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

[Alexei Starovoitov <alexei.starovoitov@gmail.com>]

On Thu, Oct 30, 2025 at 4:24 PM Roman Gushchin <roman.gushchin@linux.dev> wrote:
>
> Alexei Starovoitov <alexei.starovoitov@gmail.com> writes:
>
> > On Thu, Oct 30, 2025 at 12:06 PM Roman Gushchin
> > <roman.gushchin@linux.dev> wrote:
> >>
> >> Ok, let me summarize the options we discussed here:
> >>
> >> 1) Make the attachment details (e.g. cgroup_id) the part of struct ops
> >> itself. The attachment is happening at the reg() time.
> >>
> >>   +: It's convenient for complex stateful struct ops'es, because a
> >>       single entity represents a combination of code and data.
> >>   -: No way to attach a single struct ops to multiple entities.
> >>
> >> This approach is used by Tejun for per-cgroup sched_ext prototype.
> >
> > It's wrong. It should adopt bpf_struct_ops_link_create() approach
> > and use attr->link_create.cgroup.relative_fd to attach.
>
> This is basically what I have in v2, but Andrii and Song suggested that
> I should use attr->link_create.target_fd instead.

Yes. Of course.
link_create.cgroup.relative_fd actually points to a program.
We will need it if/when we add support for mprog style attach.

> I have a slight preference towards attr->link_create.cgroup.relative_fd
> because it makes it clear that fd is a cgroup fd and potentially opens
> a possibility to e.g. attach struct_ops to individual tasks and
> cgroups, but I'm fine with both options.

yeah. The name is confusing. It's not a cgroup fd.

> Also, as Song pointed out, fd==0 is in theory a valid target, so instead of
> using the "if (fd) {...}" check we might need a new flag. Idk if it
> really makes sense to complicate the code for it.

One option is to cgroup_get_from_fd(attr->link_create.target_fd)
and if it's not a cgroup, just ignore it in bpf_struct_ops_link_create()
But a new flag like BPF_F_CGROUP_FD maybe cleaner ?
If we ever attach st_ops to tasks there will be another BPF_F_PID_FD flag ?
Or we may try different supported kinds like bpf_fd_probe_obj() does
and don't bother with flags.

New attach_type-s are not necessary. The type of st_ops itself
reflects the purpose and hook location.