Re: [PATCH v6 3/4] mm: kasan: Add free_meta size info in struct kasan_cache

[Feng Tang <feng.tang@intel.com>]

On Sun, Sep 25, 2022 at 02:05:04AM +0800, Andrey Konovalov wrote:
> On Wed, Sep 21, 2022 at 2:03 PM Feng Tang <feng.tang@intel.com> wrote:
> >
> > Agree, it's better not touch the internal fields in slub code.
> >
> > How about the following patch, it merge the 2 functions with one flag
> > indicating in meta data or object. (I'm fine with 2 separate functions)
> 
> The overall approach sounds good. See some comments below.
> 
> > diff --git a/include/linux/kasan.h b/include/linux/kasan.h
> > index b092277bf48d..0ad05a34e708 100644
> > --- a/include/linux/kasan.h
> > +++ b/include/linux/kasan.h
> > @@ -150,11 +150,12 @@ static __always_inline void kasan_cache_create_kmalloc(struct kmem_cache *cache)
> >                 __kasan_cache_create_kmalloc(cache);
> >  }
> >
> > -size_t __kasan_metadata_size(struct kmem_cache *cache);
> > -static __always_inline size_t kasan_metadata_size(struct kmem_cache *cache)
> > +size_t __kasan_meta_size(struct kmem_cache *cache, bool in_slab_object);
> > +static __always_inline size_t kasan_meta_size(struct kmem_cache *cache,
> > +                                                       bool in_slab_object)
> 
> I would keep the name as kasan_metadata_size as it's more clear to
> external users but rename in_slab_object to in_object to make the
> declaration shorter.

Make sense to me, will do.

[...]

> > +       if (in_slab_object)
> > +               return (cache->kasan_info.alloc_meta_offset == 0 ?
> > +                       sizeof(struct kasan_alloc_meta) : 0) +
> > +                       (cache->kasan_info.free_meta_offset ?
> > +                       sizeof(struct kasan_free_meta) : 0);
> > +       else
> > +               return (cache->kasan_info.alloc_meta_offset == 0 ?
> > +                       sizeof(struct kasan_alloc_meta) : 0) +
> > +                       (cache->kasan_info.free_meta_offset ?
> > +                       sizeof(struct kasan_free_meta) : 0);
> 
> Something weird here: both if and else cases are the same.
 
Yes, will fix it. 

> The change also needs to be rebased onto [1].
> 
> Thanks!
> 
> [1] https://lore.kernel.org/linux-mm/c7b316d30d90e5947eb8280f4dc78856a49298cf.1662411799.git.andreyknvl@google.com/

I noticed this has been merged to -mm tree's 'mm-everything' branch,
so following is the patch againt that. Thanks! 

One thing I'm not very sure is, to check 'in-object' kasan's meta
size, I didn't check 'alloc_meta_offset', as from the code reading
the alloc_meta is never put inside slab object data area.


Thanks,
Feng

---8<---

diff --git a/include/linux/kasan.h b/include/linux/kasan.h
index d811b3d7d2a1..96c9d56e5510 100644
--- a/include/linux/kasan.h
+++ b/include/linux/kasan.h
@@ -302,7 +302,7 @@ static inline void kasan_unpoison_task_stack(struct task_struct *task) {}
 
 #ifdef CONFIG_KASAN_GENERIC
 
-size_t kasan_metadata_size(struct kmem_cache *cache);
+size_t kasan_metadata_size(struct kmem_cache *cache, bool in_object);
 slab_flags_t kasan_never_merge(void);
 void kasan_cache_create(struct kmem_cache *cache, unsigned int *size,
 			slab_flags_t *flags);
@@ -315,7 +315,8 @@ void kasan_record_aux_stack_noalloc(void *ptr);
 #else /* CONFIG_KASAN_GENERIC */
 
 /* Tag-based KASAN modes do not use per-object metadata. */
-static inline size_t kasan_metadata_size(struct kmem_cache *cache)
+static inline size_t kasan_metadata_size(struct kmem_cache *cache,
+						bool in_object)
 {
 	return 0;
 }
diff --git a/mm/kasan/generic.c b/mm/kasan/generic.c
index d8b5590f9484..5a806f9b9466 100644
--- a/mm/kasan/generic.c
+++ b/mm/kasan/generic.c
@@ -450,15 +450,22 @@ void kasan_init_object_meta(struct kmem_cache *cache, const void *object)
 		__memset(alloc_meta, 0, sizeof(*alloc_meta));
 }
 
-size_t kasan_metadata_size(struct kmem_cache *cache)
+size_t kasan_metadata_size(struct kmem_cache *cache, bool in_object)
 {
+	struct kasan_cache *info = &cache->kasan_info ;
+
 	if (!kasan_requires_meta())
 		return 0;
-	return (cache->kasan_info.alloc_meta_offset ?
-		sizeof(struct kasan_alloc_meta) : 0) +
-		((cache->kasan_info.free_meta_offset &&
-		  cache->kasan_info.free_meta_offset != KASAN_NO_FREE_META) ?
-		 sizeof(struct kasan_free_meta) : 0);
+
+	if (in_object)
+		return (info->free_meta_offset ?
+			0 : sizeof(struct kasan_free_meta));
+	else
+		return (info->alloc_meta_offset ?
+			sizeof(struct kasan_alloc_meta) : 0) +
+			((info->free_meta_offset &&
+			info->free_meta_offset != KASAN_NO_FREE_META) ?
+			sizeof(struct kasan_free_meta) : 0);
 }
 
 static void __kasan_record_aux_stack(void *addr, bool can_alloc)
diff --git a/mm/slub.c b/mm/slub.c
index ce8310e131b3..a75c21a0da8b 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -887,7 +887,7 @@ static void print_trailer(struct kmem_cache *s, struct slab *slab, u8 *p)
 	if (s->flags & SLAB_STORE_USER)
 		off += 2 * sizeof(struct track);
 
-	off += kasan_metadata_size(s);
+	off += kasan_metadata_size(s, false);
 
 	if (off != size_from_object(s))
 		/* Beginning of the filler is the free pointer */
@@ -1042,7 +1042,7 @@ static int check_pad_bytes(struct kmem_cache *s, struct slab *slab, u8 *p)
 		/* We also have user information there */
 		off += 2 * sizeof(struct track);
 
-	off += kasan_metadata_size(s);
+	off += kasan_metadata_size(s, false);
 
 	if (size_from_object(s) == off)
 		return 1;