[RFC PATCH bpf-next v6 06/12] bpf: Pass flags in bpf_link_create for struct_ops

[Hui Zhu <hui.zhu@linux.dev>]

From: Hui Zhu <zhuhui@kylinos.cn>

To support features like allowing overrides in cgroup hierarchies,
we need a way to pass flags from userspace to the kernel when
attaching a struct_ops.

Extend `bpf_struct_ops_link` to include a `flags` field. This field
is populated from `attr->link_create.flags` during link creation. This
will allow struct_ops implementations, such as the upcoming memory
controller ops, to interpret these flags and modify their attachment
behavior accordingly.

UAPI Change:
This patch updates the comment in include/uapi/linux/bpf.h to reflect
that the cgroup-bpf attach flags (such as BPF_F_ALLOW_OVERRIDE) are
now applicable to both BPF_PROG_ATTACH and BPF_LINK_CREATE commands.
Previously, these flags were only documented for BPF_PROG_ATTACH.

The actual flag definitions remain unchanged, so this is a compatible
extension of the existing API. Older userspace will continue to work
(by not passing flags), and newer userspace can opt-in to the new
functionality by setting appropriate flags.

Signed-off-by: Geliang Tang <geliang@kernel.org>
Signed-off-by: Hui Zhu <zhuhui@kylinos.cn>
---
 include/linux/bpf.h            | 1 +
 kernel/bpf/bpf_struct_ops.c    | 1 +
 tools/include/uapi/linux/bpf.h | 2 +-
 3 files changed, 3 insertions(+), 1 deletion(-)

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index 720055d1dbce..13c933cfc614 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -1896,6 +1896,7 @@ struct bpf_struct_ops_link {
 	struct bpf_map __rcu *map;
 	wait_queue_head_t wait_hup;
 	u64 cgroup_id;
+	u32 flags;
 };
 
 struct bpf_link_primer {
diff --git a/kernel/bpf/bpf_struct_ops.c b/kernel/bpf/bpf_struct_ops.c
index c807793e7633..0df608c88403 100644
--- a/kernel/bpf/bpf_struct_ops.c
+++ b/kernel/bpf/bpf_struct_ops.c
@@ -1392,6 +1392,7 @@ int bpf_struct_ops_link_create(union bpf_attr *attr)
 		cgroup_put(cgrp);
 	}
 #endif /* CONFIG_CGROUPS */
+	link->flags = attr->link_create.flags;
 
 	err = bpf_link_prime(&link->link, &link_primer);
 	if (err)
diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
index 3ca7d76e05f0..4e1c5d6d91ae 100644
--- a/tools/include/uapi/linux/bpf.h
+++ b/tools/include/uapi/linux/bpf.h
@@ -1185,7 +1185,7 @@ enum bpf_perf_event_type {
 	BPF_PERF_EVENT_EVENT = 6,
 };
 
-/* cgroup-bpf attach flags used in BPF_PROG_ATTACH command
+/* cgroup-bpf attach flags used in BPF_PROG_ATTACH and BPF_LINK_CREATE command
  *
  * NONE(default): No further bpf programs allowed in the subtree.
  *
-- 
2.43.0