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

From: Dmitry Vyukov
Date: Tue May 03 2016 - 13:42:39 EST

On Tue, May 3, 2016 at 9:53 AM, Luruo, Kuthonuzo
<kuthonuzo.luruo@xxxxxxx> wrote:
>> 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.

What is the maximum size that slab can allocate?
I remember seeing slabs as large as 4MB some time ago (or did I
confuse it with something else?). If there are such large objects,
that 2 bytes won't be able to hold even delta.
However, now on my desktop I don't see slabs larger than 16KB in /proc/slabinfo.