Re: [PATCH v5 2/4] mm/slub: only zero the requested size of buffer for kzalloc

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

On Wed, Sep 07, 2022 at 03:10:21PM +0800, Feng Tang wrote:
> kzalloc/kmalloc will round up the request size to a fixed size
> (mostly power of 2), so the allocated memory could be more than
> requested. Currently kzalloc family APIs will zero all the
> allocated memory.
> 
> To detect out-of-bound usage of the extra allocated memory, only
> zero the requested part, so that sanity check could be added to
> the extra space later.
> 
> For kzalloc users who will call ksize() later and utilize this
> extra space, please be aware that the space is not zeroed any
> more.

Can this break existing users?
or should we initialize extra bytes to zero when someone called ksize()?

If it is not going to break something - I think we can add a comment of this.
something like "... kzalloc() will initialize to zero only for @size bytes ..."

> Signed-off-by: Feng Tang <feng.tang@intel.com>
> ---
>  mm/slab.c | 6 +++---
>  mm/slab.h | 9 +++++++--
>  mm/slub.c | 6 +++---
>  3 files changed, 13 insertions(+), 8 deletions(-)
> 
> diff --git a/mm/slab.c b/mm/slab.c
> index a5486ff8362a..73ecaa7066e1 100644
> --- a/mm/slab.c
> +++ b/mm/slab.c
> @@ -3253,7 +3253,7 @@ slab_alloc_node(struct kmem_cache *cachep, struct list_lru *lru, gfp_t flags,
>  	init = slab_want_init_on_alloc(flags, cachep);
>  
>  out:
> -	slab_post_alloc_hook(cachep, objcg, flags, 1, &objp, init);
> +	slab_post_alloc_hook(cachep, objcg, flags, 1, &objp, init, 0);
>  	return objp;
>  }
>  
> @@ -3506,13 +3506,13 @@ int kmem_cache_alloc_bulk(struct kmem_cache *s, gfp_t flags, size_t size,
>  	 * Done outside of the IRQ disabled section.
>  	 */
>  	slab_post_alloc_hook(s, objcg, flags, size, p,
> -				slab_want_init_on_alloc(flags, s));
> +				slab_want_init_on_alloc(flags, s), 0);
>  	/* FIXME: Trace call missing. Christoph would like a bulk variant */
>  	return size;
>  error:
>  	local_irq_enable();
>  	cache_alloc_debugcheck_after_bulk(s, flags, i, p, _RET_IP_);
> -	slab_post_alloc_hook(s, objcg, flags, i, p, false);
> +	slab_post_alloc_hook(s, objcg, flags, i, p, false, 0);
>  	kmem_cache_free_bulk(s, i, p);
>  	return 0;
>  }
> diff --git a/mm/slab.h b/mm/slab.h
> index d0ef9dd44b71..20f9e2a9814f 100644
> --- a/mm/slab.h
> +++ b/mm/slab.h
> @@ -730,12 +730,17 @@ static inline struct kmem_cache *slab_pre_alloc_hook(struct kmem_cache *s,
>  
>  static inline void slab_post_alloc_hook(struct kmem_cache *s,
>  					struct obj_cgroup *objcg, gfp_t flags,
> -					size_t size, void **p, bool init)
> +					size_t size, void **p, bool init,
> +					unsigned int orig_size)
>  {
>  	size_t i;
>  
>  	flags &= gfp_allowed_mask;
>  
> +	/* If original request size(kmalloc) is not set, use object_size */
> +	if (!orig_size)
> +		orig_size = s->object_size;

I think it is more readable to pass s->object_size than zero

> +
>  	/*
>  	 * As memory initialization might be integrated into KASAN,
>  	 * kasan_slab_alloc and initialization memset must be
> @@ -746,7 +751,7 @@ static inline void slab_post_alloc_hook(struct kmem_cache *s,
>  	for (i = 0; i < size; i++) {
>  		p[i] = kasan_slab_alloc(s, p[i], flags, init);
>  		if (p[i] && init && !kasan_has_integrated_init())
> -			memset(p[i], 0, s->object_size);
> +			memset(p[i], 0, orig_size);
>  		kmemleak_alloc_recursive(p[i], s->object_size, 1,
>  					 s->flags, flags);
>  	}
> diff --git a/mm/slub.c b/mm/slub.c
> index effd994438e6..f523601d3fcf 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -3376,7 +3376,7 @@ static __always_inline void *slab_alloc_node(struct kmem_cache *s, struct list_l
>  	init = slab_want_init_on_alloc(gfpflags, s);
>  
>  out:
> -	slab_post_alloc_hook(s, objcg, gfpflags, 1, &object, init);
> +	slab_post_alloc_hook(s, objcg, gfpflags, 1, &object, init, orig_size);
>  
>  	return object;
>  }
> @@ -3833,11 +3833,11 @@ int kmem_cache_alloc_bulk(struct kmem_cache *s, gfp_t flags, size_t size,
>  	 * Done outside of the IRQ disabled fastpath loop.
>  	 */
>  	slab_post_alloc_hook(s, objcg, flags, size, p,
> -				slab_want_init_on_alloc(flags, s));
> +				slab_want_init_on_alloc(flags, s), 0);
>  	return i;
>  error:
>  	slub_put_cpu_ptr(s->cpu_slab);
> -	slab_post_alloc_hook(s, objcg, flags, i, p, false);
> +	slab_post_alloc_hook(s, objcg, flags, i, p, false, 0);
>  	kmem_cache_free_bulk(s, i, p);

>  	return 0;
>  }
> -- 
> 2.34.1
>

-- 
Thanks,
Hyeonggon