Re: [PATCH v5 3/4] mm: kasan: Add free_meta size info in struct kasan_cache

[Feng Tang]

Hi Andrey,

Thanks for reviewing this series!

On Sun, Sep 11, 2022 at 07:14:55AM +0800, Andrey Konovalov wrote:
> On Wed, Sep 7, 2022 at 9:11 AM Feng Tang <feng.tang@xxxxxxxxx> wrote:
> >
> > When kasan is enabled for slab/slub, it may save kasan' free_meta
> > data in the former part of slab object data area in slab object
> > free path, which works fine.
> >
> > There is ongoing effort to extend slub's debug function which will
> > redzone the latter part of kmalloc object area, and when both of
> > the debug are enabled, there is possible conflict, especially when
> > the kmalloc object has small size, as caught by 0Day bot [1]
> >
> > For better information for slab/slub, add free_meta's data size
> > into 'struct kasan_cache', so that its users can take right action
> > to avoid data conflict.
> >
> > [1]. https://lore.kernel.org/lkml/YuYm3dWwpZwH58Hu@xsang-OptiPlex-9020/
> > Reported-by: kernel test robot <oliver.sang@xxxxxxxxx>
> > Signed-off-by: Feng Tang <feng.tang@xxxxxxxxx>
> > Acked-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
> > ---
> >  include/linux/kasan.h | 2 ++
> >  mm/kasan/common.c     | 2 ++
> >  2 files changed, 4 insertions(+)
> >
> > diff --git a/include/linux/kasan.h b/include/linux/kasan.h
> > index b092277bf48d..293bdaa0ba09 100644
> > --- a/include/linux/kasan.h
> > +++ b/include/linux/kasan.h
> > @@ -100,6 +100,8 @@ static inline bool kasan_has_integrated_init(void)
> >  struct kasan_cache {
> >         int alloc_meta_offset;
> >         int free_meta_offset;
> > +       /* size of free_meta data saved in object's data area */
> > +       int free_meta_size_in_object;
> 
> I thinks calling this field free_meta_size is clear enough. Thanks!

Yes, the name does look long. The "in_object" was added to make it
also a flag for whether the free meta is saved inside object's data
area. 

For 'free_meta_size', the code logic in slub should be:
  
  if (info->free_meta_offset == 0 &&
	info->free_meta_size >= ...)

Thanks,
Feng