HID: bpf: allow hid_device_event hooks to inject input reports on self

This is the same logic than hid_hw_raw_request or hid_hw_output_report:
we can allow hid_bpf_try_input_report to be called from a hook on
hid_input_report if we ensure that the call can not be made twice in a
row.

There is one extra subtlety in which there is a lock in hid_input_report.
But given that we can detect if we are already in the hook, we can notify
hid_input_report to not take the lock. This is done by checking if
ctx_kern data is valid or null, and if it is equal to the dedicated
incoming data buffer.

In order to have more control on whether the lock needs to be taken or not
we introduce a new kfunc for it: hid_bpf_try_input_report()

Link: https://patch.msgid.link/[e-mail]
Acked-by: Jiri Kosina <[e-mail]>
Signed-off-by: Benjamin Tissoires <[e-mail]>

Documentation/hid/hid-bpf.rst

···
 -------------------------------------------------------------------------------------------------------
 
 .. kernel-doc:: drivers/hid/bpf/hid_bpf_dispatch.c
-   :identifiers: hid_bpf_hw_request hid_bpf_hw_output_report hid_bpf_input_report hid_bpf_allocate_context hid_bpf_release_context
+   :identifiers: hid_bpf_hw_request hid_bpf_hw_output_report hid_bpf_input_report hid_bpf_try_input_report hid_bpf_allocate_context hid_bpf_release_context
 
 General overview of a HID-BPF program
 =====================================

drivers/hid/bpf/hid_bpf_dispatch.c

···
 
 u8 *
 dispatch_hid_bpf_device_event(struct hid_device *hdev, enum hid_report_type type, u8 *data,
-			      u32 *size, int interrupt, u64 source)
+			      u32 *size, int interrupt, u64 source, bool from_bpf)
 {
 	struct hid_bpf_ctx_kern ctx_kern = {
 		.ctx = {
···
 			.size = *size,
 		},
 		.data = hdev->bpf.device_data,
+		.from_bpf = from_bpf,
 	};
 	struct hid_bpf_ops *e;
 	int ret;
···
 	return ret;
 }
 
+static int
+__hid_bpf_input_report(struct hid_bpf_ctx *ctx, enum hid_report_type type, u8 *buf,
+		       size_t size, bool lock_already_taken)
+{
+	struct hid_bpf_ctx_kern *ctx_kern;
+	int ret;
+
+	ctx_kern = container_of(ctx, struct hid_bpf_ctx_kern, ctx);
+	if (ctx_kern->from_bpf)
+		return -EDEADLOCK;
+
+	/* check arguments */
+	ret = __hid_bpf_hw_check_params(ctx, buf, &size, type);
+	if (ret)
+		return ret;
+
+	return hid_ops->hid_input_report(ctx->hid, type, buf, size, 0, (__u64)ctx, true,
+					 lock_already_taken);
+}
+
+/**
+ * hid_bpf_try_input_report - Inject a HID report in the kernel from a HID device
+ *
+ * @ctx: the HID-BPF context previously allocated in hid_bpf_allocate_context()
+ * @type: the type of the report (%HID_INPUT_REPORT, %HID_FEATURE_REPORT, %HID_OUTPUT_REPORT)
+ * @buf: a %PTR_TO_MEM buffer
+ * @buf__sz: the size of the data to transfer
+ *
+ * Returns %0 on success, a negative error code otherwise. This function will immediately
+ * fail if the device is not available, thus can be safely used in IRQ context.
+ */
+__bpf_kfunc int
+hid_bpf_try_input_report(struct hid_bpf_ctx *ctx, enum hid_report_type type, u8 *buf,
+			 const size_t buf__sz)
+{
+	struct hid_bpf_ctx_kern *ctx_kern;
+	bool from_hid_event_hook;
+
+	ctx_kern = container_of(ctx, struct hid_bpf_ctx_kern, ctx);
+	from_hid_event_hook = ctx_kern->data && ctx_kern->data == ctx->hid->bpf.device_data;
+
+	return __hid_bpf_input_report(ctx, type, buf, buf__sz, from_hid_event_hook);
+}
+
 /**
  * hid_bpf_input_report - Inject a HID report in the kernel from a HID device
  *
···
 hid_bpf_input_report(struct hid_bpf_ctx *ctx, enum hid_report_type type, u8 *buf,
 		     const size_t buf__sz)
 {
-	size_t size = buf__sz;
 	int ret;
 
 	ret = down_interruptible(&ctx->hid->driver_input_lock);
···
 		return ret;
 
 	/* check arguments */
-	ret = __hid_bpf_hw_check_params(ctx, buf, &size, type);
-	if (ret)
-		return ret;
-
-	ret = hid_ops->hid_input_report(ctx->hid, type, buf, size, 0, (__u64)ctx,
-					true /* lock_already_taken */);
+	ret = __hid_bpf_input_report(ctx, type, buf, buf__sz, true /* lock_already_taken */);
 
 	up(&ctx->hid->driver_input_lock);
 
···
 BTF_ID_FLAGS(func, hid_bpf_hw_request, KF_SLEEPABLE)
 BTF_ID_FLAGS(func, hid_bpf_hw_output_report, KF_SLEEPABLE)
 BTF_ID_FLAGS(func, hid_bpf_input_report, KF_SLEEPABLE)
+BTF_ID_FLAGS(func, hid_bpf_try_input_report)
 BTF_KFUNCS_END(hid_bpf_kfunc_ids)
 
 static const struct btf_kfunc_id_set hid_bpf_kfunc_set = {

drivers/hid/hid-core.c

···
 
 
 static int __hid_input_report(struct hid_device *hid, enum hid_report_type type,
-			      u8 *data, u32 size, int interrupt, u64 source,
+			      u8 *data, u32 size, int interrupt, u64 source, bool from_bpf,
 			      bool lock_already_taken)
 {
 	struct hid_report_enum *report_enum;
···
 	report_enum = hid->report_enum + type;
 	hdrv = hid->driver;
 
-	data = dispatch_hid_bpf_device_event(hid, type, data, &size, interrupt, source);
+	data = dispatch_hid_bpf_device_event(hid, type, data, &size, interrupt, source, from_bpf);
 	if (IS_ERR(data)) {
 		ret = PTR_ERR(data);
 		goto unlock;
···
 		     int interrupt)
 {
 	return __hid_input_report(hid, type, data, size, interrupt, 0,
+				  false, /* from_bpf */
 				  false /* lock_already_taken */);
 }
 EXPORT_SYMBOL_GPL(hid_input_report);

include/linux/hid_bpf.h

···
 	int (*hid_hw_output_report)(struct hid_device *hdev, __u8 *buf, size_t len,
 				    __u64 source, bool from_bpf);
 	int (*hid_input_report)(struct hid_device *hid, enum hid_report_type type,
-				u8 *data, u32 size, int interrupt, u64 source,
+				u8 *data, u32 size, int interrupt, u64 source, bool from_bpf,
 				bool lock_already_taken);
 	struct module *owner;
 	const struct bus_type *bus_type;
···
 
 #ifdef CONFIG_HID_BPF
 u8 *dispatch_hid_bpf_device_event(struct hid_device *hid, enum hid_report_type type, u8 *data,
-				  u32 *size, int interrupt, u64 source);
+				  u32 *size, int interrupt, u64 source, bool from_bpf);
 int dispatch_hid_bpf_raw_requests(struct hid_device *hdev,
 				  unsigned char reportnum, __u8 *buf,
 				  u32 size, enum hid_report_type rtype,
···
 #else /* CONFIG_HID_BPF */
 static inline u8 *dispatch_hid_bpf_device_event(struct hid_device *hid, enum hid_report_type type,
 						u8 *data, u32 *size, int interrupt,
-						u64 source) { return data; }
+						u64 source, bool from_bpf) { return data; }
 static inline int dispatch_hid_bpf_raw_requests(struct hid_device *hdev,
 						unsigned char reportnum, u8 *buf,
 						u32 size, enum hid_report_type rtype,