It causes a lot of out-of-bounds errors for unaligned 8 byte
allocations.

Compared to a kernel with KASAN disabled, the memory footprint increases
because all kmalloc-8 allocations now are realized as kmalloc-16, which
has twice the object size. But more meaningfully, when compared to a
kernel with generic KASAN enabled, there is no difference. Because of
redzones in generic KASAN, kmalloc-8' and kmalloc-16' object size is the
same (48 bytes). So changing the minimal SLAB alignment of the tag-based
mode doesn't have any negative impact when compared to the other
software KASAN mode.

Adjust x86 minimal SLAB alignment to match KASAN granularity size.

Signed-off-by: Maciej Wieczor-Retman <maciej.wieczor-retman@xxxxxxxxx>
---
Changelog v4:
- Extend the patch message with some more context and impact
  information.

Changelog v3:
- Fix typo in patch message 4 -> 16.
- Change define location to arch/x86/include/asm/cache.c.

 arch/x86/include/asm/cache.h | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/arch/x86/include/asm/cache.h b/arch/x86/include/asm/cache.h
index 69404eae9983..3232583b5487 100644
--- a/arch/x86/include/asm/cache.h
+++ b/arch/x86/include/asm/cache.h
@@ -21,4 +21,8 @@
 #endif
 #endif
 
+#ifdef CONFIG_KASAN_SW_TAGS
+#define ARCH_SLAB_MINALIGN (1ULL << KASAN_SHADOW_SCALE_SHIFT)
+#endif
+
 #endif /* _ASM_X86_CACHE_H */
-- 
2.50.1