Re: [PATCH bpf-next v3 4/7] bpf: Introduce css open-coded iterator kfuncs

[Andrii Nakryiko]

On Mon, Sep 25, 2023 at 3:56 AM Chuyi Zhou <zhouchuyi@xxxxxxxxxxxxx> wrote:
>
> This Patch adds kfuncs bpf_iter_css_{new,next,destroy} which allow
> creation and manipulation of struct bpf_iter_css in open-coded iterator
> style. These kfuncs actually wrapps css_next_descendant_{pre, post}.
> css_iter can be used to:
>
> 1) iterating a sepcific cgroup tree with pre/post/up order
>
> 2) iterating cgroup_subsystem in BPF Prog, like
> for_each_mem_cgroup_tree/cpuset_for_each_descendant_pre in kernel.
>
> The API design is consistent with cgroup_iter. bpf_iter_css_new accepts
> parameters defining iteration order and starting css. Here we also reuse
> BPF_CGROUP_ITER_DESCENDANTS_PRE, BPF_CGROUP_ITER_DESCENDANTS_POST,
> BPF_CGROUP_ITER_ANCESTORS_UP enums.
>
> Signed-off-by: Chuyi Zhou <zhouchuyi@xxxxxxxxxxxxx>
> ---
>  kernel/bpf/cgroup_iter.c                      | 57 +++++++++++++++++++
>  kernel/bpf/helpers.c                          |  3 +
>  .../testing/selftests/bpf/bpf_experimental.h  |  6 ++
>  3 files changed, 66 insertions(+)
>
> diff --git a/kernel/bpf/cgroup_iter.c b/kernel/bpf/cgroup_iter.c
> index 810378f04fbc..ebc3d9471f52 100644
> --- a/kernel/bpf/cgroup_iter.c
> +++ b/kernel/bpf/cgroup_iter.c
> @@ -294,3 +294,60 @@ static int __init bpf_cgroup_iter_init(void)
>  }
>
>  late_initcall(bpf_cgroup_iter_init);
> +
> +struct bpf_iter_css {
> +       __u64 __opaque[2];
> +       __u32 __opaque_int[1];
> +} __attribute__((aligned(8)));
> +

same as before, __opaque[3] only


> +struct bpf_iter_css_kern {
> +       struct cgroup_subsys_state *start;
> +       struct cgroup_subsys_state *pos;
> +       int order;
> +} __attribute__((aligned(8)));
> +
> +__bpf_kfunc int bpf_iter_css_new(struct bpf_iter_css *it,
> +               struct cgroup_subsys_state *start, enum bpf_cgroup_iter_order order)

Similarly, I wonder if we should go for a more generic "flags" argument?

> +{
> +       struct bpf_iter_css_kern *kit = (void *)it;

empty line

> +       kit->start = NULL;
> +       BUILD_BUG_ON(sizeof(struct bpf_iter_css_kern) != sizeof(struct bpf_iter_css));
> +       BUILD_BUG_ON(__alignof__(struct bpf_iter_css_kern) != __alignof__(struct bpf_iter_css));

please move this up before kit->start assignment, and separate by empty lines

> +       switch (order) {
> +       case BPF_CGROUP_ITER_DESCENDANTS_PRE:
> +       case BPF_CGROUP_ITER_DESCENDANTS_POST:
> +       case BPF_CGROUP_ITER_ANCESTORS_UP:
> +               break;
> +       default:
> +               return -EINVAL;
> +       }
> +
> +       kit->start = start;
> +       kit->pos = NULL;
> +       kit->order = order;
> +       return 0;
> +}
> +
> +__bpf_kfunc struct cgroup_subsys_state *bpf_iter_css_next(struct bpf_iter_css *it)
> +{
> +       struct bpf_iter_css_kern *kit = (void *)it;

empty line

> +       if (!kit->start)
> +               return NULL;
> +
> +       switch (kit->order) {
> +       case BPF_CGROUP_ITER_DESCENDANTS_PRE:
> +               kit->pos = css_next_descendant_pre(kit->pos, kit->start);
> +               break;
> +       case BPF_CGROUP_ITER_DESCENDANTS_POST:
> +               kit->pos = css_next_descendant_post(kit->pos, kit->start);
> +               break;
> +       default:

we know it's BPF_CGROUP_ITER_ANCESTORS_UP, so why not have that here explicitly?

> +               kit->pos = kit->pos ? kit->pos->parent : kit->start;
> +       }
> +
> +       return kit->pos;

wouldn't this implementation never return the "start" css? is that intentional?

> +}
> +
> +__bpf_kfunc void bpf_iter_css_destroy(struct bpf_iter_css *it)
> +{
> +}
> diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
> index 556262c27a75..9c3af36249a2 100644
> --- a/kernel/bpf/helpers.c
> +++ b/kernel/bpf/helpers.c
> @@ -2510,6 +2510,9 @@ BTF_ID_FLAGS(func, bpf_iter_css_task_destroy, KF_ITER_DESTROY)
>  BTF_ID_FLAGS(func, bpf_iter_task_new, KF_ITER_NEW | KF_TRUSTED_ARGS)
>  BTF_ID_FLAGS(func, bpf_iter_task_next, KF_ITER_NEXT | KF_RET_NULL)
>  BTF_ID_FLAGS(func, bpf_iter_task_destroy, KF_ITER_DESTROY)
> +BTF_ID_FLAGS(func, bpf_iter_css_new, KF_ITER_NEW | KF_TRUSTED_ARGS)
> +BTF_ID_FLAGS(func, bpf_iter_css_next, KF_ITER_NEXT | KF_RET_NULL)
> +BTF_ID_FLAGS(func, bpf_iter_css_destroy, KF_ITER_DESTROY)
>  BTF_ID_FLAGS(func, bpf_dynptr_adjust)
>  BTF_ID_FLAGS(func, bpf_dynptr_is_null)
>  BTF_ID_FLAGS(func, bpf_dynptr_is_rdonly)
> diff --git a/tools/testing/selftests/bpf/bpf_experimental.h b/tools/testing/selftests/bpf/bpf_experimental.h
> index d989775dbdb5..aa247d1d81d1 100644
> --- a/tools/testing/selftests/bpf/bpf_experimental.h
> +++ b/tools/testing/selftests/bpf/bpf_experimental.h
> @@ -174,4 +174,10 @@ extern int bpf_iter_task_new(struct bpf_iter_task *it, struct task_struct *task,
>  extern struct task_struct *bpf_iter_task_next(struct bpf_iter_task *it) __weak __ksym;
>  extern void bpf_iter_task_destroy(struct bpf_iter_task *it) __weak __ksym;
>
> +struct bpf_iter_css;
> +extern int bpf_iter_css_new(struct bpf_iter_css *it,
> +                               struct cgroup_subsys_state *start, enum bpf_cgroup_iter_order order) __weak __ksym;
> +extern struct cgroup_subsys_state *bpf_iter_css_next(struct bpf_iter_css *it) __weak __ksym;
> +extern void bpf_iter_css_destroy(struct bpf_iter_css *it) __weak __ksym;
> +
>  #endif
> --
> 2.20.1
>