[PATCH bpf-next v2 25/28] bpf/hid: Add a flag to add the program at the beginning of the list

[Benjamin Tissoires <benjamin.tissoires@redhat.com>]

When tracing the incoming events, if a bpf program is already loaded,
the next bpf program will see the potentially changed data.

Add a flag to BPF_LINK_CREATE that allows to chose the position of the
inserted program: at the beginning or at the end.

This way, we can have a tracing program that compare the raw event from
the device and the transformed stream from all the other bpf programs.

Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>

---

new in v2
---
 include/uapi/linux/bpf.h | 10 ++++++++++
 kernel/bpf/hid.c         | 11 +++++++----
 2 files changed, 17 insertions(+), 4 deletions(-)

diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index 417cf1c31579..23ebe5e96d69 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -1123,6 +1123,16 @@ enum bpf_link_type {
  */
 #define BPF_F_XDP_HAS_FRAGS	(1U << 5)
 
+/* HID flag used in BPF_LINK_CREATE command
+ *
+ * NONE(default): The bpf program will be added at the tail of the list
+ * of existing bpf program for this type.
+ *
+ * BPF_F_INSERT_HEAD: The bpf program will be added at the beginning
+ * of the list of existing bpf program for this type..
+ */
+#define BPF_F_INSERT_HEAD	(1U << 0)
+
 /* When BPF ldimm64's insn[0].src_reg != 0 then this can have
  * the following extensions:
  *
diff --git a/kernel/bpf/hid.c b/kernel/bpf/hid.c
index b3dc1cd37a3e..141eb4169079 100644
--- a/kernel/bpf/hid.c
+++ b/kernel/bpf/hid.c
@@ -416,7 +416,7 @@ static int bpf_hid_max_progs(enum bpf_hid_attach_type type)
 }
 
 static int bpf_hid_link_attach(struct hid_device *hdev, struct bpf_link *link,
-			       enum bpf_hid_attach_type type)
+			       enum bpf_hid_attach_type type, u32 flags)
 {
 	struct bpf_hid_link *hid_link =
 		container_of(link, struct bpf_hid_link, link);
@@ -443,7 +443,10 @@ static int bpf_hid_link_attach(struct hid_device *hdev, struct bpf_link *link,
 		goto out_unlock;
 	}
 
-	list_add_tail(&hid_link->node, &hdev->bpf.links[type]);
+	if (flags & BPF_F_INSERT_HEAD)
+		list_add(&hid_link->node, &hdev->bpf.links[type]);
+	else
+		list_add_tail(&hid_link->node, &hdev->bpf.links[type]);
 
 	fill_prog_array(hdev, type, run_array);
 	run_array = rcu_replace_pointer(hdev->bpf.run_array[type], run_array,
@@ -467,7 +470,7 @@ int bpf_hid_link_create(const union bpf_attr *attr, struct bpf_prog *prog)
 	struct hid_device *hdev;
 	int err;
 
-	if (attr->link_create.flags || !hid_hooks.hdev_from_fd)
+	if ((attr->link_create.flags & ~BPF_F_INSERT_HEAD) || !hid_hooks.hdev_from_fd)
 		return -EINVAL;
 
 	type = attr->link_create.attach_type;
@@ -495,7 +498,7 @@ int bpf_hid_link_create(const union bpf_attr *attr, struct bpf_prog *prog)
 		return err;
 	}
 
-	err = bpf_hid_link_attach(hdev, &hid_link->link, hid_type);
+	err = bpf_hid_link_attach(hdev, &hid_link->link, hid_type, attr->link_create.flags);
 	if (err) {
 		bpf_link_cleanup(&link_primer);
 		return err;
-- 
2.35.1