Subject: Re: [PATCH 02/12] skbuff: Proactively round up to kmalloc bucket size

From: Jakub Kicinski <[email-masked]>
Date: Thu, 22 Sep 2022 12:40:39 -0700

On Wed, 21 Sep 2022 20:10:03 -0700 Kees Cook wrote:
> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> index 974bbbbe7138..4fe4c7544c1d 100644
> --- a/net/core/skbuff.c
> +++ b/net/core/skbuff.c
> @@ -427,14 +427,15 @@ struct sk_buff *__alloc_skb(unsigned int size, gfp_t gfp_mask,
>  	 */
>  	size = SKB_DATA_ALIGN(size);
>  	size += SKB_DATA_ALIGN(sizeof(struct skb_shared_info));
> -	data = kmalloc_reserve(size, gfp_mask, node, &pfmemalloc);
> -	if (unlikely(!data))
> -		goto nodata;
> -	/* kmalloc(size) might give us more room than requested.
> +	/* kmalloc(size) might give us more room than requested, so
> +	 * allocate the true bucket size up front.
>  	 * Put skb_shared_info exactly at the end of allocated zone,
>  	 * to allow max possible filling before reallocation.
>  	 */
> -	osize = ksize(data);
> +	osize = kmalloc_size_roundup(size);
> +	data = kmalloc_reserve(osize, gfp_mask, node, &pfmemalloc);
> +	if (unlikely(!data))
> +		goto nodata;
>  	size = SKB_WITH_OVERHEAD(osize);
>  	prefetchw(data + size);

I'd rename osize here to alloc_size for consistency but one could 
argue either way :)

Acked-by: Jakub Kicinski <[email-masked]>