wlroots/session/direct.c

#define _POSIX_C_SOURCE 200809L
#include <errno.h>
#include <stdlib.h>
#include <stdio.h>
#include <stdbool.h>
#include <string.h>
#include <fcntl.h>
#include <unistd.h>
#include <signal.h>
#include <sys/ioctl.h>
#include <sys/stat.h>
#include <sys/sysmacros.h>
#include <sys/socket.h>
#include <sys/wait.h>
#include <linux/kd.h>
#include <linux/major.h>
#include <linux/vt.h>
#include <wayland-server.h>
#include <xf86drm.h>
#include <wlr/session/interface.h>
#include <wlr/util/log.h>
#ifdef HAS_LIBCAP
#include <sys/capability.h>
#endif

enum { DRM_MAJOR = 226 };

const struct session_impl session_direct;

struct direct_session {
	struct wlr_session base;
	int tty_fd;
	int old_kbmode;
	int sock;
	pid_t child;

	struct wl_event_source *vt_source;
};

enum session_message_type {
	SESSION_OPEN,
	SESSION_SETMASTER,
	SESSION_DROPMASTER,
	SESSION_END,
};

struct session_message {
	enum session_message_type type;
	char path[60];
};

static int send_message(int sock, enum session_message_type type, const char *path) {
	struct session_message msg = {
		.type = type,
	};
	struct msghdr request = {
		.msg_iov = &(struct iovec) {
			.iov_base = &msg,
			.iov_len = sizeof(msg),
		},
		.msg_iovlen = 1,
	};

	if (path) {
		snprintf(msg.path, sizeof(msg.path), "%s", path);
	}

	sendmsg(sock, &request, 0);

	int err = 0, fd = -1;
	char control[CMSG_SPACE(sizeof(fd))] = {0};
	struct msghdr reply = {
		.msg_iov = &(struct iovec) {
			.iov_base = &err,
			.iov_len = sizeof(err),
		},
		.msg_iovlen = 1,
		.msg_control = control,
		.msg_controllen = sizeof(control),
	};

	recvmsg(sock, &reply, 0);

	// The other types have no meaningful return value
	if (type != SESSION_OPEN) {
		return 0;
	}

	struct cmsghdr *cmsg = CMSG_FIRSTHDR(&reply);
	memcpy(&fd, CMSG_DATA(cmsg), sizeof(fd));

	return err ? -err : fd;
}

static int direct_session_open(struct wlr_session *base, const char *path) {
	struct direct_session *session = wl_container_of(base, session, base);

	struct stat st;
	if (stat(path, &st)) {
		return -errno;
	}

	uint32_t maj = major(st.st_rdev);
	if (maj != DRM_MAJOR && maj != INPUT_MAJOR) {
		return -EINVAL;
	}

	int fd = send_message(session->sock, SESSION_OPEN, path);
	if (fd < 0) {
		wlr_log(L_ERROR, "Failed to open %s: %s%s", path, strerror(-fd),
			fd == -EINVAL ? "; is another display server running?" : "");
		return fd;
	}

	if (maj == DRM_MAJOR) {
		session->base.drm_fd = fd;
	}

	return fd;
}

static void direct_session_close(struct wlr_session *base, int fd) {
	struct direct_session *session = wl_container_of(base, session, base);

	if (fd == session->base.drm_fd) {
		send_message(session->sock, SESSION_DROPMASTER, NULL);
		session->base.drm_fd = -1;
	}

	close(fd);
}

static bool direct_change_vt(struct wlr_session *base, unsigned vt) {
	struct direct_session *session = wl_container_of(base, session, base);
	return ioctl(session->tty_fd, VT_ACTIVATE, (int)vt) == 0;
}

static void direct_session_finish(struct wlr_session *base) {
	struct direct_session *session = wl_container_of(base, session, base);
	struct vt_mode mode = {
		.mode = VT_AUTO,
	};

	errno = 0;

	ioctl(session->tty_fd, KDSKBMODE, session->old_kbmode);
	ioctl(session->tty_fd, KDSETMODE, KD_TEXT);
	ioctl(session->tty_fd, VT_SETMODE, &mode);

	if (errno) {
		wlr_log(L_ERROR, "Failed to restore tty");
	}

	send_message(session->sock, SESSION_END, NULL);
	close(session->sock);
	wait(NULL);

	wl_event_source_remove(session->vt_source);
	close(session->tty_fd);
	free(session);
}

static int vt_handler(int signo, void *data) {
	struct direct_session *session = data;

	if (session->base.active) {
		session->base.active = false;
		wl_signal_emit(&session->base.session_signal, session);
		send_message(session->sock, SESSION_DROPMASTER, NULL);
		ioctl(session->tty_fd, VT_RELDISP, 1);
	} else {
		ioctl(session->tty_fd, VT_RELDISP, VT_ACKACQ);
		send_message(session->sock, SESSION_SETMASTER, NULL);
		session->base.active = true;
		wl_signal_emit(&session->base.session_signal, session);
	}

	return 1;
}

static bool setup_tty(struct direct_session *session, struct wl_display *display) {
	session->tty_fd = dup(STDIN_FILENO);
	if (session->tty_fd == -1) {
		wlr_log_errno(L_ERROR, "Cannot open tty");
		return false;
	}

	struct stat st;
	if (fstat(session->tty_fd, &st) == -1 || major(st.st_rdev) != TTY_MAJOR ||
			minor(st.st_rdev) == 0) {
		wlr_log(L_ERROR, "Not running from a virtual terminal");
		goto error;
	}

	int tty = minor(st.st_rdev);
	int ret, kd_mode;
	session->base.vtnr = tty;

	ret = ioctl(session->tty_fd, KDGETMODE, &kd_mode);
	if (ret) {
		wlr_log_errno(L_ERROR, "Failed to get tty mode");
		goto error;
	}

	if (kd_mode != KD_TEXT) {
		wlr_log(L_ERROR,
			"tty already in graphics mode; is another display server running?");
		goto error;
	}

	ioctl(session->tty_fd, VT_ACTIVATE, tty);
	ioctl(session->tty_fd, VT_WAITACTIVE, tty);

	if (ioctl(session->tty_fd, KDGKBMODE, &session->old_kbmode)) {
		wlr_log_errno(L_ERROR, "Failed to read keyboard mode");
		goto error;
	}

	if (ioctl(session->tty_fd, KDSKBMODE, K_OFF)) {
		wlr_log_errno(L_ERROR, "Failed to set keyboard mode");
		goto error;
	}

	if (ioctl(session->tty_fd, KDSETMODE, KD_GRAPHICS)) {
		wlr_log_errno(L_ERROR, "Failed to set graphics mode on tty");
		goto error;
	}

	struct vt_mode mode = {
		.mode = VT_PROCESS,
		.relsig = SIGUSR1,
		.acqsig = SIGUSR1,
	};

	if (ioctl(session->tty_fd, VT_SETMODE, &mode) < 0) {
		wlr_log(L_ERROR, "Failed to take control of tty");
		goto error;
	}

	struct wl_event_loop *loop = wl_display_get_event_loop(display);
	session->vt_source = wl_event_loop_add_signal(loop, SIGUSR1,
		vt_handler, session);
	if (!session->vt_source) {
		goto error;
	}

	return true;

error:
	close(session->tty_fd);
	return false;
}

static void communicate(int sock) {
	struct session_message msg;
	struct msghdr hdr = {
		.msg_iov = &(struct iovec) {
			.iov_base = &msg,
			.iov_len = sizeof(msg),
		},
		.msg_iovlen = 1,
	};

	int drm_fd = -1;

	while (recvmsg(sock, &hdr, 0) >= 0 || errno == EINTR) {
		switch (msg.type) {
		case SESSION_OPEN:
			errno = 0;
			// These are the flags that logind use
			int fd = open(msg.path, O_RDWR | O_CLOEXEC | O_NOCTTY | O_NONBLOCK);
			int e = errno;

			struct stat st;
			if (fstat(fd, &st) >= 0 && major(st.st_rdev) == DRM_MAJOR) {
				if (drmSetMaster(fd)) {
					close(fd);
					fd = -1;
					e = errno;
				}

				drm_fd = fd;
			}

			char control[CMSG_SPACE(sizeof(fd))] = {0};
			struct msghdr reply = {
				.msg_iov = &(struct iovec) {
					.iov_base = &e,
					.iov_len = sizeof(e),
				},
				.msg_iovlen = 1,
				.msg_control = &control,
				.msg_controllen = sizeof(control),
			};
			struct cmsghdr *cmsg = CMSG_FIRSTHDR(&reply);
			cmsg->cmsg_level = SOL_SOCKET;
			cmsg->cmsg_type = SCM_RIGHTS;
			cmsg->cmsg_len = CMSG_LEN(sizeof(fd));
			memcpy(CMSG_DATA(cmsg), &fd, sizeof(fd));

			sendmsg(sock, &reply, 0);
			break;
		case SESSION_SETMASTER:
			if (drm_fd != -1) {
				drmSetMaster(drm_fd);
			}

			sendmsg(sock, &(struct msghdr){0}, 0);
			break;
		case SESSION_DROPMASTER:
			if (drm_fd != -1) {
				drmDropMaster(drm_fd);
			}

			sendmsg(sock, &(struct msghdr){0}, 0);
			break;
		case SESSION_END:
			sendmsg(sock, &(struct msghdr){0}, 0);
			return;
		}
	}
}

#ifdef HAS_LIBCAP
static bool have_permissions(void) {
	cap_t cap = cap_get_proc();
	cap_flag_value_t val;

	if (!cap || cap_get_flag(cap, CAP_SYS_ADMIN, CAP_PERMITTED, &val) || val != CAP_SET) {
		wlr_log(L_ERROR, "Do not have CAP_SYS_ADMIN; cannot become DRM master");
		cap_free(cap);
		return false;
	}

	cap_free(cap);
	return true;
}
#else
static bool have_permissions(void) {
	if (geteuid() != 0) {
		wlr_log(L_ERROR, "Do not have root privileges; cannot become DRM master");
		return false;
	}

	return true;
}
#endif

static struct wlr_session *direct_session_start(struct wl_display *disp) {
	if (!have_permissions()) {
		return NULL;
	}

	int sock[2];
	if (socketpair(AF_UNIX, SOCK_SEQPACKET, 0, sock) < 0) {
		wlr_log_errno(L_ERROR, "Failed to create socket pair");
		return NULL;
	}

	pid_t pid = fork();
	if (pid < 0) {
		wlr_log_errno(L_ERROR, "Fork failed");
		goto error_sock;
	} else if (pid == 0) {
		close(sock[0]);

		communicate(sock[1]);

		_Exit(0);
	}

	close(sock[1]);
	sock[1] = -1;

	struct direct_session *session = calloc(1, sizeof(*session));
	if (!session) {
		wlr_log_errno(L_ERROR, "Allocation failed");
		goto error_child;
	}

	session->child = pid;
	session->sock = sock[0];

	if (!setup_tty(session, disp)) {
		goto error_session;
	}

	// XXX: Is it okay to trust the environment like this?
	const char *seat = getenv("XDG_SEAT");
	if (!seat) {
		seat = "seat0";
	}

	wlr_log(L_INFO, "Successfully loaded direct session");

	snprintf(session->base.seat, sizeof(session->base.seat), "%s", seat);
	session->base.drm_fd = -1;
	session->base.impl = &session_direct;
	session->base.active = true;
	wl_signal_init(&session->base.session_signal);
	return &session->base;

error_session:
	free(session);
error_child:
	send_message(sock[0], SESSION_END, NULL);
	wait(NULL);
error_sock:
	close(sock[0]);
	close(sock[1]);
	return NULL;
}

const struct session_impl session_direct = {
	.start = direct_session_start,
	.finish = direct_session_finish,
	.open = direct_session_open,
	.close = direct_session_close,
	.change_vt = direct_change_vt,
};