On 28/3/26 08:12, Jakub Kicinski wrote:

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

quoted

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.

Yep, I see these XDP progs in xdp_dummy.bpf.c.

I think the option of ip cmd to update XDP prog is -force.

# ip l set dev eth0 mtu 9000
# ip l set dev eth0 xdpdrv obj xdp_dummy.bpf.o sec xdp.frags
# ip l
6: eth0: <BROADCAST,MULTICAST,SLAVE,UP,LOWER_UP> mtu 9000 xdp qdisc mq
master bond0 state UP mode DEFAULT group default qlen 1000
    link/ether 3a:39:5f:35:17:6c brd ff:ff:ff:ff:ff:ff permaddr
e0:9d:73:c6:7a:d6
    prog/xdp id 72 name xdp_dummy_prog_ tag 614b434cd8324ecc jited
# ip -force l set dev eth0 xdpdrv obj xdp_dummy.bpf.o sec xdp.frags
# ip l
6: eth0: <BROADCAST,MULTICAST,SLAVE,UP,LOWER_UP> mtu 9000 xdp qdisc mq
master bond0 state UP mode DEFAULT group default qlen 1000
    link/ether 3a:39:5f:35:17:6c brd ff:ff:ff:ff:ff:ff permaddr
e0:9d:73:c6:7a:d6
    prog/xdp id 76 name xdp_dummy_prog_ tag 614b434cd8324ecc jited
# ip l set dev eth0 mtu 1500 xdpdrv off

I'd like to post an RFC to net-next later.

Thanks,
Leon