HID: bpf: implement HID-BPF through bpf_struct_ops

We do this implementation in several steps to not have the CI failing:
- first (this patch), we add struct_ops while keeping the existing infra
  available
- then we change the selftests, the examples and the existing in-tree
  HID-BPF programs
- then we remove the existing trace points making old HID-BPF obsolete

There are a few advantages of struct_ops over tracing:
- compatibility with sleepable programs (for hid_hw_raw_request() in
  a later patch)
- a lot simpler in the kernel: it's a simple rcu protected list
- we can add more parameters to the function called without much trouble
- the "attach" is now generic through BPF-core: the caller just needs to
  set hid_id and flags before calling __load().
- all the BPF tough part is not handled in BPF-core through generic
  processing
- hid_bpf_ctx is now only writable where it needs be

Acked-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/r/20240608-hid_bpf_struct_ops-v3-3-6ac6ade58329@kernel.org
Signed-off-by: Benjamin Tissoires <bentiss@kernel.org>

1 files changed, 60 insertions, 1 deletions

diff --git a/include/linux/hid_bpf.h b/include/linux/hid_bpf.h
index a66103618e6e..c4f4ce10b7dd 100644
--- a/include/linux/hid_bpf.h
+++ b/include/linux/hid_bpf.h
@@ -65,11 +65,12 @@ struct hid_bpf_ctx {
  * @HID_BPF_FLAG_INSERT_HEAD: insert the given program before any other program
  *                            currently attached to the device. This doesn't
  *                            guarantee that this program will always be first
- * @HID_BPF_FLAG_MAX: sentinel value, not to be used by the callers
  */
 enum hid_bpf_attach_flags {
 	HID_BPF_FLAG_NONE = 0,
 	HID_BPF_FLAG_INSERT_HEAD = _BITUL(0),
+
+	/* private: internal use only */
 	HID_BPF_FLAG_MAX,
 };
 
@@ -112,6 +113,60 @@ struct hid_ops {
 
 extern struct hid_ops *hid_ops;
 
+/**
+ * struct hid_bpf_ops - A BPF struct_ops of callbacks allowing to attach HID-BPF
+ *			programs to a HID device
+ * @hid_id: the HID uniq ID to attach to. This is writeable before ``load()``, and
+ *	    cannot be changed after
+ * @flags: flags used while attaching the struct_ops to the device. Currently only
+ *	   available value is %0 or ``BPF_F_BEFORE``.
+ *	   Writeable only before ``load()``
+ */
+struct hid_bpf_ops {
+	/* hid_id needs to stay first so we can easily change it
+	 * from userspace.
+	 */
+	int			hid_id;
+	u32			flags;
+
+	/* private: do not show up in the docs */
+	struct list_head	list;
+
+	/* public: rest should show up in the docs */
+
+	/**
+	 * @hid_device_event: called whenever an event is coming in from the device
+	 *
+	 * It has the following arguments:
+	 *
+	 * ``ctx``: The HID-BPF context as &struct hid_bpf_ctx
+	 *
+	 * Return: %0 on success and keep processing; a positive
+	 * value to change the incoming size buffer; a negative
+	 * error code to interrupt the processing of this event
+	 *
+	 * Context: Interrupt context.
+	 */
+	int (*hid_device_event)(struct hid_bpf_ctx *ctx, enum hid_report_type report_type);
+
+	/**
+	 * @hid_rdesc_fixup: called when the probe function parses the report descriptor
+	 * of the HID device
+	 *
+	 * It has the following arguments:
+	 *
+	 * ``ctx``: The HID-BPF context as &struct hid_bpf_ctx
+	 *
+	 * Return: %0 on success and keep processing; a positive
+	 * value to change the incoming size buffer; a negative
+	 * error code to interrupt the processing of this device
+	 */
+	int (*hid_rdesc_fixup)(struct hid_bpf_ctx *ctx);
+
+	/* private: do not show up in the docs */
+	struct hid_device *hdev;
+};
+
 struct hid_bpf_prog_list {
 	u16 prog_idx[HID_BPF_MAX_PROGS_PER_DEV];
 	u8 prog_cnt;
@@ -129,6 +184,10 @@ struct hid_bpf {
 	bool destroyed;			/* prevents the assignment of any progs */
 
 	spinlock_t progs_lock;		/* protects RCU update of progs */
+
+	struct hid_bpf_ops *rdesc_ops;
+	struct list_head prog_list;
+	struct mutex prog_list_lock;	/* protects prog_list update */
 };
 
 /* specific HID-BPF link when a program is attached to a device */