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

[Amery Hung <ameryhung@gmail.com>]

On Thu, Oct 30, 2025 at 11:09 AM Song Liu <song@kernel.org> wrote:
>
> 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.

+1.

In TCP congestion control and BPF qdisc's model:

During link_create, both adds the struct_ops to a list, and the
struct_ops can be indexed by name. The struct_ops are not "active" by
this time.
Then, each has their own interface to 'apply' the struct_ops to a
socket or queue: setsockopt() or netlink.

But maybe cgroup-related struct_ops are different.

-Amery

>
> Thanks,
> Song
>