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

["Toke Høiland-Jørgensen" <toke@redhat.com>]

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