On Wed, Apr 17, 2024 at 11:07 PM Kui-Feng Lee <sinquersw@xxxxxxxxx> wrote:
>
>
>
> On 4/17/24 22:11, Alexei Starovoitov wrote:
> > On Wed, Apr 17, 2024 at 9:31 PM Kui-Feng Lee <sinquersw@xxxxxxxxx> wrote:
> >>
> >>
> >>
> >> On 4/17/24 20:30, Alexei Starovoitov wrote:
> >>> On Fri, Apr 12, 2024 at 2:08 PM Kui-Feng Lee <thinker.li@xxxxxxxxx> 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.
You don't sound convinced :)
Please add support for nested structs on top of your "nelems" approach
and prototype the same without "nelems" and let's compare the two.
