[PATCH bpf-next V1] xdp: bpf_xdp_metadata use NODEV for no device support

[Jesper Dangaard Brouer <brouer@xxxxxxxxxx>]

With our XDP-hints kfunc approach, where individual drivers overload the
default implementation, it can be hard for API users to determine
whether or not the current device driver have this kfunc available.

Change the default implementations to use an errno (ENODEV), that
drivers shouldn't return, to make it possible for BPF runtime to
determine if bpf kfunc for xdp metadata isn't implemented by driver.

This is intended to ease supporting and troubleshooting setups. E.g.
when users on mailing list report -19 (ENODEV) as an error, then we can
immediately tell them their kernel is too old.

Signed-off-by: Jesper Dangaard Brouer <brouer@xxxxxxxxxx>
---
 net/core/xdp.c |    8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/net/core/xdp.c b/net/core/xdp.c
index 26483935b7a4..7bb5984ae4f7 100644
--- a/net/core/xdp.c
+++ b/net/core/xdp.c
@@ -722,10 +722,12 @@ __diag_ignore_all("-Wmissing-prototypes",
  * @timestamp: Return value pointer.
  *
  * Returns 0 on success or ``-errno`` on error.
+ *
+ *  -ENODEV (19): means device driver doesn't implement kfunc
  */
 __bpf_kfunc int bpf_xdp_metadata_rx_timestamp(const struct xdp_md *ctx, u64 *timestamp)
 {
-	return -EOPNOTSUPP;
+	return -ENODEV;
 }
 
 /**
@@ -734,10 +736,12 @@ __bpf_kfunc int bpf_xdp_metadata_rx_timestamp(const struct xdp_md *ctx, u64 *tim
  * @hash: Return value pointer.
  *
  * Returns 0 on success or ``-errno`` on error.
+ *
+ *  -ENODEV (19): means device driver doesn't implement kfunc
  */
 __bpf_kfunc int bpf_xdp_metadata_rx_hash(const struct xdp_md *ctx, u32 *hash)
 {
-	return -EOPNOTSUPP;
+	return -ENODEV;
 }
 
 __diag_pop();