HID: bpf: move HID-BPF report descriptor fixup earlier

Currently, hid_bpf_rdesc_fixup() is called once the match between the
HID device and the driver is done. This can be problematic in case
the driver selected by the kernel would change the report descriptor
after the fact.

To give a chance for hid_bpf_rdesc_fixup() to provide hints on to how
to select a dedicated driver or not, move the call to that BPF hook
earlier in the .probe() process, when we get the first match.

However, this means that we might get called more than once (typically
once for hid-generic, and once for hid-vendor-specific). So we store the
result of HID-BPF fixup in struct hid_device. Basically, this means that
->bpf_rdesc can replace ->dev_rdesc when it was used in the code.

In order to not grow struct hid_device, some fields are re-ordered. This
was the output of pahole for the first 128 bytes:
struct hid_device {
	__u8 *                     dev_rdesc;            /*     0     8 */
	unsigned int               dev_rsize;            /*     8     4 */

	/* XXX 4 bytes hole, try to pack */

	__u8 *                     rdesc;                /*    16     8 */
	unsigned int               rsize;                /*    24     4 */

	/* XXX 4 bytes hole, try to pack */

	struct hid_collection *    collection;           /*    32     8 */
	unsigned int               collection_size;      /*    40     4 */
	unsigned int               maxcollection;        /*    44     4 */
	unsigned int               maxapplication;       /*    48     4 */
	__u16                      bus;                  /*    52     2 */
	__u16                      group;                /*    54     2 */
	__u32                      vendor;               /*    56     4 */
	__u32                      product;              /*    60     4 */
	/* --- cacheline 1 boundary (64 bytes) --- */
	__u32                      version;              /*    64     4 */
	enum hid_type              type;                 /*    68     4 */
	unsigned int               country;              /*    72     4 */

	/* XXX 4 bytes hole, try to pack */

	struct hid_report_enum     report_enum[3];       /*    80  6216 */

Basically, we got three holes of 4 bytes. We can reorder things a little
and makes those 3 holes a continuous 12 bytes hole, which can be replaced
by the new pointer and the new unsigned int we need.

In terms of code allocation, when not using HID-BPF, we are back to kernel
v6.2 in hid_open_report(). These multiple kmemdup() calls will be fixed
in a later commit.

Link: https://patch.msgid.link/20241001-hid-bpf-hid-generic-v3-1-2ef1019468df@kernel.org
Signed-off-by: Benjamin Tissoires <bentiss@kernel.org>

1 files changed, 26 insertions, 4 deletions

diff --git a/drivers/hid/hid-core.c b/drivers/hid/hid-core.c
index 30de92d0bf0f..d6bf933623e8 100644
--- a/drivers/hid/hid-core.c
+++ b/drivers/hid/hid-core.c
@@ -698,6 +698,14 @@ static void hid_close_report(struct hid_device *device)
 	device->status &= ~HID_STAT_PARSED;
 }
 
+static inline void hid_free_bpf_rdesc(struct hid_device *hdev)
+{
+	/* bpf_rdesc is either equal to dev_rdesc or allocated by call_hid_bpf_rdesc_fixup() */
+	if (hdev->bpf_rdesc != hdev->dev_rdesc)
+		kfree(hdev->bpf_rdesc);
+	hdev->bpf_rdesc = NULL;
+}
+
 /*
  * Free a device structure, all reports, and all fields.
  */
@@ -707,6 +715,7 @@ void hiddev_free(struct kref *ref)
 	struct hid_device *hid = container_of(ref, struct hid_device, ref);
 
 	hid_close_report(hid);
+	hid_free_bpf_rdesc(hid);
 	kfree(hid->dev_rdesc);
 	kfree(hid);
 }
@@ -1221,13 +1230,12 @@ int hid_open_report(struct hid_device *device)
 	if (WARN_ON(device->status & HID_STAT_PARSED))
 		return -EBUSY;
 
-	start = device->dev_rdesc;
+	start = device->bpf_rdesc;
 	if (WARN_ON(!start))
 		return -ENODEV;
-	size = device->dev_rsize;
+	size = device->bpf_rsize;
 
-	/* call_hid_bpf_rdesc_fixup() ensures we work on a copy of rdesc */
-	buf = call_hid_bpf_rdesc_fixup(device, start, &size);
+	buf = kmemdup(start, size, GFP_KERNEL);
 	if (buf == NULL)
 		return -ENOMEM;
 
@@ -2684,6 +2692,18 @@ static int __hid_device_probe(struct hid_device *hdev, struct hid_driver *hdrv)
 	const struct hid_device_id *id;
 	int ret;
 
+	if (!hdev->bpf_rsize) {
+		/* in case a bpf program gets detached, we need to free the old one */
+		hid_free_bpf_rdesc(hdev);
+
+		/* keep this around so we know we called it once */
+		hdev->bpf_rsize = hdev->dev_rsize;
+
+		/* call_hid_bpf_rdesc_fixup will always return a valid pointer */
+		hdev->bpf_rdesc = call_hid_bpf_rdesc_fixup(hdev, hdev->dev_rdesc,
+							   &hdev->bpf_rsize);
+	}
+
 	if (!hid_check_device_match(hdev, hdrv, &id))
 		return -ENODEV;
 
@@ -2940,9 +2960,11 @@ static void hid_remove_device(struct hid_device *hdev)
 		hid_debug_unregister(hdev);
 		hdev->status &= ~HID_STAT_ADDED;
 	}
+	hid_free_bpf_rdesc(hdev);
 	kfree(hdev->dev_rdesc);
 	hdev->dev_rdesc = NULL;
 	hdev->dev_rsize = 0;
+	hdev->bpf_rsize = 0;
 }
 
 /**