On Fri, 27 Mar 2026 14:42:19 +0800 Leon Hwang wrote:

quoted

It may be worth adding a selftest to
tools/testing/selftests/drivers/net/xdp.py
which sets MTU to 9k, tries to attach a non-frag-capable prog
if that fails attaches a frag-capable prog and then checks if
replacing the capable prog with non-capable fails.
Drivers may be buggy in this regard.  

Do you mean adding these two tests to xdp.py?

1. Verify the failure of attaching non-frag-capable prog to mtu=9k
   driver.
2. Verify the failure of updating frag-capable prog with non-frag-
   capable prog via libbpf.c::bpf_link__update_program().

Not directly via libbpf just ip -f link, it uses libbpf internally
but testing with  ip link is simpler. Also - there are already XDP
progs which return XDP_PASS in frag-capable and non-capable mode,
so just use those.

As for test #2, it may require a helper to attach prog then update prog
using libbpf's APIs.

See above.