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

[Luruo, Kuthonuzo]

> 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.

Kuthonuzo