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SubjectRe: [PATCH v8 16/20] kasan: add hooks implementation for tag-based mode
On Fri, Sep 21, 2018 at 1:37 PM, Dmitry Vyukov <dvyukov@google.com> wrote:
> On Wed, Sep 19, 2018 at 8:54 PM, Andrey Konovalov <andreyknvl@google.com> wrote:

>> + /*
>> + * Since it's desirable to only call object contructors ones during
>
> s/ones/once/

Will fix.

>
>> + * slab allocation, we preassign tags to all such objects.
>
> While we are here, it can make sense to mention that we can't repaint
> objects with ctors after reallocation (even for
> non-SLAB_TYPESAFE_BY_RCU) because the ctor code can memorize pointer
> to the object somewhere (e.g. in the object itself). Then if we
> repaint it, the old memorized pointer will become invalid.

Will mention.

>> - kasan_unpoison_shadow(object, size);
>> + /* See the comment in kasan_init_slab_obj regarding preassigned tags */
>> + if (IS_ENABLED(CONFIG_KASAN_SW_TAGS) &&
>> + (cache->ctor || cache->flags & SLAB_TYPESAFE_BY_RCU)) {
>> +#ifdef CONFIG_SLAB
>> + struct page *page = virt_to_page(object);
>> +
>> + tag = (u8)obj_to_index(cache, page, (void *)object);
>> +#else
>> + tag = get_tag(object);
>> +#endif
>
> This kinda _almost_ matches the chunk of code in kasan_init_slab_obj,
> but not exactly. Wonder if there is some nice way to unify this code?
>
> Maybe something like:
>
> static u8 tag_for_object(struct kmem_cache *cache, const void *object, new bool)
> {
> if (!IS_ENABLED(CONFIG_KASAN_SW_TAGS) ||
> !cache->ctor && !(cache->flags & SLAB_TYPESAFE_BY_RCU))
> return random_tag();
> #ifdef CONFIG_SLAB
> struct page *page = virt_to_page(object);
> return (u8)obj_to_index(cache, page, (void *)object);
> #else
> return new ? random_tag() : get_tag(object);
> #endif
> }
>
> Then we can call this in both places.

Will do, however I think it's better to do the CONFIG_KASAN_SW_TAGS
check outside this helper function.

> As a side effect this will assign tags to pointers during slab
> initialization even if we don't have ctors, but it should be fine (?).

We don't have to assign tag in this case, can just leave 0xff.

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