input, fb, vt, sysfs: the kernel calls Xorg makes
Xorg on a framebuffer -- the fbdev video driver, the evdev input driver, libpciaccess -- asks the kernel a fixed set of questions before it draws anything, and gives up on the first wrong answer. These are those answers. evdev: EVIOCGVERSION, GID, GNAME, GPHYS, GUNIQ, GPROP, GBIT, GKEY, GABS, GREP and the accepted setters. Xorg and libinput decide what a device *is* from the capability bitmaps; /dev/input/eventN answered only EVIOCGRAB, which reads as no device at all. Each device's bitmaps are its defaults plus whatever it has reported, so a USB tablet makes the pointer absolute. fbdev: FBIOPUT_VSCREENINFO succeeds for the mode already set, as on Linux -- the fbdev driver sets it every time it starts -- and still refuses any other. FBIOPAN_DISPLAY to the origin; FBIOBLANK. The console: KDSETMODE(KD_GRAPHICS) stops it drawing on a terminal, KDSKBMODE(K_OFF) stops keys reaching its tty, and VT_PROCESS now does the switch handshake: leaving the terminal sends relsig and waits for VT_RELDISP, arriving sends acqsig. A terminal whose owner has died goes back to the kernel whole, so a crashed X cannot leave the console dead. /sys: boot_vga, a writable enable, and graphics/fb0 under the display device, which is how the fbdev driver pairs a card with /dev/fb0. ports/linux-headers: <linux/*.h> into musl's prefix, from sabotage- linux's kernel-headers 4.19.88-2. Its checksum is recorded from the first download; upstream publishes none. cmd/xready makes each call and checks the answer, including the switch handshake on tty2 and an owner that dies holding it. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01S1giNvrHTrPMPw7V6x1kQ4
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Nexoniarz
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14 changed files with 860 additions and 32 deletions
233
cmd/xready/main.c
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233
cmd/xready/main.c
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// xready: the kernel calls Xorg makes before it draws anything, checked one
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// by one -- the evdev queries its input driver decides what a device is from,
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// the framebuffer calls its fbdev driver sets the mode with, and the console
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// calls it takes a terminal with, including the switch handshake.
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//
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// And the part of /sys libpciaccess reads to find the card to use.
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//
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// Uses tty2 for the terminal part, and puts everything back before it exits.
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// A musl program, linked dynamically, like evtest.
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#include <dirent.h>
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#include <errno.h>
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#include <fcntl.h>
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#include <linux/fb.h>
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#include <linux/input.h>
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#include <linux/kd.h>
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#include <linux/vt.h>
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#include <signal.h>
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#include <stdio.h>
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#include <string.h>
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#include <sys/ioctl.h>
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#include <sys/wait.h>
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#include <time.h>
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#include <unistd.h>
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static int bad;
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#define CHECK(cond, ...) do { \
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if (!(cond)) { printf("xready: FAIL " __VA_ARGS__); printf("\n"); bad++; } \
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} while (0)
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#define HAS(bits, n) ((bits)[(n) / 8] & (1 << ((n) % 8)))
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static void
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evdev(void)
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{
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unsigned char ev[EV_MAX / 8 + 1], key[KEY_MAX / 8 + 1], rel[REL_MAX / 8 + 1];
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char name[64];
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struct input_id id;
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int v, fd;
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fd = open("/dev/input/event0", O_RDONLY | O_NONBLOCK);
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CHECK(fd >= 0, "open event0: %s", strerror(errno));
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if (fd < 0)
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return;
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CHECK(ioctl(fd, EVIOCGVERSION, &v) == 0 && v == EV_VERSION, "EVIOCGVERSION");
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CHECK(ioctl(fd, EVIOCGID, &id) == 0 && id.bustype == BUS_I8042, "EVIOCGID");
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CHECK(ioctl(fd, EVIOCGNAME(sizeof name), name) > 0 && strstr(name, "keyboard"),
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"EVIOCGNAME");
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memset(ev, 0, sizeof ev);
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CHECK(ioctl(fd, EVIOCGBIT(0, sizeof ev), ev) > 0 && HAS(ev, EV_KEY) &&
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!HAS(ev, EV_REL), "keyboard EVIOCGBIT(0)");
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memset(key, 0, sizeof key);
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CHECK(ioctl(fd, EVIOCGBIT(EV_KEY, sizeof key), key) > 0 && HAS(key, KEY_A) &&
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HAS(key, KEY_ENTER) && !HAS(key, BTN_LEFT), "keyboard EVIOCGBIT(EV_KEY)");
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CHECK(ioctl(fd, EVIOCGKEY(sizeof key), key) > 0, "EVIOCGKEY");
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CHECK(ioctl(fd, EVIOCSCLOCKID, &(int){CLOCK_MONOTONIC}) == 0, "EVIOCSCLOCKID");
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close(fd);
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fd = open("/dev/input/event1", O_RDONLY | O_NONBLOCK);
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CHECK(fd >= 0, "open event1: %s", strerror(errno));
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if (fd < 0)
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return;
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memset(ev, 0, sizeof ev);
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CHECK(ioctl(fd, EVIOCGBIT(0, sizeof ev), ev) > 0 && HAS(ev, EV_REL),
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"pointer EVIOCGBIT(0)");
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memset(key, 0, sizeof key);
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CHECK(ioctl(fd, EVIOCGBIT(EV_KEY, sizeof key), key) > 0 && HAS(key, BTN_LEFT) &&
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!HAS(key, KEY_A), "pointer EVIOCGBIT(EV_KEY)");
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memset(rel, 0, sizeof rel);
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CHECK(ioctl(fd, EVIOCGBIT(EV_REL, sizeof rel), rel) > 0 && HAS(rel, REL_X) &&
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HAS(rel, REL_WHEEL), "pointer EVIOCGBIT(EV_REL)");
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unsigned char prop[4] = {0};
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CHECK(ioctl(fd, EVIOCGPROP(sizeof prop), prop) >= 0 && HAS(prop, INPUT_PROP_POINTER),
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"EVIOCGPROP");
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struct input_absinfo abs;
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CHECK(ioctl(fd, EVIOCGABS(ABS_X), &abs) == 0 && abs.maximum > 0, "EVIOCGABS");
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close(fd);
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}
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static void
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fbdev(void)
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{
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struct fb_var_screeninfo var, want;
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int fd = open("/dev/fb0", O_RDWR);
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CHECK(fd >= 0, "open fb0: %s", strerror(errno));
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if (fd < 0)
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return;
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CHECK(ioctl(fd, FBIOGET_VSCREENINFO, &var) == 0, "FBIOGET_VSCREENINFO");
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want = var;
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want.activate = FB_ACTIVATE_TEST;
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CHECK(ioctl(fd, FBIOPUT_VSCREENINFO, &want) == 0 && want.xres == var.xres,
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"FBIOPUT_VSCREENINFO, the current mode");
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want = var;
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want.xres++;
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CHECK(ioctl(fd, FBIOPUT_VSCREENINFO, &want) < 0 && errno == EINVAL,
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"FBIOPUT_VSCREENINFO, another mode");
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want = var;
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CHECK(ioctl(fd, FBIOPAN_DISPLAY, &want) == 0, "FBIOPAN_DISPLAY");
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CHECK(ioctl(fd, FBIOBLANK, FB_BLANK_UNBLANK) == 0, "FBIOBLANK");
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close(fd);
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}
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static void
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pci(void)
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{
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DIR *d = opendir("/sys/bus/pci/devices");
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struct dirent *e;
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char path[300], buf[8];
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int vgas = 0;
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CHECK(d != 0, "opendir /sys/bus/pci/devices");
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if (!d)
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return;
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while ((e = readdir(d)) != 0) {
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if (e->d_name[0] == '.')
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continue;
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snprintf(path, sizeof path, "/sys/bus/pci/devices/%s/boot_vga", e->d_name);
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int fd = open(path, O_RDONLY);
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CHECK(fd >= 0, "open %s", path);
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if (fd < 0)
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continue;
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memset(buf, 0, sizeof buf);
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read(fd, buf, sizeof buf - 1);
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close(fd);
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if (buf[0] != '1')
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continue;
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vgas++;
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snprintf(path, sizeof path, "/sys/bus/pci/devices/%s/graphics/fb0", e->d_name);
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CHECK(access(path, F_OK) == 0, "%s", path);
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snprintf(path, sizeof path, "/sys/bus/pci/devices/%s/enable", e->d_name);
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fd = open(path, O_RDWR);
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CHECK(fd >= 0 && write(fd, "1", 1) == 1, "write %s", path);
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if (fd >= 0)
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close(fd);
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}
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closedir(d);
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CHECK(vgas == 1, "one boot_vga device (found %d)", vgas);
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}
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static volatile sig_atomic_t rel, acq;
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static void on_rel(int s) { (void)s; rel++; }
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static void on_acq(int s) { (void)s; acq++; }
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static int
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active(int fd)
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{
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struct vt_stat st;
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return ioctl(fd, VT_GETSTATE, &st) == 0 ? st.v_active : -1;
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}
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static void
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settle(void)
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{
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usleep(50000); // let a signal be delivered
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}
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static void
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console(void)
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{
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int fd = open("/dev/tty2", O_RDWR), mode;
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CHECK(fd >= 0, "open tty2: %s", strerror(errno));
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if (fd < 0)
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return;
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int was = active(fd);
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CHECK(was > 0, "VT_GETSTATE");
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char kbtype = 0;
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CHECK(ioctl(fd, KDGKBTYPE, &kbtype) == 0 && kbtype == KB_101, "KDGKBTYPE");
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signal(SIGUSR1, on_rel);
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signal(SIGUSR2, on_acq);
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struct vt_mode vm = { .mode = VT_PROCESS, .relsig = SIGUSR1, .acqsig = SIGUSR2 };
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CHECK(ioctl(fd, VT_SETMODE, &vm) == 0, "VT_SETMODE");
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CHECK(ioctl(fd, KDSETMODE, KD_GRAPHICS) == 0, "KDSETMODE");
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CHECK(ioctl(fd, KDGETMODE, &mode) == 0 && mode == KD_GRAPHICS, "KDGETMODE");
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CHECK(ioctl(fd, KDSKBMODE, K_OFF) == 0, "KDSKBMODE");
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CHECK(ioctl(fd, KDGKBMODE, &mode) == 0 && mode == K_OFF, "KDGKBMODE");
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// On to tty2: it happens, and we are told to redraw.
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CHECK(ioctl(fd, VT_ACTIVATE, 2) == 0 && active(fd) == 2, "VT_ACTIVATE 2");
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settle();
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CHECK(acq == 1, "acqsig on arrival (got %d)", (int)acq);
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// Away from it: we are asked first, and nothing moves until we answer.
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CHECK(ioctl(fd, VT_ACTIVATE, was) == 0, "VT_ACTIVATE back");
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settle();
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CHECK(rel == 1, "relsig on leaving (got %d)", (int)rel);
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CHECK(active(fd) == 2, "switch waits for VT_RELDISP (active %d)", active(fd));
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CHECK(ioctl(fd, VT_RELDISP, 0) == 0 && active(fd) == 2, "VT_RELDISP 0 refuses");
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CHECK(ioctl(fd, VT_ACTIVATE, was) == 0, "VT_ACTIVATE again");
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settle();
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CHECK(rel == 2 && ioctl(fd, VT_RELDISP, 1) == 0 && active(fd) == was,
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"VT_RELDISP 1 lets it go (active %d)", active(fd));
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vm.mode = VT_AUTO;
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ioctl(fd, KDSKBMODE, K_UNICODE);
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ioctl(fd, KDSETMODE, KD_TEXT);
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ioctl(fd, VT_SETMODE, &vm);
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// A display server that dies holding the terminal must not take the
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// console with it.
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pid_t pid = fork();
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if (pid == 0) {
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struct vt_mode cm = { .mode = VT_PROCESS, .relsig = SIGUSR1, .acqsig = SIGUSR2 };
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signal(SIGUSR2, SIG_IGN);
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ioctl(fd, VT_SETMODE, &cm);
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ioctl(fd, KDSETMODE, KD_GRAPHICS);
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ioctl(fd, KDSKBMODE, K_OFF);
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ioctl(fd, VT_ACTIVATE, 2);
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_exit(0);
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}
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waitpid(pid, 0, 0);
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CHECK(active(fd) == 2, "the child took tty2");
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CHECK(ioctl(fd, VT_ACTIVATE, was) == 0 && active(fd) == was,
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"switch away from a dead owner happens at once (active %d)", active(fd));
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CHECK(ioctl(fd, KDGETMODE, &mode) == 0 && mode == KD_TEXT,
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"a dead owner's terminal is text again");
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close(fd);
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}
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int
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main(void)
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{
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evdev();
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fbdev();
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pci();
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console();
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printf(bad ? "xready: %d failed\n" : "xready: ok\n", bad);
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return bad != 0;
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}
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