[RFC PATCH bpf-next v1 3/7] xdp: add bpf_xdp_metadata_rx_csum() RX metadata kfunc

[Vladimir Vdovin <deliran@verdict.gg>]

Add a device-bound RX-metadata kfunc that reports the hardware
checksum verdict (enum xdp_csum_status: XDP_CSUM_NONE / XDP_CSUM_VERIFIED)
through a new xmo_rx_csum operation, so an XDP program can make an
informed decision (e.g. call bpf_xdp_assert_rx_csum()) instead of trusting
blindly.  Wire it into the XDP_METADATA_KFUNC machinery and advertise it
via NETDEV_XDP_RX_METADATA_CSUM.

Signed-off-by: Vladimir Vdovin <deliran@verdict.gg>
---
 Documentation/netlink/specs/netdev.yaml |  5 +++++
 include/net/xdp.h                       | 12 ++++++++++++
 include/uapi/linux/netdev.h             |  3 +++
 net/core/xdp.c                          | 23 +++++++++++++++++++++++
 tools/include/uapi/linux/netdev.h       |  3 +++
 5 files changed, 46 insertions(+)

diff --git a/Documentation/netlink/specs/netdev.yaml b/Documentation/netlink/specs/netdev.yaml
index 5f143da7458c..86017f7402d9 100644
--- a/Documentation/netlink/specs/netdev.yaml
+++ b/Documentation/netlink/specs/netdev.yaml
@@ -61,6 +61,11 @@ definitions:
         doc: |
           Device is capable of exposing receive packet VLAN tag via
           bpf_xdp_metadata_rx_vlan_tag().
+      -
+        name: csum
+        doc: |
+          Device is capable of exposing receive packet checksum status via
+          bpf_xdp_metadata_rx_csum().
   -
     type: flags
     name: xsk-flags
diff --git a/include/net/xdp.h b/include/net/xdp.h
index 5a1e2cc9c312..40f6fba41962 100644
--- a/include/net/xdp.h
+++ b/include/net/xdp.h
@@ -597,6 +597,10 @@ void xdp_attachment_setup(struct xdp_attachment_info *info,
 			   NETDEV_XDP_RX_METADATA_VLAN_TAG, \
 			   bpf_xdp_metadata_rx_vlan_tag, \
 			   xmo_rx_vlan_tag) \
+	XDP_METADATA_KFUNC(XDP_METADATA_KFUNC_RX_CSUM, \
+			   NETDEV_XDP_RX_METADATA_CSUM, \
+			   bpf_xdp_metadata_rx_csum, \
+			   xmo_rx_csum) \
 
 enum xdp_rx_metadata {
 #define XDP_METADATA_KFUNC(name, _, __, ___) name,
@@ -654,12 +658,20 @@ enum xdp_rss_hash_type {
 	XDP_RSS_TYPE_L4_IPV6_SCTP_EX = XDP_RSS_TYPE_L4_IPV6_SCTP | XDP_RSS_L3_DYNHDR,
 };
 
+/* Checksum status reported by bpf_xdp_metadata_rx_csum(). */
+enum xdp_csum_status {
+	XDP_CSUM_NONE = 0,	/* HW did not validate the checksum */
+	XDP_CSUM_VERIFIED,	/* HW validated the L4 checksum; it is correct */
+};
+
 struct xdp_metadata_ops {
 	int	(*xmo_rx_timestamp)(const struct xdp_md *ctx, u64 *timestamp);
 	int	(*xmo_rx_hash)(const struct xdp_md *ctx, u32 *hash,
 			       enum xdp_rss_hash_type *rss_type);
 	int	(*xmo_rx_vlan_tag)(const struct xdp_md *ctx, __be16 *vlan_proto,
 				   u16 *vlan_tci);
+	int	(*xmo_rx_csum)(const struct xdp_md *ctx,
+			       enum xdp_csum_status *csum_status);
 };
 
 #ifdef CONFIG_NET
diff --git a/include/uapi/linux/netdev.h b/include/uapi/linux/netdev.h
index 2f3ab75e8cc0..99cda716f0ee 100644
--- a/include/uapi/linux/netdev.h
+++ b/include/uapi/linux/netdev.h
@@ -47,11 +47,14 @@ enum netdev_xdp_act {
  *   hash via bpf_xdp_metadata_rx_hash().
  * @NETDEV_XDP_RX_METADATA_VLAN_TAG: Device is capable of exposing receive
  *   packet VLAN tag via bpf_xdp_metadata_rx_vlan_tag().
+ * @NETDEV_XDP_RX_METADATA_CSUM: Device is capable of exposing receive packet
+ *   checksum status via bpf_xdp_metadata_rx_csum().
  */
 enum netdev_xdp_rx_metadata {
 	NETDEV_XDP_RX_METADATA_TIMESTAMP = 1,
 	NETDEV_XDP_RX_METADATA_HASH = 2,
 	NETDEV_XDP_RX_METADATA_VLAN_TAG = 4,
+	NETDEV_XDP_RX_METADATA_CSUM = 8,
 };
 
 /**
diff --git a/net/core/xdp.c b/net/core/xdp.c
index 63ee36ec93de..7f4b5c6f7c87 100644
--- a/net/core/xdp.c
+++ b/net/core/xdp.c
@@ -964,6 +964,29 @@ __bpf_kfunc int bpf_xdp_metadata_rx_vlan_tag(const struct xdp_md *ctx,
 	return -EOPNOTSUPP;
 }
 
+/**
+ * bpf_xdp_metadata_rx_csum - Read the device's RX checksum verdict.
+ * @ctx: XDP context pointer.
+ * @csum_status: Destination pointer for the checksum status.
+ *
+ * Report what the hardware concluded about the packet's checksum, so the
+ * program can decide whether to assert it (e.g. via bpf_xdp_assert_rx_csum()
+ * before a cpumap redirect) instead of having the stack validate it again.
+ *
+ * On ``XDP_CSUM_VERIFIED`` the device has checked the L4 checksum and it is
+ * correct. ``XDP_CSUM_NONE`` means the device did not validate it.
+ *
+ * Return:
+ * * Returns 0 on success or ``-errno`` on error.
+ * * ``-EOPNOTSUPP`` : device driver doesn't implement kfunc
+ * * ``-ENODATA``    : checksum information is not available
+ */
+__bpf_kfunc int bpf_xdp_metadata_rx_csum(const struct xdp_md *ctx,
+					 enum xdp_csum_status *csum_status)
+{
+	return -EOPNOTSUPP;
+}
+
 /**
  * bpf_xdp_assert_rx_csum - Assert the packet's L4 checksum is correct.
  * @ctx: XDP context pointer.
diff --git a/tools/include/uapi/linux/netdev.h b/tools/include/uapi/linux/netdev.h
index 2f3ab75e8cc0..99cda716f0ee 100644
--- a/tools/include/uapi/linux/netdev.h
+++ b/tools/include/uapi/linux/netdev.h
@@ -47,11 +47,14 @@ enum netdev_xdp_act {
  *   hash via bpf_xdp_metadata_rx_hash().
  * @NETDEV_XDP_RX_METADATA_VLAN_TAG: Device is capable of exposing receive
  *   packet VLAN tag via bpf_xdp_metadata_rx_vlan_tag().
+ * @NETDEV_XDP_RX_METADATA_CSUM: Device is capable of exposing receive packet
+ *   checksum status via bpf_xdp_metadata_rx_csum().
  */
 enum netdev_xdp_rx_metadata {
 	NETDEV_XDP_RX_METADATA_TIMESTAMP = 1,
 	NETDEV_XDP_RX_METADATA_HASH = 2,
 	NETDEV_XDP_RX_METADATA_VLAN_TAG = 4,
+	NETDEV_XDP_RX_METADATA_CSUM = 8,
 };
 
 /**
-- 
2.47.0