From: Roman Gushchin <roman.gushchin@linux.dev>

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@linux.dev>
---
 include/linux/bpf.h         |  1 +
 kernel/bpf/bpf_struct_ops.c | 15 +++++++++++++++
 2 files changed, 16 insertions(+)

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index 899dd911dc82..720055d1dbce 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -1895,6 +1895,7 @@ 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 de01cf3025b3..c807793e7633 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;
@@ -1377,6 +1378,20 @@ 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)) {
+			err = PTR_ERR(cgrp);
+			goto err_out;
+		}
+
+		link->cgroup_id = cgroup_id(cgrp);
+		cgroup_put(cgrp);
+	}
+#endif /* CONFIG_CGROUPS */
 
 	err = bpf_link_prime(&link->link, &link_primer);
 	if (err)
-- 
2.43.0