[PATCH net] net: stmmac: set XDP metadata length on the page_pool RX path

stmmac_rx() initializes the XDP buffer with the metadata area valid, so
an XDP program can reserve metadata with bpf_xdp_adjust_meta() and write
it in [xdp->data_meta, xdp->data), for example to export the RX hardware
timestamp obtained from the metadata kfunc. When such a frame is passed
to the stack, the skb is built in place around the same page with
napi_build_skb(), but skb_metadata_set() is never called.

napi_build_skb() clears skb_shared_info up to dataref, which includes
meta_len, so skb_metadata_len() stays zero. The stack and TC BPF then see
an empty metadata range even though the bytes are present in the skb
headroom. The AF_XDP zero-copy path already records the length in
stmmac_construct_skb_zc().

Record the metadata length after reserving the headroom and adding the
frame data. No copy is needed because the skb is built over the buffer the
XDP program wrote to. With the length recorded, the software VLAN
stripping path can move the metadata together with the MAC header, as it
already does for zero-copy frames.

Fixes: e3f9c3e34840 ("net: stmmac: add Rx HWTS metadata to XDP receive pkt")
Signed-off-by: Lorenzo Bianconi <redacted>
---
 drivers/net/ethernet/stmicro/stmmac/stmmac_main.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
index b038472b9817..f97f6dc08789 100644
--- a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
+++ b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c

@@ -6012,7 +6012,7 @@ static int stmmac_rx(struct stmmac_priv *priv, int limit, u32 queue)
 		}
 
 		if (!skb) {
-			unsigned int head_pad_len;
+			unsigned int metasize;
 
 			/* XDP program may expand or reduce tail */
 			buf1_len = ctx.xdp.data_end - ctx.xdp.data;

@@ -6029,10 +6029,14 @@ static int stmmac_rx(struct stmmac_priv *priv, int limit, u32 queue)
 			}
 
 			/* XDP program may adjust header */
-			head_pad_len = ctx.xdp.data - ctx.xdp.data_hard_start;
-			skb_reserve(skb, head_pad_len);
+			skb_reserve(skb,
+				    ctx.xdp.data - ctx.xdp.data_hard_start);
 			skb_put(skb, buf1_len);
 			skb_mark_for_recycle(skb);
+
+			metasize = ctx.xdp.data - ctx.xdp.data_meta;
+			if (metasize)
+				skb_metadata_set(skb, metasize);
 			buf->page = NULL;
 		} else if (buf1_len) {
 			dma_sync_single_for_cpu(priv->device, buf->addr,

---
base-commit: 21f57ec673a1917547c808cfa81890485fffa03b
change-id: 20261102-stmmac_meatada_pp_path-0c1b9299e2fb

Best regards,
-- 
Lorenzo Bianconi [off-list ref]