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

Leon Hwang
2026-03-25T01:42:42Z

On 25/3/26 00:18, Toke Høiland-Jørgensen wrote:
> Leon Hwang <leon.hwang@linux.dev> 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
>

Indeed, it failed to 'make test'.

Will drop this patch in v2, as it wasn't an issue.

Thanks,
Leon