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

Toke Høiland-Jørgensen
2026-03-25T10:38:58Z

Leon Hwang <leon.hwang@linux.dev> writes:

> 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.

Great, thanks!

-Toke