Commit bb8be68d authored by Benjamin Tissoires's avatar Benjamin Tissoires Committed by Jiri Kosina
Browse files

hid: bpf: hid_bpf_helpers: add helper for having read/write udev properties 

We want udev-hid-bpf to be able to set udev properties by printing them
out after the BPF object has been loaded. This allows to make a query to
the device, and set a udev prop based on the answer.

Because the way udev works, the properties are cleared on bind/unbind,
and we need a way to store them. After several attempts to keep the
property alive without re-running the udev-hid-bpf tool to communicate
with the device, it came out that HID-BPF maps are pinned in the bpffs
and we can then query them.

So the following would export a UDEV property in the bpffs:
   EXPORT_UDEV_PROP(HID_FOO, 32);

   SEC("syscall")
   int probe(struct hid_bpf_probe_args *ctx)
   {
     const char *foo = "foo";
     UDEV_PROP_SPRINTF(HID_FOO, "%s", foo);

     return 0;
   }

Then, we can debug it with a simple cat:
   sudo cat /sys/fs/bpf/hid/.../UDEV_PROP_HID_FOO
0: {['f','o','o',],}

This way, the property is always accessible without talking to the
device

Link: https://gitlab.freedesktop.org/libevdev/udev-hid-bpf/-/merge_requests/220


Signed-off-by: default avatarBenjamin Tissoires <bentiss@kernel.org>
Signed-off-by: default avatarJiri Kosina <jkosina@suse.com>
parent fc778f21

drivers/hid/bpf/progs/hid_bpf_helpers.h

+38 −0
Original line number Diff line number Diff line
@@ -340,6 +340,44 @@ DEFINE_GUARD(bpf_spin, struct bpf_spin_lock, bpf_spin_lock, bpf_spin_unlock); 
#define hid_bpf_cpu_to_be32(x)	bpf_htonl(x)

#define hid_bpf_cpu_to_be64(x)	bpf_cpu_to_be64(x)



/*

 * The following macros are helpers for exporting udev properties:

 *

 * EXPORT_UDEV_PROP(name, len) generates:

 *  - a map with a single element UDEV_PROP_##name, of size len

 *  - a const global declaration of that len: SIZEOF_##name

 *

 * udev_prop_ptr(name) retrieves the data pointer behind the map.

 *

 * UDEV_PROP_SPRINTF(name, fmt, ...) writes data into the udev property.

 *

 *  Can be used as such:

 *  EXPORT_UDEV_PROP(HID_FOO, 32);

 *

 *  SEC("syscall")

 *  int probe(struct hid_bpf_probe_args *ctx)

 *  {

 *    const char *foo = "foo";

 *    UDEV_PROP_SPRINTF(HID_FOO, "%s", foo);

 *

 *    return 0;

 *  }

 */

#define EXPORT_UDEV_PROP(name, len) \

	const __u32 SIZEOF_##name = len; \

	struct COMBINE(udev_prop, __LINE__) { \

		__uint(type, BPF_MAP_TYPE_ARRAY); \

		__uint(max_entries, 1); \

		__type(key, __u32); \

		__type(value, __u8[len]); \

	} UDEV_PROP_##name SEC(".maps");



#define udev_prop_ptr(name) \

	bpf_map_lookup_elem(&UDEV_PROP_##name, &(__u32){0})



#define UDEV_PROP_SPRINTF(name, fmt, ...) \

	BPF_SNPRINTF(udev_prop_ptr(name), SIZEOF_##name, fmt, ##__VA_ARGS__)



static inline __maybe_unused __u16 field_start_byte(struct hid_rdesc_field *field)

{

	return field->bits_start / 8;