Re: [PATCH bpf-next v2 00/11] Enable BPF programs to declare arrays of kptr, bpf_rb_root, and bpf_list_head.

[Kui-Feng Lee <sinquersw@gmail.com>]

On 4/17/24 22:11, Alexei Starovoitov wrote:
> On Wed, Apr 17, 2024 at 9:31 PM Kui-Feng Lee <sinquersw@gmail.com> wrote:
>>
>>
>>
>> On 4/17/24 20:30, Alexei Starovoitov wrote:
>>> On Fri, Apr 12, 2024 at 2:08 PM Kui-Feng Lee <thinker.li@gmail.com> wrote:
>>>>
>>>> The arrays of kptr, bpf_rb_root, and bpf_list_head didn't work as
>>>> global variables. This was due to these types being initialized and
>>>> verified in a special manner in the kernel. This patchset allows BPF
>>>> programs to declare arrays of kptr, bpf_rb_root, and bpf_list_head in
>>>> the global namespace.
>>>>
>>>> The main change is to add "nelems" to btf_fields. The value of
>>>> "nelems" represents the number of elements in the array if a btf_field
>>>> represents an array. Otherwise, "nelem" will be 1. The verifier
>>>> verifies these types based on the information provided by the
>>>> btf_field.
>>>>
>>>> The value of "size" will be the size of the entire array if a
>>>> btf_field represents an array. Dividing "size" by "nelems" gives the
>>>> size of an element. The value of "offset" will be the offset of the
>>>> beginning for an array. By putting this together, we can determine the
>>>> offset of each element in an array. For example,
>>>>
>>>>       struct bpf_cpumask __kptr * global_mask_array[2];
>>>
>>> Looks like this patch set enables arrays only.
>>> Meaning the following is supported already:
>>>
>>> +private(C) struct bpf_spin_lock glock_c;
>>> +private(C) struct bpf_list_head ghead_array1 __contains(foo, node2);
>>> +private(C) struct bpf_list_head ghead_array2 __contains(foo, node2);
>>>
>>> while this support is added:
>>>
>>> +private(C) struct bpf_spin_lock glock_c;
>>> +private(C) struct bpf_list_head ghead_array1[3] __contains(foo, node2);
>>> +private(C) struct bpf_list_head ghead_array2[2] __contains(foo, node2);
>>>
>>> Am I right?
>>>
>>> What about the case when bpf_list_head is wrapped in a struct?
>>> private(C) struct foo {
>>>     struct bpf_list_head ghead;
>>> } ghead;
>>>
>>> that's not enabled in this patch. I think.
>>>
>>> And the following:
>>> private(C) struct foo {
>>>     struct bpf_list_head ghead;
>>> } ghead[2];
>>>
>>>
>>> or
>>>
>>> private(C) struct foo {
>>>     struct bpf_list_head ghead[2];
>>> } ghead;
>>>
>>> Won't work either.
>>
>> No, they don't work.
>> We had a discussion about this in the other day.
>> I proposed to have another patch set to work on struct types.
>> Do you prefer to handle it in this patch set?
>>
>>>
>>> I think eventually we want to support all such combinations and
>>> the approach proposed in this patch with 'nelems'
>>> won't work for wrapper structs.
>>>
>>> I think it's better to unroll/flatten all structs and arrays
>>> and represent them as individual elements in the flattened
>>> structure. Then there will be no need to special case array with 'nelems'.
>>> All special BTF types will be individual elements with unique offset.
>>>
>>> Does this make sense?
>>
>> That means it will creates 10 btf_field(s) for an array having 10
>> elements. The purpose of adding "nelems" is to avoid the repetition. Do
>> you prefer to expand them?
> 
> It's not just expansion, but a common way to handle nested structs too.
> 
> I suspect by delaying nested into another patchset this approach
> will become useless.
> 
> So try adding nested structs in all combinations as a follow up and
> I suspect you're realize that "nelems" approach doesn't really help.
> You'd need to flatten them all.
> And once you do there is no need for "nelems".

For me, "nelems" is more like a choice of avoiding repetition of
information, not a necessary. Before adding "nelems", I had considered
to expand them as well. But, eventually, I chose to add "nelems".

Since you think this repetition is not a problem, I will expand array as
individual elements.