Re: [PATCH] net: linearizing skb when downgrade gso_size

[Willem de Bruijn <willemdebruijn.kernel@gmail.com>]

Fred Li wrote:
> Here is a patch that linearizing skb when downgrade
> gso_size and sg should disabled, If there are no issues,
> I will submit a formal patch shortly.

Target bpf.

Probably does not need quite as many direct CCs. 
 
> Signed-off-by: Fred Li <dracodingfly@gmail.com>
> ---
>  include/linux/skbuff.h | 22 ++++++++++++++++++++++
>  net/core/filter.c      | 16 ++++++++++++----
>  net/core/skbuff.c      | 19 ++-----------------
>  3 files changed, 36 insertions(+), 21 deletions(-)
> 
> diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
> index 5f11f9873341..99b7fc1e826a 100644
> --- a/include/linux/skbuff.h
> +++ b/include/linux/skbuff.h
> @@ -2400,6 +2400,28 @@ static inline unsigned int skb_headlen(const struct sk_buff *skb)
>  	return skb->len - skb->data_len;
>  }
>  
> +static inline bool skb_is_nonsg(const struct sk_buff *skb)
> +{

is_nonsg does not cover the functionality, which is fairly subtle.
But maybe we don't need this function at all, see below..

> +	struct sk_buff *list_skb = skb_shinfo(skb)->frag_list;
> +	struct sk_buff *check_skb;

No need for separate check_skb

> +	for (check_skb = list_skb; check_skb; check_skb = check_skb->next) {
> +		if (skb_headlen(check_skb) && !check_skb->head_frag) {
> +			/* gso_size is untrusted, and we have a frag_list with
> +                         * a linear non head_frag item.
> +                         *
> +                         * If head_skb's headlen does not fit requested gso_size,
> +                         * it means that the frag_list members do NOT terminate
> +                         * on exact gso_size boundaries. Hence we cannot perform
> +                         * skb_frag_t page sharing. Therefore we must fallback to
> +                         * copying the frag_list skbs; we do so by disabling SG.
> +                         */
> +			return true;
> +		}
> +	}
> +
> +	return false;
> +}
> +
>  static inline unsigned int __skb_pagelen(const struct sk_buff *skb)
>  {
>  	unsigned int i, len = 0;
> diff --git a/net/core/filter.c b/net/core/filter.c
> index df4578219e82..c0e6e7f28635 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -3525,13 +3525,21 @@ static int bpf_skb_net_grow(struct sk_buff *skb, u32 off, u32 len_diff,
>  	if (skb_is_gso(skb)) {
>  		struct skb_shared_info *shinfo = skb_shinfo(skb);
>  
> -		/* Due to header grow, MSS needs to be downgraded. */
> -		if (!(flags & BPF_F_ADJ_ROOM_FIXED_GSO))
> -			skb_decrease_gso_size(shinfo, len_diff);
> -
>  		/* Header must be checked, and gso_segs recomputed. */
>  		shinfo->gso_type |= gso_type;
>  		shinfo->gso_segs = 0;
> +
> +		/* Due to header grow, MSS needs to be downgraded.
> +		 * There is BUG_ON When segment the frag_list with
> +		 * head_frag true so linearize skb after downgrade
> +		 * the MSS.
> +		 */
> +		if (!(flags & BPF_F_ADJ_ROOM_FIXED_GSO)) {
> +			skb_decrease_gso_size(shinfo, len_diff);
> +			if (skb_is_nonsg(skb))
> +				return skb_linearize(skb) ? : 0;
> +		}
> +

No need for ternary statement.

Instead of the complex test in skb_is_nonsg, can we just assume that
alignment will be off if having frag_list and changing gso_size.

The same will apply to bpf_skb_net_shrink too.

Not sure that it is okay to linearize inside a BPF helper function.
Hopefully bpf experts can chime in on that.

>  	}
>  
>  	return 0;
> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> index b1dab1b071fc..81e018185527 100644
> --- a/net/core/skbuff.c
> +++ b/net/core/skbuff.c
> @@ -4458,23 +4458,8 @@ struct sk_buff *skb_segment(struct sk_buff *head_skb,
>  
>  	if ((skb_shinfo(head_skb)->gso_type & SKB_GSO_DODGY) &&
>  	    mss != GSO_BY_FRAGS && mss != skb_headlen(head_skb)) {
> -		struct sk_buff *check_skb;
> -
> -		for (check_skb = list_skb; check_skb; check_skb = check_skb->next) {
> -			if (skb_headlen(check_skb) && !check_skb->head_frag) {
> -				/* gso_size is untrusted, and we have a frag_list with
> -				 * a linear non head_frag item.
> -				 *
> -				 * If head_skb's headlen does not fit requested gso_size,
> -				 * it means that the frag_list members do NOT terminate
> -				 * on exact gso_size boundaries. Hence we cannot perform
> -				 * skb_frag_t page sharing. Therefore we must fallback to
> -				 * copying the frag_list skbs; we do so by disabling SG.
> -				 */
> -				features &= ~NETIF_F_SG;
> -				break;
> -			}
> -		}
> +		if (skb_is_nonsg(head_skb))
> +			features &= ~NETIF_F_SG;
>  	}
>  
>  	__skb_push(head_skb, doffset);
> -- 
> 2.33.0
>