Re: [PATCH net 1/3] eth: bnxt: fix truesize for mb-xdp-pass case

[Jakub Kicinski <kuba@kernel.org>]

On Wed, 26 Feb 2025 06:18:35 +0000 Taehee Yoo wrote:
> When mb-xdp is set and return is XDP_PASS, packet is converted from
> xdp_buff to sk_buff with xdp_update_skb_shared_info() in
> bnxt_xdp_build_skb().
> bnxt_xdp_build_skb() passes incorrect truesize argument to
> xdp_update_skb_shared_info().
> truesize is calculated as BNXT_RX_PAGE_SIZE * sinfo->nr_frags but
> sinfo->nr_frags should not be used because sinfo->nr_frags is not yet
> updated.

"not yet updated" sounds misleading the problem is that calling
build_skb() wipes the shared info back to 0, but it was initialized.

> so it should use num_frags instead.

num_frags may be stale, tho. The program can trim the skb effectively
discarding the fragments. Maybe we should fix that also..

Could you follow up and switch to xdp_build_skb_from_buff() in net-next?
It does all the right things already.

> How to reproduce:
> <Node A>
> ip link set $interface1 xdp obj xdp_pass.o
> ip link set $interface1 mtu 9000 up
> ip a a 10.0.0.1/24 dev $interface1
> <Node B>
> ip link set $interfac2 mtu 9000 up
> ip a a 10.0.0.2/24 dev $interface2
> ping 10.0.0.1 -s 65000

Would you be willing to turn this into a selftest?
The xdp program I added for HDS recently is a PASS so we can reuse it.
29b036be1b0bfcf
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