[PATCH net-next v2] selftests/drivers/net: Add an xdp test to xdp.py

[Leon Hwang <leon.huangfu@shopee.com>]

In "bpf: Disallow freplace on XDP with mismatched xdp_has_frags values" [1],
this XDP test is suggested to add to xdp.py.

1. Verify the failure of updating frag-capable prog with non-frag-capable
   prog, when the frag-capable prog attaches to mtu=9k driver.

The test has been verified against Mellanox CX6 and Intel 82599ES NICs.

With dropping other tests, here is the test log.

 # ethtool -i eth0
 driver: mlx5_core
 version: 6.19.0-061900-generic

 # NETIF=eth0 python3 xdp.py
 TAP version 13
 1..1
 ok 1 xdp.test_xdp_native_update_mb_to_sb
 # Totals: pass:1 fail:0 xfail:0 xpass:0 skip:0 error:0

 # ethtool -i eth0
 driver: ixgbe
 version: 6.19.0-061900-generic

 # NETIF=eth0 python3 xdp.py
 TAP version 13
 1..1
 # CMD: ip  link set dev eth0 xdpdrv obj /path/to/tools/testing/selftests/net/lib/xdp_dummy.bpf.o sec xdp.frags
 #   EXIT: 2
 #   STDERR: RTNETLINK answers: Invalid argument
 ok 1 xdp.test_xdp_native_update_mb_to_sb # SKIP device does not support multi-buffer XDP
 # Totals: pass:0 fail:0 xfail:0 xpass:0 skip:1 error:0

Links:
[1] https://lore.kernel.org/bpf/20260326124205.1a3bb825@kernel.org/

Cc: Leon Hwang <leon.hwang@linux.dev>
Signed-off-by: Leon Hwang <leon.huangfu@shopee.com>
---
v1 -> v2:
- Update the category in subject from selftests/net to selftests/drivers/net.
- Drop _exec_cmd helper.
- Recover the original MTU using cfg.dev['mtu'] (per Kuba).
- Drop the unnecessary shell=True arg (per Kuba).
- Print error using ksft_pr (per Kuba).
- Drop the sb_to_mb case (per Kuba).
- Skip test when device supports sb XDP with mtu 9000 (per Kuba,Gemini/sashiko).
v1: https://lore.kernel.org/netdev/20260401052746.314667-1-leon.huangfu@shopee.com/
---
 tools/testing/selftests/drivers/net/xdp.py | 28 +++++++++++++++++++++-
 1 file changed, 27 insertions(+), 1 deletion(-)

diff --git a/tools/testing/selftests/drivers/net/xdp.py b/tools/testing/selftests/drivers/net/xdp.py
index d86446569f89..864ca4097b87 100755
--- a/tools/testing/selftests/drivers/net/xdp.py
+++ b/tools/testing/selftests/drivers/net/xdp.py
@@ -13,7 +13,7 @@ from enum import Enum
 
 from lib.py import ksft_run, ksft_exit, ksft_eq, ksft_ge, ksft_ne, ksft_pr
 from lib.py import KsftNamedVariant, ksft_variants
-from lib.py import KsftFailEx, NetDrvEpEnv
+from lib.py import KsftFailEx, KsftSkipEx, NetDrvEpEnv
 from lib.py import EthtoolFamily, NetdevFamily, NlError
 from lib.py import bkg, cmd, rand_port, wait_port_listen
 from lib.py import ip, defer
@@ -693,6 +693,31 @@ def test_xdp_native_qstats(cfg, act):
             ksft_ge(after['tx-packets'], before['tx-packets'])
 
 
+def test_xdp_native_update_mb_to_sb(cfg):
+    obj = cfg.net_lib_dir / "xdp_dummy.bpf.o"
+    mtu = 9000
+
+    ip(f"link set dev {cfg.ifname} mtu {mtu}")
+    defer(ip, f"link set dev {cfg.ifname} mtu {cfg.dev['mtu']} xdpdrv off")
+
+    attach = cmd(f"ip link set dev {cfg.ifname} xdpdrv obj {obj} sec xdp", fail=False)
+    if attach.ret == 0:
+        raise KsftSkipEx(f"device supports single-buffer XDP with mtu {mtu}")
+
+    attach = cmd(f"ip link set dev {cfg.ifname} xdpdrv obj {obj} sec xdp.frags", fail=False)
+    if attach.ret != 0:
+        ksft_pr(attach)
+        raise KsftSkipEx("device does not support multi-buffer XDP")
+
+    # Verify updating mb -> mb program works.
+    cmd(f"ip -force link set dev {cfg.ifname} xdpdrv obj {obj} sec xdp.frags")
+
+    # Verify updating mb -> sb program does not work.
+    update = cmd(f"ip -force link set dev {cfg.ifname} xdpdrv obj {obj} sec xdp", fail=False)
+    if update.ret == 0:
+        raise KsftFailEx("device unexpectedly updates non-multi-buffer XDP")
+
+
 def main():
     """
     Main function to execute the XDP tests.
@@ -718,6 +743,7 @@ def main():
                 test_xdp_native_adjst_head_grow_data,
                 test_xdp_native_adjst_head_shrnk_data,
                 test_xdp_native_qstats,
+                test_xdp_native_update_mb_to_sb,
             ],
             args=(cfg,))
     ksft_exit()
-- 
2.53.0