[PATCH bpf-next v2 08/28] HID: allow to change the report descriptor from an eBPF program

[Benjamin Tissoires <benjamin.tissoires@xxxxxxxxxx>]

Make use of BPF_HID_ATTACH_RDESC_FIXUP so we can trigger an rdesc fixup
in the bpf world.

Whenever the program gets attached/detached, the device is reconnected
meaning that userspace will see it disappearing and reappearing with
the new report descriptor.

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

---

changes in v2:
- split the series by bpf/libbpf/hid/selftests and samples
---
 drivers/hid/hid-bpf.c  | 60 ++++++++++++++++++++++++++++++++++++++++++
 drivers/hid/hid-core.c |  3 ++-
 include/linux/hid.h    |  6 +++++
 3 files changed, 68 insertions(+), 1 deletion(-)

diff --git a/drivers/hid/hid-bpf.c b/drivers/hid/hid-bpf.c
index 8120e598de9f..510e24f4307c 100644
--- a/drivers/hid/hid-bpf.c
+++ b/drivers/hid/hid-bpf.c
@@ -50,6 +50,14 @@ static struct hid_device *hid_bpf_fd_to_hdev(int fd)
 	return hdev;
 }
 
+static int hid_reconnect(struct hid_device *hdev)
+{
+	if (!test_and_set_bit(ffs(HID_STAT_REPROBED), &hdev->status))
+		return device_reprobe(&hdev->dev);
+
+	return 0;
+}
+
 static int hid_bpf_link_attach(struct hid_device *hdev, enum bpf_hid_attach_type type)
 {
 	int err = 0;
@@ -71,6 +79,17 @@ static int hid_bpf_link_attach(struct hid_device *hdev, enum bpf_hid_attach_type
 	return err;
 }
 
+static void hid_bpf_link_attached(struct hid_device *hdev, enum bpf_hid_attach_type type)
+{
+	switch (type) {
+	case BPF_HID_ATTACH_RDESC_FIXUP:
+		hid_reconnect(hdev);
+		break;
+	default:
+		/* do nothing */
+	}
+}
+
 static void hid_bpf_array_detached(struct hid_device *hdev, enum bpf_hid_attach_type type)
 {
 	switch (type) {
@@ -78,6 +97,9 @@ static void hid_bpf_array_detached(struct hid_device *hdev, enum bpf_hid_attach_
 		kfree(hdev->bpf.ctx);
 		hdev->bpf.ctx = NULL;
 		break;
+	case BPF_HID_ATTACH_RDESC_FIXUP:
+		hid_reconnect(hdev);
+		break;
 	default:
 		/* do nothing */
 	}
@@ -98,6 +120,9 @@ static int hid_bpf_run_progs(struct hid_device *hdev, enum bpf_hid_attach_type t
 	case BPF_HID_ATTACH_DEVICE_EVENT:
 		event = HID_BPF_DEVICE_EVENT;
 		break;
+	case BPF_HID_ATTACH_RDESC_FIXUP:
+		event = HID_BPF_RDESC_FIXUP;
+		break;
 	default:
 		return -EINVAL;
 	}
@@ -138,11 +163,46 @@ u8 *hid_bpf_raw_event(struct hid_device *hdev, u8 *data, int *size)
 	return hdev->bpf.ctx->data;
 }
 
+u8 *hid_bpf_report_fixup(struct hid_device *hdev, u8 *rdesc, unsigned int *size)
+{
+	struct hid_bpf_ctx *ctx = NULL;
+	int ret;
+
+	if (bpf_hid_link_empty(&hdev->bpf, BPF_HID_ATTACH_RDESC_FIXUP))
+		goto ignore_bpf;
+
+	ctx = bpf_hid_allocate_ctx(hdev, HID_MAX_DESCRIPTOR_SIZE);
+	if (IS_ERR(ctx))
+		goto ignore_bpf;
+
+	ret = hid_bpf_run_progs(hdev, BPF_HID_ATTACH_RDESC_FIXUP, ctx, rdesc, *size);
+	if (ret)
+		goto ignore_bpf;
+
+	*size = ctx->size;
+
+	if (!*size) {
+		rdesc = NULL;
+		goto unlock;
+	}
+
+	rdesc = kmemdup(ctx->data, *size, GFP_KERNEL);
+
+ unlock:
+	kfree(ctx);
+	return rdesc;
+
+ ignore_bpf:
+	kfree(ctx);
+	return kmemdup(rdesc, *size, GFP_KERNEL);
+}
+
 int __init hid_bpf_module_init(void)
 {
 	struct bpf_hid_hooks hooks = {
 		.hdev_from_fd = hid_bpf_fd_to_hdev,
 		.link_attach = hid_bpf_link_attach,
+		.link_attached = hid_bpf_link_attached,
 		.array_detached = hid_bpf_array_detached,
 	};
 
diff --git a/drivers/hid/hid-core.c b/drivers/hid/hid-core.c
index a80bffe6ce4a..0eb8189faaee 100644
--- a/drivers/hid/hid-core.c
+++ b/drivers/hid/hid-core.c
@@ -1213,7 +1213,8 @@ int hid_open_report(struct hid_device *device)
 		return -ENODEV;
 	size = device->dev_rsize;
 
-	buf = kmemdup(start, size, GFP_KERNEL);
+	/* hid_bpf_report_fixup() ensures we work on a copy of rdesc */
+	buf = hid_bpf_report_fixup(device, start, &size);
 	if (buf == NULL)
 		return -ENOMEM;
 
diff --git a/include/linux/hid.h b/include/linux/hid.h
index 8fd79011f461..66d949d10b78 100644
--- a/include/linux/hid.h
+++ b/include/linux/hid.h
@@ -1213,10 +1213,16 @@ do {									\
 
 #ifdef CONFIG_BPF
 u8 *hid_bpf_raw_event(struct hid_device *hdev, u8 *rd, int *size);
+u8 *hid_bpf_report_fixup(struct hid_device *hdev, u8 *rdesc, unsigned int *size);
 int hid_bpf_module_init(void);
 void hid_bpf_module_exit(void);
 #else
 static inline u8 *hid_bpf_raw_event(struct hid_device *hdev, u8 *rd, int *size) { return rd; }
+static inline u8 *hid_bpf_report_fixup(struct hid_device *hdev, u8 *rdesc,
+				       unsigned int *size)
+{
+	return kmemdup(rdesc, *size, GFP_KERNEL);
+}
 static inline int hid_bpf_module_init(void) { return 0; }
 static inline void hid_bpf_module_exit(void) {}
 #endif
-- 
2.35.1