Re: [PATCH 1/5] mm/sl[au]b: Unify __ksize()

[Vlastimil Babka <vbabka@suse.cz>]

On 2/21/22 11:53, Hyeonggon Yoo wrote:
> Only SLOB need to implement __ksize() separately because SLOB records
> size in object header for kmalloc objects. Unify SLAB/SLUB's __ksize().
> 
> Signed-off-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>
> ---
>  mm/slab.c        | 23 -----------------------
>  mm/slab_common.c | 29 +++++++++++++++++++++++++++++
>  mm/slub.c        | 16 ----------------
>  3 files changed, 29 insertions(+), 39 deletions(-)
> 
> diff --git a/mm/slab.c b/mm/slab.c
> index ddf5737c63d9..eb73d2499480 100644
> --- a/mm/slab.c
> +++ b/mm/slab.c
> @@ -4199,27 +4199,4 @@ void __check_heap_object(const void *ptr, unsigned long n,
>  }
>  #endif /* CONFIG_HARDENED_USERCOPY */
>  
> -/**
> - * __ksize -- Uninstrumented ksize.
> - * @objp: pointer to the object
> - *
> - * Unlike ksize(), __ksize() is uninstrumented, and does not provide the same
> - * safety checks as ksize() with KASAN instrumentation enabled.
> - *
> - * Return: size of the actual memory used by @objp in bytes
> - */
> -size_t __ksize(const void *objp)
> -{
> -	struct kmem_cache *c;
> -	size_t size;
>  
> -	BUG_ON(!objp);
> -	if (unlikely(objp == ZERO_SIZE_PTR))
> -		return 0;
> -
> -	c = virt_to_cache(objp);
> -	size = c ? c->object_size : 0;

This comes from commit a64b53780ec3 ("mm/slab: sanity-check page type when
looking up cache") by Kees and virt_to_cache() is an implicit check for
folio slab flag ...

> -
> -	return size;
> -}
> -EXPORT_SYMBOL(__ksize);
> diff --git a/mm/slab_common.c b/mm/slab_common.c
> index 23f2ab0713b7..488997db0d97 100644
> --- a/mm/slab_common.c
> +++ b/mm/slab_common.c
> @@ -1245,6 +1245,35 @@ void kfree_sensitive(const void *p)
>  }
>  EXPORT_SYMBOL(kfree_sensitive);
>  
> +#ifndef CONFIG_SLOB
> +/**
> + * __ksize -- Uninstrumented ksize.
> + * @objp: pointer to the object
> + *
> + * Unlike ksize(), __ksize() is uninstrumented, and does not provide the same
> + * safety checks as ksize() with KASAN instrumentation enabled.
> + *
> + * Return: size of the actual memory used by @objp in bytes
> + */
> +size_t __ksize(const void *object)
> +{
> +	struct folio *folio;
> +
> +	if (unlikely(object == ZERO_SIZE_PTR))
> +		return 0;
> +
> +	folio = virt_to_folio(object);
> +
> +#ifdef CONFIG_SLUB
> +	if (unlikely(!folio_test_slab(folio)))
> +		return folio_size(folio);
> +#endif
> +
> +	return slab_ksize(folio_slab(folio)->slab_cache);

... and here in the common version you now for SLAB trust that the folio
will be a slab folio, thus undoing the intention of that commit. Maybe
that's not good and we should keep the folio_test_slab() for both cases?
Although maybe it's also strange that prior this patch, SLAB would return 0
if the test fails, and SLUB would return folio_size(). Probably because with
SLUB this can be a large kmalloc here and with SLAB not. So we could keep
doing that in the unified version, or KASAN devs (CC'd) could advise
something better?

> +}
> +EXPORT_SYMBOL(__ksize);
> +#endif
> +
>  /**
>   * ksize - get the actual amount of memory allocated for a given object
>   * @objp: Pointer to the object
> diff --git a/mm/slub.c b/mm/slub.c
> index 261474092e43..3a4458976ab7 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -4526,22 +4526,6 @@ void __check_heap_object(const void *ptr, unsigned long n,
>  }
>  #endif /* CONFIG_HARDENED_USERCOPY */
>  
> -size_t __ksize(const void *object)
> -{
> -	struct folio *folio;
> -
> -	if (unlikely(object == ZERO_SIZE_PTR))
> -		return 0;
> -
> -	folio = virt_to_folio(object);
> -
> -	if (unlikely(!folio_test_slab(folio)))
> -		return folio_size(folio);
> -
> -	return slab_ksize(folio_slab(folio)->slab_cache);
> -}
> -EXPORT_SYMBOL(__ksize);
> -
>  void kfree(const void *x)
>  {
>  	struct folio *folio;