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

From: Toke Høiland-Jørgensen <[email-masked]>
Date: Tue, 24 Mar 2026 17:18:54 +0100

Leon Hwang <[email-masked]> writes:

> xdp_has_frags was introduced by the commit
> c2f2cdbeffda ("bpf: introduce BPF_F_XDP_HAS_FRAGS flag in prog_flags loading the ebpf program").
>
> 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.

The problem you describe is not actually a problem, though? A
frags-aware program can run on a non-frags interface just fine.

You're messing with long-standing behaviour (since 5.18!) to solve a
non-existent problem. In a way that completely breaks the frags handling
in libxdp[0]:

    Running tests from ./test-libxdp.sh
     [test_link_so]                PASS
     [test_link_a]                 PASS
     [test_old_dispatcher]         PASS
     [test_xdp_devbound]           PASS
     [test_xdp_frags]              FAIL
          Kernel supports XDP programs with frags
          check_load_frags:	FAILED
          check_load_nofrags_success:	PASSED
          check_load_nofrags_fail:	PASSED
          check_load_frags_multi:	FAILED
          check_load_mix_big:	FAILED
          check_load_mix_small:	FAILED
          Test test_xdp_frags exited with return code: 1

Please don't do that.

-Toke

[0] Run 'make test' here: https://github.com/xdp-project/xdp-tools