Re: [PATCH 16/16] mm/sl[au]b: check if large object is valid in __ksize()

[Hyeonggon Yoo <42.hyeyoo@gmail.com>]

On Tue, Jul 12, 2022 at 05:13:44PM +0200, Christoph Lameter wrote:
> On Tue, 12 Jul 2022, Hyeonggon Yoo wrote:
> 
> > __ksize() returns size of objects allocated from slab allocator.
> > When invalid object is passed to __ksize(), returning zero
> > prevents further memory corruption and makes caller be able to
> > check if there is an error.
> >
> > If address of large object is not beginning of folio or size of
> > the folio is too small, it must be invalid. Return zero in such cases.
> 
> Why return 0 if there is an error and why bother the callers with these
> checks. BUG()?

I thought BUG should be used when there is no other solution.

> > diff --git a/mm/slab_common.c b/mm/slab_common.c
> > index 1f8db7959366..0d6cbe9d7ad0 100644
> > --- a/mm/slab_common.c
> > +++ b/mm/slab_common.c
> > @@ -1013,8 +1013,12 @@ size_t __ksize(const void *object)
> >
> >  	folio = virt_to_folio(object);
> >
> > -	if (unlikely(!folio_test_slab(folio)))
> > +	if (unlikely(!folio_test_slab(folio))) {
> > +		if (WARN_ON(object != folio_address(folio) ||
> > +				folio_size(folio) <= KMALLOC_MAX_CACHE_SIZE))
> 
> Hmmm... This may change things a bit. Before this patch it was possible to
> determine the storage size of order-0 pages using ksize(). Now this
> returns 0?
> 
> I guess this is an error since the order-0 page cannot come from slab
> allocations.

comment in ksize() says:
	"The caller must guarantee that objp points to a valid object
	previously allocated with either kmalloc() or kmem_cache_alloc()."

It should not be used on order-0 page that is not allocated from slab. No?