[PATCH net] net: make dev_xdp_sb_prog_count() see programs attached through a link

[Jakub Kicinski <kuba@kernel.org>]

ethtool refuses to enable tcp-data-split on a device running
a single-buffer XDP program (which can't deal with the payload
landing in a separate buffer). dev_xdp_sb_prog_count() only looks
at xdp_state[].prog though, ignoring bpf_link integration.
Get the prog pointer from dev_xdp_prog(), which will consult both.

bpf_xdp_link_update() needs a bit of a touch-up, too, since
ethtool calls dev_xdp_sb_prog_count() with just the instance
lock (for ops-locked devices) - we now have to make sure the
link updates happen under that lock.

Spotted by AI while reviewing the XDP propagation series,
verified and tested on netdevsim with a C test along these lines:

  link = bpf_program__attach_xdp(prog, ifindex);
  system("ethtool -G eth0 tcp-data-split on")

Fixes: 197258f0ef68 ("net: ethtool: add hds_config member in ethtool_netdev_state")
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
---
CC: daniel@iogearbox.net
CC: hawk@kernel.org
CC: john.fastabend@gmail.com
CC: sdf@fomichev.me
CC: ap420073@gmail.com
CC: bpf@vger.kernel.org
---
 net/core/dev.c | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/net/core/dev.c b/net/core/dev.c
index 79be99b52f20..030c444aee45 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -10367,13 +10367,15 @@ EXPORT_SYMBOL_GPL(dev_xdp_prog_count);
 
 u8 dev_xdp_sb_prog_count(struct net_device *dev)
 {
+	struct bpf_prog *prog;
 	u8 count = 0;
 	int i;
 
-	for (i = 0; i < __MAX_XDP_MODE; i++)
-		if (dev->xdp_state[i].prog &&
-		    !dev->xdp_state[i].prog->aux->xdp_has_frags)
+	for (i = 0; i < __MAX_XDP_MODE; i++) {
+		prog = dev_xdp_prog(dev, i);
+		if (prog && !prog->aux->xdp_has_frags)
 			count++;
+	}
 	return count;
 }
 
@@ -10753,11 +10755,12 @@ static int bpf_xdp_link_update(struct bpf_link *link, struct bpf_prog *new_prog,
 	bpf_op = dev_xdp_bpf_op(xdp_link->dev, mode);
 	err = dev_xdp_install(xdp_link->dev, mode, bpf_op, NULL,
 			      xdp_link->flags, new_prog);
+	if (!err)
+		old_prog = xchg(&link->prog, new_prog);
 	netdev_unlock_ops(xdp_link->dev);
 	if (err)
 		goto out_unlock;
 
-	old_prog = xchg(&link->prog, new_prog);
 	bpf_prog_put(old_prog);
 
 out_unlock:
-- 
2.55.0