Re: [PATCH v2 02/23] bpf: initial support for attaching struct ops to cgroups

[Song Liu <song@kernel.org>]

On Thu, Oct 30, 2025 at 10:22 AM Roman Gushchin
<roman.gushchin@linux.dev> wrote:
>
> Song Liu <song@kernel.org> writes:
>
> > On Mon, Oct 27, 2025 at 4:17 PM Roman Gushchin <roman.gushchin@linux.dev> wrote:
> > [...]
> >>  struct bpf_struct_ops_value {
> >>         struct bpf_struct_ops_common_value common;
> >> @@ -1359,6 +1360,18 @@ int bpf_struct_ops_link_create(union bpf_attr *attr)
> >>         }
> >>         bpf_link_init(&link->link, BPF_LINK_TYPE_STRUCT_OPS, &bpf_struct_ops_map_lops, NULL,
> >>                       attr->link_create.attach_type);
> >> +#ifdef CONFIG_CGROUPS
> >> +       if (attr->link_create.cgroup.relative_fd) {
> >> +               struct cgroup *cgrp;
> >> +
> >> +               cgrp = cgroup_get_from_fd(attr->link_create.cgroup.relative_fd);
> >
> > We should use "target_fd" here, not relative_fd.
> >
> > Also, 0 is a valid fd, so we cannot use target_fd == 0 to attach to
> > global memcg.
>
> Yep, but then we need somehow signal there is a cgroup fd passed,
> so that struct ops'es which are not attached to cgroups keep working
> as previously. And we can't use link_create.attach_type.
>
> Should I use link_create.flags? E.g. something like add new flag
>
> @@ -1224,6 +1224,7 @@ enum bpf_perf_event_type {
>  #define BPF_F_AFTER            (1U << 4)
>  #define BPF_F_ID               (1U << 5)
>  #define BPF_F_PREORDER         (1U << 6)
> +#define BPF_F_CGROUP           (1U << 7)
>  #define BPF_F_LINK             BPF_F_LINK /* 1 << 13 */
>
>  /* If BPF_F_STRICT_ALIGNMENT is used in BPF_PROG_LOAD command, the
>
> and then do something like this:
>
> int bpf_struct_ops_link_create(union bpf_attr *attr)
> {
>         <...>
>         if (attr->link_create.flags & BPF_F_CGROUP) {
>                 struct cgroup *cgrp;
>
>                 cgrp = cgroup_get_from_fd(attr->link_create.target_fd);
>                 if (IS_ERR(cgrp)) {
>                         err = PTR_ERR(cgrp);
>                         goto err_out;
>                 }
>
>                 link->cgroup_id = cgroup_id(cgrp);
>                 cgroup_put(cgrp);
>         }
>
> Does it sound right?

I believe adding a flag (BPF_F_CGROUP or some other name), is the
right solution for this.

OTOH, I am not sure whether we want to add cgroup fd/id to the
bpf link. I personally prefer the model used by TCP congestion
control: the link attaches the struct_ops to a global list, then each
user picks a struct_ops from the list. But I do agree this might be
an overkill for cgroup use cases.

Thanks,
Song