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Miller" , David Ahern , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Simon Horman , Martin KaFai Lau , netdev@vger.kernel.org, bpf@vger.kernel.org, kernel-team@cloudflare.com Subject: Re: [PATCH net v2] net: Clear skb metadata in LWT BPF xmit Message-ID: References: <20260514-bpf-lwt-drop-skb-metadata-v2-1-458664edc2b5@cloudflare.com> Precedence: bulk X-Mailing-List: bpf@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260514-bpf-lwt-drop-skb-metadata-v2-1-458664edc2b5@cloudflare.com> On 2026-05-14 12:47:09, Jakub Sitnicki wrote: > skb metadata is meant for passing information between XDP and TC. > LWT programs cannot access the __sk_buff->data_meta pseudo-pointer. > > However, LWT_XMIT programs can call helpers that move packet data. > One such helper is bpf_skb_change_head(), which uses skb_data_move() to > move both packet data and metadata after skb_push(). skb_data_move() > expects metadata to sit immediately before skb->data. This is the layout > that commit 8989d328dfe7 ("net: Helper to move packet data and metadata > after skb_push/pull") preserves for TC(X) programs when skb_push() or > skb_pull() moves packet data. > > The IP output path can run LWT xmit before neighbour output has built the > outgoing L2 header. For forwarded, or otherwise RX-originated, packets > skb->data points at the network header while skb_mac_header() can still > point at the old L2 header. If such an skb still carries XDP metadata, > skb_data_move() sees metadata ending at skb_mac_header(), not immediately > before skb->data, and warns: > > WARNING: CPU: 21 PID: 454557 at include/linux/skbuff.h:4609 skb_data_move+0x47/0x90 > CPU: 21 UID: 0 PID: 454557 Comm: napi/iconduit-g Tainted: G O 6.18.21 #1 > RIP: 0010:skb_data_move+0x47/0x90 > Call Trace: > > bpf_skb_change_head+0xe6/0x1a0 > bpf_prog_...+0x213/0x2e3 > run_lwt_bpf.isra.0+0x1d3/0x360 > bpf_xmit+0x46/0xe0 > lwtunnel_xmit+0xa1/0xf0 > ip_finish_output2+0x1e7/0x5e0 > ip_output+0x63/0x100 > __netif_receive_skb_one_core+0x85/0xa0 > process_backlog+0x9c/0x150 > __napi_poll+0x2b/0x190 > net_rx_action+0x40b/0x7f0 > handle_softirqs+0xd2/0x270 > do_softirq+0x3f/0x60 > > > Drop skb metadata in the LWT BPF xmit path, just before invoking the > program. Doing it late, rather than in ip[6]_finish_output2(), preserves > metadata for tc egress on the outgoing device when no LWT BPF program is > attached - the only scenario where carrying tc-layer metadata through the > IP output path could be meaningful. Non-BPF LWT encap types do not run > helpers that touch metadata, so the BPF xmit op is the only site that needs > the clear. > > Fixes: 8989d328dfe7 ("net: Helper to move packet data and metadata after skb_push/pull") > Signed-off-by: Jakub Sitnicki > --- > Changes in v2: > - Clear metadata in bpf_xmit to allow access from tc(x) egress (Daniel) > - Add WARNING snippet to the description > - Link to v1: https://lore.kernel.org/r/20260428-wip-skb-local-storage-from-scratch-v1-1-8f7ca9b378ce@cloudflare.com I was able to reproduce the problem with a C program. After applying your patch I could no longer repro. Tested-by: Chris J Arges