udev works: /sys device tree, uevent netlink, datagram sockets with per-message credentials

Kernel devices (input, mem, tty, graphics, block) get /sys/devices/virtual
entries with dev/uevent/subsystem, /sys/class and /sys/dev links, input
capabilities, and writable uevent files that announce on the netlink socket.
eudev now runs, processes a full trigger and settles.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EVRhxi9zpdW2uDUN4FVcao

include/dev/kdev.h

@@ -0,0 +1,44 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+//
+// The devices the kernel implements itself, as /sys and udev see them.
+//
+// /dev holds them already (kernel/fs/devfs.c). This is the other half of what
+// udev needs: for each node a place in /sys/devices, the class and dev/ links
+// that lead back to it, the facts udev's rules read (MAJOR, MINOR, DEVNAME,
+// input capabilities), and a way to announce it on the uevent netlink socket.
+#pragma once
+
+#include "core/types.h"
+
+#define KDEV_NAME     24
+#define KDEV_DEVNAME  32
+#define KDEV_BLK0     256       // kdev indexes from here are block devices
+
+enum { KD_NONE, KD_KEYBOARD, KD_MOUSE };
+
+struct kdev {
+  char  subsys[16];             // class: input, mem, tty, graphics, block
+  char  name[KDEV_NAME];        // directory name under the parent
+  char  devname[KDEV_DEVNAME];  // path below /dev, "" for a device with no node
+  short major, minor;
+  int   block;
+  int   parent;                 // kdev index of the device it hangs off, or -1
+  int   cap;                    // KD_*: what an input device can do
+};
+
+int  kdev_nsubsys(void);
+const char *kdev_subsys(int s);
+
+// Index of the Nth device that exists, 0.. until -1.
+int  kdev_nth(int n);
+int  kdev_get(int i, struct kdev *k);
+// The numbers stat() shows for its node, which are what /sys must show too.
+void kdev_numbers(const struct kdev *k, int *maj, int *min);
+
+// "devices/virtual/input/input0/event0": where it lives below /sys.
+int  kdev_path(int i, char *buf, int cap);
+
+// Announce a device on the uevent socket: action is "add", "remove", "change".
+void kdev_uevent(int i, const char *action);
+// What reading a device's `uevent` file says.
+int  kdev_uevent_text(int i, char *buf, int cap);