RE: [PATCH] kasan: improve double-free detection

From: Luruo, Kuthonuzo
Date: Tue May 03 2016 - 03:53:26 EST

> I missed that Alexander already landed patches that reduce header size
> to 16 bytes.
> It is not OK to increase them again. Please leave state as bitfield
> and update it with CAS (if we introduce helper functions for state
> manipulation, they will hide the CAS loop, which is nice).

Available CAS primitives/compiler do not support CAS with bitfield. I propose
to change kasan_alloc_meta to:

struct kasan_alloc_meta {
struct kasan_track track;
u16 size_delta; /* object_size - alloc size */
u8 state; /* enum kasan_state */
u8 reserved1;
u32 reserved2;

This shrinks _used_ meta object by 1 byte wrt the original. (btw, patch v1 does
not increase overall alloc meta object size). "Alloc size", where needed, is
easily calculated as a delta from cache->object_size.