Subject: [PATCH net v2 1/8] xsk: reject sw-csum UMEM binding to IFF_TX_SKB_NO_LINEAR devices

From: Jason Xing <[email-masked]>
Date: Mon, 20 Apr 2026 16:27:58 +0800

From: Jason Xing <[email-masked]>

skb_checksum_help() is a common helper that writes the folded
16-bit checksum back via skb->data + csum_start + csum_offset,
i.e. it relies on the skb's linear head and fails (with WARN_ONCE
and -EINVAL) when skb_headlen() is 0.

AF_XDP generic xmit takes two very different paths depending on the
netdev. Drivers that advertise IFF_TX_SKB_NO_LINEAR (e.g. virtio_net)
skip the "copy payload into a linear head" step on purpose as a
performance optimisation: xsk_build_skb_zerocopy() only attaches UMEM
pages as frags and never calls skb_put(), so skb_headlen() stays 0
for the whole skb. For these skbs there is simply no linear area for
skb_checksum_help() to write the csum into - the sw-csum fallback is
structurally inapplicable.

The patch tries to catch this and reject the combination with error at
setup time. Rejecting at bind() converts this silent per-packet failure
into a synchronous, actionable -EOPNOTSUPP at setup time. HW csum and
launch_time metadata on IFF_TX_SKB_NO_LINEAR drivers are unaffected
because they do not call skb_checksum_help().

Without the patch, every descriptor carrying 'XDP_TX_METADATA |
XDP_TXMD_FLAGS_CHECKSUM' produces:
1) a WARN_ONCE "offset (N) >= skb_headlen() (0)" from skb_checksum_help(),
2) sendmsg() returning -EINVAL without consuming the descriptor
   (invalid_descs is not incremented),
3) a wedged TX ring: __xsk_generic_xmit() does not advance the
    consumer on non-EOVERFLOW errors, so the next sendmsg() re-reads
    the same descriptor and re-hits the same WARN until the socket
    is closed.

Closes: https://lore.kernel.org/all/[email-masked]/#t
Fixes: 30c3055f9c0d ("xsk: wrap generic metadata handling onto separate function")
Signed-off-by: Jason Xing <[email-masked]>
---
 net/xdp/xsk_buff_pool.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/net/xdp/xsk_buff_pool.c b/net/xdp/xsk_buff_pool.c
index 37b7a68b89b3..c2521b6547e3 100644
--- a/net/xdp/xsk_buff_pool.c
+++ b/net/xdp/xsk_buff_pool.c
@@ -169,6 +169,9 @@ int xp_assign_dev(struct xsk_buff_pool *pool,
 	if (force_zc && force_copy)
 		return -EINVAL;
 
+	if (pool->tx_sw_csum && (netdev->priv_flags & IFF_TX_SKB_NO_LINEAR))
+		return -EOPNOTSUPP;
+
 	if (xsk_get_pool_from_qid(netdev, queue_id))
 		return -EBUSY;
 
-- 
2.41.3