Subject: Re: [PATCH bpf-next 1/3] bpf: Disallow freplace on XDP with mismatched xdp_has_frags values

From: Leon Hwang <[email-masked]>
Date: Fri, 27 Mar 2026 14:42:19 +0800

On 27/3/26 03:42, Jakub Kicinski wrote:
> On Tue, 24 Mar 2026 23:04:42 +0800 Leon Hwang wrote:
>> The commit f45d5b6ce2e8 ("bpf: generalise tail call map compatibility check")
>> was to ensure backwards compatibility against tail calls. However, it
>> missed that XDP progs can be extended by freplace progs, which could break
>> the backwards compatibility, e.g. xdp_has_frags=true freplace progs are
>> allowed to attach to xdp_has_frags=false XDP progs.
>>
>> To avoid breaking the backwards compatibility via freplace, disallow
>> freplace on XDP programs with different xdp_has_frags values.
> 
> 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().

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

Thanks,
Leon