Re: [PATCH bpf-next v3 03/15] libbpf: honor autocreate flag for struct_ops maps

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On Mon, Mar 4, 2024 at 2:52 PM Eduard Zingerman <eddyz87@xxxxxxxxx> wrote:
>
> Skip load steps for struct_ops maps not marked for automatic creation.
> This should allow to load bpf object in situations like below:
>
>     SEC("struct_ops/foo") int BPF_PROG(foo) { ... }
>     SEC("struct_ops/bar") int BPF_PROG(bar) { ... }
>
>     struct test_ops___v1 {
>         int (*foo)(void);
>     };
>
>     struct test_ops___v2 {
>         int (*foo)(void);
>         int (*does_not_exist)(void);
>     };
>
>     SEC(".struct_ops.link")
>     struct test_ops___v1 map_for_old = {
>         .test_1 = (void *)foo
>     };
>
>     SEC(".struct_ops.link")
>     struct test_ops___v2 map_for_new = {
>         .test_1 = (void *)foo,
>         .does_not_exist = (void *)bar
>     };
>
> Suppose program is loaded on old kernel that does not have definition
> for 'does_not_exist' struct_ops member. After this commit it would be
> possible to load such object file after the following tweaks:
>
>     bpf_program__set_autoload(skel->progs.bar, false);
>     bpf_map__set_autocreate(skel->maps.map_for_new, false);
>
> Acked-by: David Vernet <void@xxxxxxxxxxxxx>
> Signed-off-by: Eduard Zingerman <eddyz87@xxxxxxxxx>
> ---
>  tools/lib/bpf/libbpf.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index 2c0cb72bc7a4..25c452c20d7d 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -1209,7 +1209,7 @@ static int bpf_object__init_kern_struct_ops_maps(struct bpf_object *obj)
>         for (i = 0; i < obj->nr_maps; i++) {
>                 map = &obj->maps[i];
>
> -               if (!bpf_map__is_struct_ops(map))
> +               if (!bpf_map__is_struct_ops(map) || !map->autocreate)

tbh, I'd keep them as separate checks (they check very different
aspects, so feels appropriate to not combine them in one check)

>                         continue;
>
>                 err = bpf_map__init_kern_struct_ops(map);
> @@ -8136,7 +8136,7 @@ static int bpf_object_prepare_struct_ops(struct bpf_object *obj)
>         int i;
>
>         for (i = 0; i < obj->nr_maps; i++)
> -               if (bpf_map__is_struct_ops(&obj->maps[i]))
> +               if (bpf_map__is_struct_ops(&obj->maps[i]) && obj->maps[i].autocreate)

especially here, this becomes a bit convoluted

how about we make it a bit more verbose, but also straightforward

map = &obj->maps[i];
if (!bpf_map__is_struct_ops(map))
    continue;
if (!map->autocreate)
    continue;
bpf_map_prepare_vdata(...)


It will also mirror the structure in bpf_object__init_kern_struct_ops_maps()

>                         bpf_map_prepare_vdata(&obj->maps[i]);
>
>         return 0;
> --
> 2.43.0
>





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