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

[Martin KaFai Lau]

On 10/27/25 4:17 PM, Roman Gushchin wrote:
> When a struct ops is being attached and a bpf link is created,
> allow to pass a cgroup fd using bpf attr, so that struct ops
> can be attached to a cgroup instead of globally.
>
> Attached struct ops doesn't hold a reference to the cgroup,
> only preserves cgroup id.
>
> Signed-off-by: Roman Gushchin <roman.gushchin@xxxxxxxxx>
> ---
>   include/linux/bpf.h         |  1 +
>   kernel/bpf/bpf_struct_ops.c | 13 +++++++++++++
>   2 files changed, 14 insertions(+)
>
> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index eae907218188..7205b813e25f 100644
> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h
> @@ -1849,6 +1849,7 @@ struct bpf_struct_ops_link {
>   	struct bpf_link link;
>   	struct bpf_map __rcu *map;
>   	wait_queue_head_t wait_hup;
> +	u64 cgroup_id;
>   };
>   
>   struct bpf_link_primer {
> diff --git a/kernel/bpf/bpf_struct_ops.c b/kernel/bpf/bpf_struct_ops.c
> index 45cc5ee19dc2..58664779a2b6 100644
> --- a/kernel/bpf/bpf_struct_ops.c
> +++ b/kernel/bpf/bpf_struct_ops.c
> @@ -13,6 +13,7 @@
>   #include <linux/btf_ids.h>
>   #include <linux/rcupdate_wait.h>
>   #include <linux/poll.h>
> +#include <linux/cgroup.h>
>   
>   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);
> +		if (IS_ERR(cgrp))
> +			return PTR_ERR(cgrp);
> +
> +		link->cgroup_id = cgroup_id(cgrp);

Not sure storing the cgroup_id or storing the memcg/cgroup pointer is 
better here. Regardless, link->cgroup_id should be cleared in 
bpf_struct_ops_map_link_detach(). The cgroup_id probably is useful to 
bpf_struct_ops_map_link_show_fdinfo().

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