Re: [PATCH v3] net: linearizing skb when downgrade gso_size

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

Fred Li wrote:
> Linearizing skb when downgrade gso_size because it may
> trigger the BUG_ON when segment skb as described in [1].
> 
> v3 changes:
>   linearize skb if having frag_list as Willem de Bruijn suggested[2].
> 
> [1] https://lore.kernel.org/all/20240626065555.35460-2-dracodingfly@gmail.com/
> [2] https://lore.kernel.org/all/668d5cf1ec330_1c18c32947@willemb.c.googlers.com.notmuch/
> 
> Signed-off-by: Fred Li <dracodingfly@gmail.com>

A fix needs a Fixed tag.

This might be the original commit that introduced gso_size adjustment,
commit 6578171a7ff0c ("bpf: add bpf_skb_change_proto helper")

Unless support for frag_list came later.

> ---
>  net/core/filter.c | 16 ++++++++++++----
>  1 file changed, 12 insertions(+), 4 deletions(-)
> 
> diff --git a/net/core/filter.c b/net/core/filter.c
> index df4578219e82..70919b532d68 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.
> +		 */

Super tiny nit: no capitalization of When in the middle of a sentence.

> +		if (!(flags & BPF_F_ADJ_ROOM_FIXED_GSO)) {
> +			skb_decrease_gso_size(shinfo, len_diff);
> +			if (shinfo->frag_list)
> +				return skb_linearize(skb);

I previously asked whether it was safe to call pskb_expand_head from
within a BPF external function. There are actually plenty of existing
examples of this, so this is fine.

> +		}
> +
>  	}
>  
>  	return 0;
> -- 
> 2.33.0
>