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

[Leon Hwang]

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