terminal: add evdev elements to idev

The evdev-element provides linux evdev interfaces as idev-elements. This
way, all real input hardware devices on linux can be used with the idev
interface.

We use libevdev to interface with the kernel. It's a simple wrapper
library around the kernel evdev API that takes care to resync devices
after kernel-queue overflows, which is a rather non-trivial task.
Furthermore, it's a well tested interface used by all other major input
users (Xorg, weston, libinput, ...).
Last but not least, it provides nice keycode to keyname lookup tables (and
vice versa), which is really nice for debugging input problems.

4 files changed, 1016 insertions, 0 deletions

diff --git a/src/libsystemd-terminal/idev-evdev.c b/src/libsystemd-terminal/idev-evdev.c
new file mode 100644
index 0000000000..c93ede8dc9
--- /dev/null
+++ b/src/libsystemd-terminal/idev-evdev.c
@@ -0,0 +1,938 @@
+/*-*- Mode: C; c-basic-offset: 8; indent-tabs-mode: nil -*-*/
+
+/***
+  This file is part of systemd.
+
+  Copyright (C) 2014 David Herrmann <dh.herrmann@gmail.com>
+
+  systemd is free software; you can redistribute it and/or modify it
+  under the terms of the GNU Lesser General Public License as published by
+  the Free Software Foundation; either version 2.1 of the License, or
+  (at your option) any later version.
+
+  systemd is distributed in the hope that it will be useful, but
+  WITHOUT ANY WARRANTY; without even the implied warranty of
+  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+  Lesser General Public License for more details.
+
+  You should have received a copy of the GNU Lesser General Public License
+  along with systemd; If not, see <http://www.gnu.org/licenses/>.
+***/
+
+#include <fcntl.h>
+#include <inttypes.h>
+#include <libevdev/libevdev.h>
+#include <libudev.h>
+#include <stdbool.h>
+#include <stdlib.h>
+#include <systemd/sd-bus.h>
+#include <systemd/sd-event.h>
+#include <unistd.h>
+#include "bus-util.h"
+#include "hashmap.h"
+#include "idev.h"
+#include "idev-internal.h"
+#include "macro.h"
+#include "udev-util.h"
+#include "util.h"
+
+typedef struct idev_evdev idev_evdev;
+typedef struct unmanaged_evdev unmanaged_evdev;
+typedef struct managed_evdev managed_evdev;
+
+struct idev_evdev {
+        struct idev_element element;
+        struct libevdev *evdev;
+        int fd;
+        sd_event_source *fd_src;
+        sd_event_source *idle_src;
+
+        bool unsync : 1;                /* not in-sync with kernel */
+        bool resync : 1;                /* re-syncing with kernel */
+};
+
+struct unmanaged_evdev {
+        struct idev_evdev evdev;
+        char *devnode;
+};
+
+struct managed_evdev {
+        struct idev_evdev evdev;
+        dev_t devnum;
+
+        sd_bus_slot *slot_pause_device;
+        sd_bus_slot *slot_resume_device;
+        sd_bus_slot *slot_take_device;
+
+        bool requested : 1;             /* TakeDevice() was sent */
+        bool acquired : 1;              /* TakeDevice() was successful */
+};
+
+#define idev_evdev_from_element(_e) container_of((_e), idev_evdev, element)
+#define unmanaged_evdev_from_element(_e) \
+        container_of(idev_evdev_from_element(_e), unmanaged_evdev, evdev)
+#define managed_evdev_from_element(_e) \
+        container_of(idev_evdev_from_element(_e), managed_evdev, evdev)
+
+#define IDEV_EVDEV_INIT(_vtable, _session) ((idev_evdev){ \
+                .element = IDEV_ELEMENT_INIT((_vtable), (_session)), \
+                .fd = -1, \
+        })
+
+#define IDEV_EVDEV_NAME_MAX (8 + DECIMAL_STR_MAX(unsigned) * 2)
+
+static const idev_element_vtable unmanaged_evdev_vtable;
+static const idev_element_vtable managed_evdev_vtable;
+
+static int idev_evdev_resume(idev_evdev *evdev, int dev_fd);
+static void idev_evdev_pause(idev_evdev *evdev, bool release);
+
+/*
+ * Virtual Evdev Element
+ * The virtual evdev element is the base class of all other evdev elements. It
+ * uses libevdev to access the kernel evdev API. It supports asynchronous
+ * access revocation, re-syncing if events got dropped and more.
+ * This element cannot be used by itself. There must be a wrapper around it
+ * which opens a file-descriptor and passes it to the virtual evdev element.
+ */
+
+static void idev_evdev_name(char *out, dev_t devnum) {
+        /* @out must be at least of size IDEV_EVDEV_NAME_MAX */
+        sprintf(out, "evdev/%u:%u", major(devnum), minor(devnum));
+}
+
+static int idev_evdev_raise(idev_evdev *evdev, struct input_event *event) {
+        idev_data data = {
+                .type = IDEV_DATA_EVDEV,
+                .resync = evdev->resync,
+                .evdev = {
+                        .event = *event,
+                },
+        };
+
+        return idev_element_feed(&evdev->element, &data);
+}
+
+static void idev_evdev_hup(idev_evdev *evdev) {
+        /*
+         * On HUP, we close the current fd via idev_evdev_pause(). This drops
+         * the event-sources from the main-loop and effectively puts the
+         * element asleep. If the HUP is part of a hotplug-event, a following
+         * udev-notification will destroy the element. Otherwise, the HUP is
+         * either result of access-revokation or a serious error.
+         * For unmanaged devices, we should never receive HUP (except for
+         * unplug-events). But if we do, something went seriously wrong and we
+         * shouldn't try to be clever.
+         * Instead, we simply stay asleep and wait for the device to be
+         * disabled and then re-enabled (or closed and re-opened). This will
+         * re-open the device node and restart the device.
+         * For managed devices, a HUP usually means our device-access was
+         * revoked. In that case, we simply put the device asleep and wait for
+         * logind to notify us once the device is alive again. logind also
+         * passes us a new fd. Hence, we don't have to re-enable the device.
+         *
+         * Long story short: The only thing we have to do here, is close() the
+         * file-descriptor and remove it from the main-loop. Everything else is
+         * handled via additional events we receive.
+         */
+
+        idev_evdev_pause(evdev, true);
+}
+
+static int idev_evdev_io(idev_evdev *evdev) {
+        idev_element *e = &evdev->element;
+        struct input_event ev;
+        unsigned int flags;
+        int r, error = 0;
+
+        /*
+         * Read input-events via libevdev until the input-queue is drained. In
+         * case we're disabled, don't do anything. The input-queue might
+         * overflow, but we don't care as we have to resync after wake-up,
+         * anyway.
+         * TODO: libevdev should give us a hint how many events to read. We
+         * really want to avoid starvation, so we shouldn't read forever in
+         * case we cannot keep up with the kernel.
+         * TODO: Make sure libevdev always reports SYN_DROPPED to us, regardless
+         * whether any event was synced afterwards.
+         * TODO: Forward SYN_DROPPED to attached devices.
+         */
+
+        flags = LIBEVDEV_READ_FLAG_NORMAL;
+        while (e->enabled) {
+                if (evdev->unsync) {
+                        /* immediately resync, even if in sync right now */
+                        evdev->unsync = false;
+                        evdev->resync = false;
+                        flags = LIBEVDEV_READ_FLAG_NORMAL;
+                        r = libevdev_next_event(evdev->evdev, flags | LIBEVDEV_READ_FLAG_FORCE_SYNC, &ev);
+                        if (r < 0 && r != -EAGAIN) {
+                                r = 0;
+                                goto error;
+                        } else if (r != LIBEVDEV_READ_STATUS_SYNC) {
+                                log_debug("idev-evdev: %s/%s: cannot force resync: %d",
+                                          e->session->name, e->name, r);
+                        }
+                } else {
+                        r = libevdev_next_event(evdev->evdev, flags, &ev);
+                }
+
+                if (evdev->resync && r == -EAGAIN) {
+                        /* end of re-sync */
+                        evdev->resync = false;
+                        flags = LIBEVDEV_READ_FLAG_NORMAL;
+                } else if (r == -EAGAIN) {
+                        /* no data available */
+                        break;
+                } else if (r < 0) {
+                        /* read error */
+                        goto error;
+                } else if (r == LIBEVDEV_READ_STATUS_SYNC) {
+                        if (evdev->resync) {
+                                /* sync-event */
+                                r = idev_evdev_raise(evdev, &ev);
+                                if (r != 0) {
+                                        error = r;
+                                        break;
+                                }
+                        } else {
+                                /* start of sync */
+                                evdev->resync = true;
+                                flags = LIBEVDEV_READ_FLAG_SYNC;
+                        }
+                } else {
+                        /* normal event */
+                        r = idev_evdev_raise(evdev, &ev);
+                        if (r != 0) {
+                                error = r;
+                                break;
+                        }
+                }
+        }
+
+        if (error < 0)
+                log_debug("idev-evdev: %s/%s: error on data event: %s",
+                          e->session->name, e->name, strerror(-error));
+        return error;
+
+error:
+        idev_evdev_hup(evdev);
+        return r;
+}
+
+static int idev_evdev_event_fn(sd_event_source *s, int fd, uint32_t revents, void *userdata) {
+        idev_evdev *evdev = userdata;
+
+        /* fetch data as long as EPOLLIN is signalled */
+        if (revents & EPOLLIN)
+                return idev_evdev_io(evdev);
+
+        if (revents & (EPOLLHUP | EPOLLERR))
+                idev_evdev_hup(evdev);
+
+        return 0;
+}
+
+static int idev_evdev_idle_fn(sd_event_source *s, void *userdata) {
+        idev_evdev *evdev = userdata;
+
+        /*
+         * The idle-event is raised whenever we have to re-sync the libevdev
+         * state from the kernel. We simply call into idev_evdev_io() which
+         * flushes the state and re-syncs it if @unsync is set.
+         * State has to be synced whenever our view of the kernel device is
+         * out of date. This is the case when we open the device, if the
+         * kernel's receive buffer overflows, or on other exceptional
+         * situations. Events during re-syncs must be forwarded to the upper
+         * layers so they can update their view of the device. However, such
+         * events must only be handled passively, as they might be out-of-order
+         * and/or re-ordered. Therefore, we mark them as 'sync' events.
+         */
+
+        if (!evdev->unsync)
+                return 0;
+
+        return idev_evdev_io(evdev);
+}
+
+static void idev_evdev_destroy(idev_evdev *evdev) {
+        assert(evdev);
+        assert(evdev->fd < 0);
+
+        libevdev_free(evdev->evdev);
+        evdev->evdev = NULL;
+}
+
+static void idev_evdev_enable(idev_evdev *evdev) {
+        assert(evdev);
+        assert(evdev->fd_src);
+        assert(evdev->idle_src);
+
+        sd_event_source_set_enabled(evdev->fd_src, SD_EVENT_ON);
+        sd_event_source_set_enabled(evdev->idle_src, SD_EVENT_ONESHOT);
+}
+
+static void idev_evdev_disable(idev_evdev *evdev) {
+        assert(evdev);
+        assert(evdev->fd_src);
+        assert(evdev->idle_src);
+
+        sd_event_source_set_enabled(evdev->fd_src, SD_EVENT_OFF);
+        sd_event_source_set_enabled(evdev->idle_src, SD_EVENT_OFF);
+}
+
+static int idev_evdev_resume(idev_evdev *evdev, int dev_fd) {
+        idev_element *e = &evdev->element;
+        _cleanup_close_ int fd = dev_fd;
+        int r, flags;
+
+        if (fd < 0 || evdev->fd == fd) {
+                fd = -1;
+                if (evdev->fd >= 0 && e->n_open > 0 && e->enabled)
+                        idev_evdev_enable(evdev);
+
+                return 0;
+        }
+
+        idev_evdev_pause(evdev, true);
+        log_debug("idev-evdev: %s/%s: resume", e->session->name, e->name);
+
+        r = fd_nonblock(fd, true);
+        if (r < 0)
+                return r;
+
+        r = fd_cloexec(fd, true);
+        if (r < 0)
+                return r;
+
+        flags = fcntl(fd, F_GETFL, 0);
+        if (flags < 0)
+                return r;
+
+        flags &= O_ACCMODE;
+        if (flags == O_WRONLY)
+                return -EACCES;
+
+        evdev->element.readable = true;
+        evdev->element.writable = true;
+        if (flags == O_RDONLY)
+                evdev->element.writable = false;
+        else if (flags == O_WRONLY)
+                evdev->element.readable = false;
+
+        /*
+         * TODO: We *MUST* re-sync the device so we get a delta of the changed
+         * state while we didn't read events from the device. This works just
+         * fine with libevdev_change_fd(), however, libevdev_new_from_fd() (or
+         * libevdev_set_fd()) don't pass us events for the initial device
+         * state. So even if we force a re-sync, we will not get the delta for
+         * the initial device state.
+         * We really need to fix libevdev to support that!
+         */
+        if (evdev->evdev)
+                r = libevdev_change_fd(evdev->evdev, fd);
+        else
+                r = libevdev_new_from_fd(fd, &evdev->evdev);
+
+        if (r < 0)
+                return r;
+
+        r = sd_event_add_io(e->session->context->event,
+                            &evdev->fd_src,
+                            fd,
+                            EPOLLHUP | EPOLLERR | EPOLLIN,
+                            idev_evdev_event_fn,
+                            evdev);
+        if (r < 0)
+                return r;
+
+        r = sd_event_add_defer(e->session->context->event,
+                               &evdev->idle_src,
+                               idev_evdev_idle_fn,
+                               evdev);
+        if (r < 0) {
+                evdev->fd_src = sd_event_source_unref(evdev->fd_src);
+                return r;
+        }
+
+        if (e->n_open < 1 || !e->enabled) {
+                sd_event_source_set_enabled(evdev->fd_src, SD_EVENT_OFF);
+                sd_event_source_set_enabled(evdev->idle_src, SD_EVENT_OFF);
+        }
+
+        evdev->unsync = true;
+        evdev->fd = fd;
+
+        fd = -1;
+        return 0;
+}
+
+static void idev_evdev_pause(idev_evdev *evdev, bool release) {
+        idev_element *e = &evdev->element;
+
+        if (evdev->fd < 0)
+                return;
+
+        log_debug("idev-evdev: %s/%s: pause", e->session->name, e->name);
+
+        if (release) {
+                evdev->idle_src = sd_event_source_unref(evdev->idle_src);
+                evdev->fd_src = sd_event_source_unref(evdev->fd_src);
+                evdev->fd = safe_close(evdev->fd);
+        } else {
+                idev_evdev_disable(evdev);
+        }
+}
+
+/*
+ * Unmanaged Evdev Element
+ * The unmanaged evdev element opens the evdev node for a given input device
+ * directly (/dev/input/eventX) and thus needs sufficient privileges. It opens
+ * the device only if we really require it and releases it as soon as we're
+ * disabled or closed.
+ * The unmanaged element can be used in all situations where you have direct
+ * access to input device nodes. Unlike managed evdev elements, it can be used
+ * outside of user sessions and in emergency situations where logind is not
+ * available.
+ */
+
+static void unmanaged_evdev_resume(idev_element *e) {
+        unmanaged_evdev *eu = unmanaged_evdev_from_element(e);
+        int r, fd;
+
+        /*
+         * Unmanaged devices can be acquired on-demand. Therefore, don't
+         * acquire it unless someone opened the device *and* we're enabled.
+         */
+        if (e->n_open < 1 || !e->enabled)
+                return;
+
+        fd = eu->evdev.fd;
+        if (fd < 0) {
+                fd = open(eu->devnode, O_RDWR | O_CLOEXEC | O_NOCTTY | O_NONBLOCK);
+                if (fd < 0) {
+                        if (errno != EACCES && errno != EPERM) {
+                                log_debug("idev-evdev: %s/%s: cannot open node %s: %m",
+                                          e->session->name, e->name, eu->devnode);
+                                return;
+                        }
+
+                        fd = open(eu->devnode, O_RDONLY | O_CLOEXEC | O_NOCTTY | O_NONBLOCK);
+                        if (fd < 0) {
+                                log_debug("idev-evdev: %s/%s: cannot open node %s: %m",
+                                          e->session->name, e->name, eu->devnode);
+                                return;
+                        }
+
+                        e->readable = true;
+                        e->writable = false;
+                } else {
+                        e->readable = true;
+                        e->writable = true;
+                }
+        }
+
+        r = idev_evdev_resume(&eu->evdev, fd);
+        if (r < 0)
+                log_debug("idev-evdev: %s/%s: cannot resume: %s",
+                          e->session->name, e->name, strerror(-r));
+}
+
+static void unmanaged_evdev_pause(idev_element *e) {
+        unmanaged_evdev *eu = unmanaged_evdev_from_element(e);
+
+        /*
+         * Release the device if the device is disabled or there is no-one who
+         * opened it. This guarantees we stay only available if we're opened
+         * *and* enabled.
+         */
+
+        idev_evdev_pause(&eu->evdev, true);
+}
+
+static int unmanaged_evdev_new(idev_element **out, idev_session *s, struct udev_device *ud) {
+        _cleanup_(idev_element_freep) idev_element *e = NULL;
+        char name[IDEV_EVDEV_NAME_MAX];
+        unmanaged_evdev *eu;
+        const char *devnode;
+        dev_t devnum;
+        int r;
+
+        assert_return(s, -EINVAL);
+        assert_return(ud, -EINVAL);
+
+        devnode = udev_device_get_devnode(ud);
+        devnum = udev_device_get_devnum(ud);
+        if (!devnode || devnum == 0)
+                return -ENODEV;
+
+        idev_evdev_name(name, devnum);
+
+        eu = new0(unmanaged_evdev, 1);
+        if (!eu)
+                return -ENOMEM;
+
+        e = &eu->evdev.element;
+        eu->evdev = IDEV_EVDEV_INIT(&unmanaged_evdev_vtable, s);
+
+        eu->devnode = strdup(devnode);
+        if (!eu->devnode)
+                return -ENOMEM;
+
+        r = idev_element_add(e, name);
+        if (r < 0)
+                return r;
+
+        if (out)
+                *out = e;
+        e = NULL;
+        return 0;
+}
+
+static void unmanaged_evdev_free(idev_element *e) {
+        unmanaged_evdev *eu = unmanaged_evdev_from_element(e);
+
+        idev_evdev_destroy(&eu->evdev);
+        free(eu->devnode);
+        free(eu);
+}
+
+static const idev_element_vtable unmanaged_evdev_vtable = {
+        .free                   = unmanaged_evdev_free,
+        .enable                 = unmanaged_evdev_resume,
+        .disable                = unmanaged_evdev_pause,
+        .open                   = unmanaged_evdev_resume,
+        .close                  = unmanaged_evdev_pause,
+};
+
+/*
+ * Managed Evdev Element
+ * The managed evdev element uses systemd-logind to acquire evdev devices. This
+ * means, we do not open the device node /dev/input/eventX directly. Instead,
+ * logind passes us a file-descriptor whenever our session is activated. Thus,
+ * we don't need access to the device node directly.
+ * Furthermore, whenever the session is put asleep, logind revokes the
+ * file-descriptor so we loose access to the device.
+ * Managed evdev elements should be preferred over unmanaged elements whenever
+ * you run inside a user session with exclusive device access.
+ */
+
+static int managed_evdev_take_device_fn(sd_bus *bus,
+                                        sd_bus_message *reply,
+                                        void *userdata,
+                                        sd_bus_error *ret_error) {
+        managed_evdev *em = userdata;
+        idev_element *e = &em->evdev.element;
+        idev_session *s = e->session;
+        int r, paused, fd;
+
+        em->slot_take_device = sd_bus_slot_unref(em->slot_take_device);
+
+        if (sd_bus_message_is_method_error(reply, NULL)) {
+                const sd_bus_error *error = sd_bus_message_get_error(reply);
+
+                log_debug("idev-evdev: %s/%s: TakeDevice failed: %s: %s",
+                          s->name, e->name, error->name, error->message);
+                return 0;
+        }
+
+        em->acquired = true;
+
+        r = sd_bus_message_read(reply, "hb", &fd, &paused);
+        if (r < 0) {
+                log_debug("idev-evdev: %s/%s: erroneous TakeDevice reply", s->name, e->name);
+                return 0;
+        }
+
+        /* If the device is paused, ignore it; we will get the next fd via
+         * ResumeDevice signals. */
+        if (paused)
+                return 0;
+
+        fd = fcntl(fd, F_DUPFD_CLOEXEC, 3);
+        if (fd < 0) {
+                log_debug("idev-evdev: %s/%s: cannot duplicate evdev fd: %m", s->name, e->name);
+                return 0;
+        }
+
+        r = idev_evdev_resume(&em->evdev, fd);
+        if (r < 0)
+                log_debug("idev-evdev: %s/%s: cannot resume: %s",
+                          s->name, e->name, strerror(-r));
+
+        return 0;
+}
+
+static void managed_evdev_resume(idev_element *e) {
+        _cleanup_bus_message_unref_ sd_bus_message *m = NULL;
+        managed_evdev *em = managed_evdev_from_element(e);
+        idev_session *s = e->session;
+        idev_context *c = s->context;
+        int r;
+
+        /*
+         * Acquiring managed devices is heavy, so do it only once we're
+         * enabled *and* opened by someone.
+         */
+        if (e->n_open < 1 || !e->enabled)
+                return;
+
+        /* bail out if already pending */
+        if (em->requested)
+                return;
+
+        r = sd_bus_message_new_method_call(c->sysbus,
+                                           &m,
+                                           "org.freedesktop.login1",
+                                           s->path,
+                                           "org.freedesktop.login1.Session",
+                                           "TakeDevice");
+        if (r < 0)
+                goto error;
+
+        r = sd_bus_message_append(m, "uu", major(em->devnum), minor(em->devnum));
+        if (r < 0)
+                goto error;
+
+        r = sd_bus_call_async(c->sysbus,
+                              &em->slot_take_device,
+                              m,
+                              managed_evdev_take_device_fn,
+                              em,
+                              0);
+        if (r < 0)
+                goto error;
+
+        em->requested = true;
+        return;
+
+error:
+        log_debug("idev-evdev: %s/%s: cannot send TakeDevice request: %s",
+                  s->name, e->name, strerror(-r));
+}
+
+static void managed_evdev_pause(idev_element *e) {
+        _cleanup_bus_message_unref_ sd_bus_message *m = NULL;
+        managed_evdev *em = managed_evdev_from_element(e);
+        idev_session *s = e->session;
+        idev_context *c = s->context;
+        int r;
+
+        /*
+         * Releasing managed devices is heavy. Once acquired, we get
+         * notifications for sleep/wake-up events, so there's no reason to
+         * release it if disabled but opened. However, if a device is closed,
+         * we release it immediately as we don't care for sleep/wake-up events
+         * then (even if we're actually enabled).
+         */
+
+        idev_evdev_pause(&em->evdev, false);
+
+        if (e->n_open > 0 || !em->requested)
+                return;
+
+        /*
+         * If TakeDevice() is pending or was successful, make sure to
+         * release the device again. We don't care for return-values,
+         * so send it without waiting or callbacks.
+         * If a failed TakeDevice() is pending, but someone else took
+         * the device on the same bus-connection, we might incorrectly
+         * release their device. This is an unlikely race, though.
+         * Furthermore, you really shouldn't have two users of the
+         * controller-API on the same session, on the same devices, *AND* on
+         * the same bus-connection. So we don't care for that race..
+         */
+
+        idev_evdev_pause(&em->evdev, true);
+        em->requested = false;
+
+        if (!em->acquired && !em->slot_take_device)
+                return;
+
+        em->slot_take_device = sd_bus_slot_unref(em->slot_take_device);
+        em->acquired = false;
+
+        r = sd_bus_message_new_method_call(c->sysbus,
+                                           &m,
+                                           "org.freedesktop.login1",
+                                           s->path,
+                                           "org.freedesktop.login1.Session",
+                                           "ReleaseDevice");
+        if (r >= 0) {
+                r = sd_bus_message_append(m, "uu", major(em->devnum), minor(em->devnum));
+                if (r >= 0)
+                        r = sd_bus_send(c->sysbus, m, NULL);
+        }
+
+        if (r < 0 && r != -ENOTCONN)
+                log_debug("idev-evdev: %s/%s: cannot send ReleaseDevice: %s",
+                          s->name, e->name, strerror(-r));
+}
+
+static int managed_evdev_pause_device_fn(sd_bus *bus,
+                                         sd_bus_message *signal,
+                                         void *userdata,
+                                         sd_bus_error *ret_error) {
+        managed_evdev *em = userdata;
+        idev_element *e = &em->evdev.element;
+        idev_session *s = e->session;
+        idev_context *c = s->context;
+        uint32_t major, minor;
+        const char *mode;
+        int r;
+
+        /*
+         * We get PauseDevice() signals from logind whenever a device we
+         * requested was, or is about to be, paused. Arguments are major/minor
+         * number of the device and the mode of the operation.
+         * In case the event is not about our device, we ignore it. Otherwise,
+         * we treat it as asynchronous access-revocation (as if we got HUP on
+         * the device fd). Note that we might have already treated the HUP
+         * event via EPOLLHUP, whichever comes first.
+         *
+         * @mode can be one of the following:
+         *   "pause": The device is about to be paused. We must react
+         *            immediately and respond with PauseDeviceComplete(). Once
+         *            we replied, logind will pause the device. Note that
+         *            logind might apply any kind of timeout and force pause
+         *            the device if we don't respond in a timely manner. In
+         *            this case, we will receive a second PauseDevice event
+         *            with @mode set to "force" (or similar).
+         *   "force": The device was disabled forecfully by logind. Access is
+         *            already revoked. This is just an asynchronous
+         *            notification so we can put the device asleep (in case
+         *            we didn't already notice the access revocation).
+         *    "gone": This is like "force" but is sent if the device was
+         *            paused due to a device-removal event.
+         *
+         * We always handle PauseDevice signals as "force" as we properly
+         * support asynchronous access revocation, anyway. But in case logind
+         * sent mode "pause", we also call PauseDeviceComplete() to immediately
+         * acknowledge the request.
+         */
+
+        r = sd_bus_message_read(signal, "uus", &major, &minor, &mode);
+        if (r < 0) {
+                log_debug("idev-evdev: %s/%s: erroneous PauseDevice signal",
+                          s->name, e->name);
+                return 0;
+        }
+
+        /* not our device? */
+        if (makedev(major, minor) != em->devnum)
+                return 0;
+
+        idev_evdev_pause(&em->evdev, true);
+
+        if (streq(mode, "pause")) {
+                _cleanup_bus_message_unref_ sd_bus_message *m = NULL;
+
+                /*
+                 * Sending PauseDeviceComplete() is racy if logind triggers the
+                 * timeout. That is, if we take too long and logind pauses the
+                 * device by sending a forced PauseDevice, our
+                 * PauseDeviceComplete call will be stray. That's fine, though.
+                 * logind ignores such stray calls. Only if logind also sent a
+                 * further PauseDevice() signal, it might match our call
+                 * incorrectly to the newer PauseDevice(). That's fine, too, as
+                 * we handle that event asynchronously, anyway. Therefore,
+                 * whatever happens, we're fine. Yay!
+                 */
+
+                r = sd_bus_message_new_method_call(c->sysbus,
+                                                   &m,
+                                                   "org.freedesktop.login1",
+                                                   s->path,
+                                                   "org.freedesktop.login1.Session",
+                                                   "PauseDeviceComplete");
+                if (r >= 0) {
+                        r = sd_bus_message_append(m, "uu", major, minor);
+                        if (r >= 0)
+                                r = sd_bus_send(c->sysbus, m, NULL);
+                }
+
+                if (r < 0)
+                        log_debug("idev-evdev: %s/%s: cannot send PauseDeviceComplete: %s",
+                                  s->name, e->name, strerror(-r));
+        }
+
+        return 0;
+}
+
+static int managed_evdev_resume_device_fn(sd_bus *bus,
+                                          sd_bus_message *signal,
+                                          void *userdata,
+                                          sd_bus_error *ret_error) {
+        managed_evdev *em = userdata;
+        idev_element *e = &em->evdev.element;
+        idev_session *s = e->session;
+        uint32_t major, minor;
+        int r, fd;
+
+        /*
+         * We get ResumeDevice signals whenever logind resumed a previously
+         * paused device. The arguments contain the major/minor number of the
+         * related device and a new file-descriptor for the freshly opened
+         * device-node.
+         * If the signal is not about our device, we simply ignore it.
+         * Otherwise, we take the file-descriptor and immediately resume the
+         * device.
+         */
+
+        r = sd_bus_message_read(signal, "uuh", &major, &minor, &fd);
+        if (r < 0) {
+                log_debug("idev-evdev: %s/%s: erroneous ResumeDevice signal",
+                          s->name, e->name);
+                return 0;
+        }
+
+        /* not our device? */
+        if (makedev(major, minor) != em->devnum)
+                return 0;
+
+        fd = fcntl(fd, F_DUPFD_CLOEXEC, 3);
+        if (fd < 0) {
+                log_debug("idev-evdev: %s/%s: cannot duplicate evdev fd: %m",
+                          s->name, e->name);
+                return 0;
+        }
+
+        r = idev_evdev_resume(&em->evdev, fd);
+        if (r < 0)
+                log_debug("idev-evdev: %s/%s: cannot resume: %s",
+                          s->name, e->name, strerror(-r));
+
+        return 0;
+}
+
+static int managed_evdev_setup_bus(managed_evdev *em) {
+        idev_element *e = &em->evdev.element;
+        idev_session *s = e->session;
+        idev_context *c = s->context;
+        _cleanup_free_ char *match = NULL;
+        int r;
+
+        match = strjoin("type='signal',"
+                        "sender='org.freedesktop.login1',"
+                        "interface='org.freedesktop.login1.Session',"
+                        "member='PauseDevice',"
+                        "path='", s->path, "'",
+                        NULL);
+        if (!match)
+                return -ENOMEM;
+
+        r = sd_bus_add_match(c->sysbus,
+                             &em->slot_pause_device,
+                             match,
+                             managed_evdev_pause_device_fn,
+                             em);
+        if (r < 0)
+                return r;
+
+        free(match);
+        match = strjoin("type='signal',"
+                        "sender='org.freedesktop.login1',"
+                        "interface='org.freedesktop.login1.Session',"
+                        "member='ResumeDevice',"
+                        "path='", s->path, "'",
+                        NULL);
+        if (!match)
+                return -ENOMEM;
+
+        r = sd_bus_add_match(c->sysbus,
+                             &em->slot_resume_device,
+                             match,
+                             managed_evdev_resume_device_fn,
+                             em);
+        if (r < 0)
+                return r;
+
+        return 0;
+}
+
+static int managed_evdev_new(idev_element **out, idev_session *s, struct udev_device *ud) {
+        _cleanup_(idev_element_freep) idev_element *e = NULL;
+        char name[IDEV_EVDEV_NAME_MAX];
+        managed_evdev *em;
+        dev_t devnum;
+        int r;
+
+        assert_return(s, -EINVAL);
+        assert_return(s->context->sysbus, -EINVAL);
+        assert_return(s->managed, -EINVAL);
+        assert_return(s->context->sysbus, -EINVAL);
+        assert_return(ud, -EINVAL);
+
+        devnum = udev_device_get_devnum(ud);
+        if (devnum == 0)
+                return -ENODEV;
+
+        idev_evdev_name(name, devnum);
+
+        em = new0(managed_evdev, 1);
+        if (!em)
+                return -ENOMEM;
+
+        e = &em->evdev.element;
+        em->evdev = IDEV_EVDEV_INIT(&managed_evdev_vtable, s);
+        em->devnum = devnum;
+
+        r = managed_evdev_setup_bus(em);
+        if (r < 0)
+                return r;
+
+        r = idev_element_add(e, name);
+        if (r < 0)
+                return r;
+
+        if (out)
+                *out = e;
+        e = NULL;
+        return 0;
+}
+
+static void managed_evdev_free(idev_element *e) {
+        managed_evdev *em = managed_evdev_from_element(e);
+
+        em->slot_resume_device = sd_bus_slot_unref(em->slot_resume_device);
+        em->slot_pause_device = sd_bus_slot_unref(em->slot_pause_device);
+        idev_evdev_destroy(&em->evdev);
+        free(em);
+}
+
+static const idev_element_vtable managed_evdev_vtable = {
+        .free                   = managed_evdev_free,
+        .enable                 = managed_evdev_resume,
+        .disable                = managed_evdev_pause,
+        .open                   = managed_evdev_resume,
+        .close                  = managed_evdev_pause,
+};
+
+/*
+ * Generic Constructor
+ * Instead of relying on the caller to choose between managed and unmanaged
+ * evdev devices, the idev_evdev_new() constructor does that for you (by
+ * looking at s->managed).
+ */
+
+bool idev_is_evdev(idev_element *e) {
+        return e && (e->vtable == &unmanaged_evdev_vtable ||
+                     e->vtable == &managed_evdev_vtable);
+}
+
+idev_element *idev_find_evdev(idev_session *s, dev_t devnum) {
+        char name[IDEV_EVDEV_NAME_MAX];
+
+        assert_return(s, NULL);
+        assert_return(devnum != 0, NULL);
+
+        idev_evdev_name(name, devnum);
+        return idev_find_element(s, name);
+}
+
+int idev_evdev_new(idev_element **out, idev_session *s, struct udev_device *ud) {
+        assert_return(s, -EINVAL);
+        assert_return(ud, -EINVAL);
+
+        return s->managed ? managed_evdev_new(out, s, ud) : unmanaged_evdev_new(out, s, ud);
+}
diff --git a/src/libsystemd-terminal/idev-internal.h b/src/libsystemd-terminal/idev-internal.h
index bffefbb9c1..3301ebf6e4 100644
--- a/src/libsystemd-terminal/idev-internal.h
+++ b/src/libsystemd-terminal/idev-internal.h
@@ -22,6 +22,8 @@
 #pragma once
 
 #include <inttypes.h>
+#include <libudev.h>
+#include <linux/input.h>
 #include <stdbool.h>
 #include <stdlib.h>
 #include <systemd/sd-bus.h>
@@ -37,6 +39,14 @@ typedef struct idev_element             idev_element;
 typedef struct idev_element_vtable      idev_element_vtable;
 
 /*
+ * Evdev Elements
+ */
+
+bool idev_is_evdev(idev_element *e);
+idev_element *idev_find_evdev(idev_session *s, dev_t devnum);
+int idev_evdev_new(idev_element **out, idev_session *s, struct udev_device *ud);
+
+/*
  * Element Links
  */
 
diff --git a/src/libsystemd-terminal/idev.c b/src/libsystemd-terminal/idev.c
index 5e3080797a..2316a66529 100644
--- a/src/libsystemd-terminal/idev.c
+++ b/src/libsystemd-terminal/idev.c
@@ -20,6 +20,8 @@
 ***/
 
 #include <inttypes.h>
+#include <libudev.h>
+#include <linux/input.h>
 #include <stdbool.h>
 #include <stdlib.h>
 #include <systemd/sd-bus.h>
@@ -31,6 +33,7 @@
 #include "login-shared.h"
 #include "macro.h"
 #include "set.h"
+#include "udev-util.h"
 #include "util.h"
 
 static void element_open(idev_element *e);
@@ -522,6 +525,51 @@ void idev_session_disable(idev_session *s) {
         }
 }
 
+int idev_session_add_evdev(idev_session *s, struct udev_device *ud) {
+        idev_element *e;
+        dev_t devnum;
+        int r;
+
+        assert_return(s, -EINVAL);
+        assert_return(ud, -EINVAL);
+
+        devnum = udev_device_get_devnum(ud);
+        if (devnum == 0)
+                return 0;
+
+        e = idev_find_evdev(s, devnum);
+        if (e)
+                return 0;
+
+        r = idev_evdev_new(&e, s, ud);
+        if (r < 0)
+                return r;
+
+        r = session_add_element(s, e);
+        if (r != 0)
+                return r;
+
+        return 0;
+}
+
+int idev_session_remove_evdev(idev_session *s, struct udev_device *ud) {
+        idev_element *e;
+        dev_t devnum;
+
+        assert(s);
+        assert(ud);
+
+        devnum = udev_device_get_devnum(ud);
+        if (devnum == 0)
+                return 0;
+
+        e = idev_find_evdev(s, devnum);
+        if (!e)
+                return 0;
+
+        return session_remove_element(s, e);
+}
+
 /*
  * Contexts
  */
diff --git a/src/libsystemd-terminal/idev.h b/src/libsystemd-terminal/idev.h
index 6f618f37af..c98fb1bfb0 100644
--- a/src/libsystemd-terminal/idev.h
+++ b/src/libsystemd-terminal/idev.h
@@ -26,6 +26,8 @@
 #pragma once
 
 #include <inttypes.h>
+#include <libudev.h>
+#include <linux/input.h>
 #include <stdbool.h>
 #include <stdlib.h>
 #include <systemd/sd-bus.h>
@@ -33,6 +35,7 @@
 #include "util.h"
 
 typedef struct idev_data                idev_data;
+typedef struct idev_data_evdev          idev_data_evdev;
 
 typedef struct idev_event               idev_event;
 typedef struct idev_device              idev_device;
@@ -44,6 +47,7 @@ typedef struct idev_context             idev_context;
  */
 
 enum {
+        IDEV_ELEMENT_EVDEV,
         IDEV_ELEMENT_CNT
 };
 
@@ -52,17 +56,30 @@ enum {
 };
 
 /*
+ * Evdev Elements
+ */
+
+struct idev_data_evdev {
+        struct input_event event;
+};
+
+/*
  * Data Packets
  */
 
 enum {
         IDEV_DATA_RESYNC,
+        IDEV_DATA_EVDEV,
         IDEV_DATA_CNT
 };
 
 struct idev_data {
         unsigned int type;
         bool resync : 1;
+
+        union {
+                idev_data_evdev evdev;
+        };
 };
 
 /*
@@ -122,6 +139,9 @@ bool idev_session_is_enabled(idev_session *s);
 void idev_session_enable(idev_session *s);
 void idev_session_disable(idev_session *s);
 
+int idev_session_add_evdev(idev_session *s, struct udev_device *ud);
+int idev_session_remove_evdev(idev_session *s, struct udev_device *ud);
+
 /*
  * Contexts
  */