Re: [PATCH] kasan: unpoison use memset to init unaligned object size

[Yee Lee]

On Tue, 2021-06-22 at 17:03 +0300, Andrey Konovalov wrote:
> On Mon, Jun 21, 2021 at 6:45 PM <yee.lee@xxxxxxxxxxxx> wrote:
> > 
> > From: Yee Lee <yee.lee@xxxxxxxxxxxx>
> > 
> > This patch adds a memset to initialize object of unaligned size.
> > Duing to the MTE granulrity, the integrated initialization using
> > hwtag instruction will force clearing out bytes in granular size,
> > which may cause undesired effect, such as overwriting to the
> > redzone
> > of SLUB debug. In this patch, for the unaligned object size,
> > function
> > uses memset to initailize context instead of the hwtag instruction.
> > 
> > Signed-off-by: Yee Lee <yee.lee@xxxxxxxxxxxx>
> > ---
> >  mm/kasan/kasan.h | 5 ++++-
> >  1 file changed, 4 insertions(+), 1 deletion(-)
> > 
> > diff --git a/mm/kasan/kasan.h b/mm/kasan/kasan.h
> > index 8f450bc28045..d8faa64614b7 100644
> > --- a/mm/kasan/kasan.h
> > +++ b/mm/kasan/kasan.h
> > @@ -387,8 +387,11 @@ static inline void kasan_unpoison(const void
> > *addr, size_t size, bool init)
> > 
> >         if (WARN_ON((unsigned long)addr & KASAN_GRANULE_MASK))
> >                 return;
> > +       if (init && ((unsigned long)size & KASAN_GRANULE_MASK)) {
> > +               init = false;
> > +               memset((void *)addr, 0, size);
> > +       }
> 
> With this implementation, we loose the benefit of setting tags and
> initializing memory with the same instructions.
> 
> Perhaps a better implementation would be to call
> hw_set_mem_tag_range() with the size rounded down, and then
> separately
> deal with the leftover memory.

Yes, this fully takes the advantage of hw instruction. 
However, the leftover memory needs one more hw_set_mem_tag_range() for
protection as well.

If the extra path is only executed as CONFIG_SLUB_DEBUG, the
performance lost would be less concerned.

> 
> >         size = round_up(size, KASAN_GRANULE_SIZE);
> > -
> >         hw_set_mem_tag_range((void *)addr, size, tag, init);
> >  }
> > 
> > --
> > 2.18.0
> >