Re: [PATCH net v1 1/2] net/mlx5e: RX, Fix generating skb from non-linear xdp_buff for legacy RQ

[Tariq Toukan <ttoukan.linux@gmail.com>]

On 10/09/2025 6:41, 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@gmail.com>
> ---

Hi,

Thanks for your patch!

>   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;
> +	wi -= nr_frags_free;
> +	truesize -= nr_frags_free * frag_info->frag_stride;
> +

New code section better be under if (prog), rather than running 
unconditionally.
Also move all needed new local vars under if (prog) to minimize their scope.

>   	skb = mlx5e_build_linear_skb(
>   		rq, mxbuf->xdp.data_hard_start, rq->buff.frame0_sz,
>   		mxbuf->xdp.data - mxbuf->xdp.data_hard_start,