netdev - Re: [PATCH net v1 1/2] net/mlx5e: RX, Fix generating skb from non-linear xdp

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Message-ID: <x4b26sfgbwuxodwbkk5gl5ohczmalycr3qxo2xwctiygzvvydh@fu26veserybx>
Date: Wed, 10 Sep 2025 16:23:56 +0000
From: Dragos Tatulea <dtatulea@...dia.com>
To: Amery Hung <ameryhung@...il.com>, netdev@...r.kernel.org
Cc: bpf@...r.kernel.org, andrew+netdev@...n.ch, davem@...emloft.net, 
	edumazet@...gle.com, pabeni@...hat.com, kuba@...nel.org, martin.lau@...nel.org, 
	noren@...dia.com, saeedm@...dia.com, tariqt@...dia.com, mbloch@...dia.com, 
	cpaasch@...nai.com, kernel-team@...a.com
Subject: Re: [PATCH net v1 1/2] net/mlx5e: RX, Fix generating skb from
 non-linear xdp_buff for legacy RQ

On Tue, Sep 09, 2025 at 08:41:02PM -0700, Amery Hung wrote:
> XDP programs can release xdp_buff fragments when calling
> bpf_xdp_adjust_tail(). The driver currently assumes the number of
> fragments to be unchanged and may generate skb with wrong truesize or
> containing invalid frags. Fix the bug by generating skb according to
> xdp_buff after the XDP program runs.
> 
> Fixes: ea5d49bdae8b ("net/mlx5e: Add XDP multi buffer support to the non-linear legacy RQ")
> Signed-off-by: Amery Hung <ameryhung@...il.com>
> ---
>  drivers/net/ethernet/mellanox/mlx5/core/en_rx.c | 9 +++++++++
>  1 file changed, 9 insertions(+)
> 
> diff --git a/drivers/net/ethernet/mellanox/mlx5/core/en_rx.c b/drivers/net/ethernet/mellanox/mlx5/core/en_rx.c
> index b8c609d91d11..1d3eacfd0325 100644
> --- a/drivers/net/ethernet/mellanox/mlx5/core/en_rx.c
> +++ b/drivers/net/ethernet/mellanox/mlx5/core/en_rx.c
> @@ -1729,6 +1729,7 @@ mlx5e_skb_from_cqe_nonlinear(struct mlx5e_rq *rq, struct mlx5e_wqe_frag_info *wi
>  	struct mlx5e_wqe_frag_info *head_wi = wi;
>  	u16 rx_headroom = rq->buff.headroom;
>  	struct mlx5e_frag_page *frag_page;
> +	u8 nr_frags_free, old_nr_frags;
>  	struct skb_shared_info *sinfo;
>  	u32 frag_consumed_bytes;
>  	struct bpf_prog *prog;
> @@ -1772,17 +1773,25 @@ mlx5e_skb_from_cqe_nonlinear(struct mlx5e_rq *rq, struct mlx5e_wqe_frag_info *wi
>  		wi++;
>  	}
>  
> +	old_nr_frags = sinfo->nr_frags;
> +
>  	prog = rcu_dereference(rq->xdp_prog);
>  	if (prog && mlx5e_xdp_handle(rq, prog, mxbuf)) {
>  		if (__test_and_clear_bit(MLX5E_RQ_FLAG_XDP_XMIT, rq->flags)) {
>  			struct mlx5e_wqe_frag_info *pwi;
>  
> +			wi -= old_nr_frags - sinfo->nr_frags;
> +
>  			for (pwi = head_wi; pwi < wi; pwi++)
>  				pwi->frag_page->frags++;
>  		}
>  		return NULL; /* page/packet was consumed by XDP */
>  	}
>  
> +	nr_frags_free = old_nr_frags - sinfo->nr_frags;
Just double checking that my understanding is correct:
bpf_xdp_adjust_tail() can increase the tail only up to fragment limit,
right? So this operation can always be >= 0.

If yes:
Reviewed-by: Dragos Tatulea <dtatulea@...dia.com>
 
Thanks,
Dragos