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

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Message-ID: <CAHzjS_sLqPZFqsGXB+wVzRE=Z9sQ-ZFMjy8T__50D4z44yqctg@mail.gmail.com>
Date: Wed, 29 Oct 2025 11:01:00 -0700
From: Song Liu <song@...nel.org>
To: Roman Gushchin <roman.gushchin@...ux.dev>
Cc: Andrew Morton <akpm@...ux-foundation.org>, linux-kernel@...r.kernel.org, 
	Alexei Starovoitov <ast@...nel.org>, Suren Baghdasaryan <surenb@...gle.com>, Michal Hocko <mhocko@...nel.org>, 
	Shakeel Butt <shakeel.butt@...ux.dev>, Johannes Weiner <hannes@...xchg.org>, 
	Andrii Nakryiko <andrii@...nel.org>, JP Kobryn <inwardvessel@...il.com>, linux-mm@...ck.org, 
	cgroups@...r.kernel.org, bpf@...r.kernel.org, 
	Martin KaFai Lau <martin.lau@...nel.org>, Song Liu <song@...nel.org>, 
	Kumar Kartikeya Dwivedi <memxor@...il.com>, Tejun Heo <tj@...nel.org>
Subject: Re: [PATCH v2 02/23] bpf: initial support for attaching struct ops to cgroups

On Mon, Oct 27, 2025 at 4:17 PM Roman Gushchin <roman.gushchin@...ux.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.

Thanks,
Song

> +               if (IS_ERR(cgrp))
> +                       return PTR_ERR(cgrp);
> +
> +               link->cgroup_id = cgroup_id(cgrp);
> +               cgroup_put(cgrp);
> +       }
> +#endif /* CONFIG_CGROUPS */
>
>         err = bpf_link_prime(&link->link, &link_primer);
>         if (err)
> --
> 2.51.0
>