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readme: create Ctrl-Alt-Del and improve SysRq
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‎README.adoc

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@@ -3242,24 +3242,144 @@ sendkey shift-pgup
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sendkey shift-pgdown
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....
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https://en.wikipedia.org/wiki/Magic_SysRq_key Those can be tested through the monitor with:
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===== Ctrl Alt Del
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Reboot guest:
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....
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Ctrl-Alt-Del
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....
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Enabled from our link:rootfs_overlay/etc/inittab[]:
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....
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::ctrlaltdel:/sbin/reboot
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....
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Under the hood, behaviour is controlled by the `reboot` syscall:
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....
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man 2 reboot
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....
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`reboot` calls can set either of the these behaviours for `Ctrl-Alt-Del`:
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* do a hard shutdown syscall. Set in ublibc C code with:
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+
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....
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reboot(RB_ENABLE_CAD)
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....
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+
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or from procfs with:
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+
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....
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echo 1 > /proc/sys/kernel/ctrl-alt-del
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....
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* send a SIGINT to the init process. This is what BusyBox' init does, and it then execs the string set in `inittab`.
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+
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Set in uclibc C code with:
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+
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....
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reboot(RB_DISABLE_CAD)
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....
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+
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or from procfs with:
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+
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....
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echo 0 > /proc/sys/kernel/ctrl-alt-del
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....
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Minimal example:
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....
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./run -e 'init=/ctrl_alt_del.out' -x
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....
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When you hit `Ctrl-Alt-Del` in the guest, our tiny init handles a `SIGINT` sent by the kernel and outputs to stdout:
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....
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cad
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....
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To map between `man 2 reboot` and the uclibc `RB_*` magic constants see:
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....
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less out/x86_64/buildroot/build/uclibc-*/include/sys/reboot.h
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....
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The procfs mechanism is documented at:
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....
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less linux/Documentation/sysctl/kernel.txt
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....
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which says:
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....
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When the value in this file is 0, ctrl-alt-del is trapped and
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sent to the init(1) program to handle a graceful restart.
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When, however, the value is > 0, Linux's reaction to a Vulcan
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Nerve Pinch (tm) will be an immediate reboot, without even
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syncing its dirty buffers.
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Note: when a program (like dosemu) has the keyboard in 'raw'
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mode, the ctrl-alt-del is intercepted by the program before it
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ever reaches the kernel tty layer, and it's up to the program
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to decide what to do with it.
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....
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===== SysRq
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https://en.wikipedia.org/wiki/Magic_SysRq_key
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We cannot test these actual shortcuts on QEMU since the host captures them at a lower level, but from:
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....
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./qemumonitor
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....
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we can for example crash the system with:
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....
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sendkey alt-sysrq-c
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....
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or you can try the much more boring method of:
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Same but boring because no magic key:
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....
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echo c > /proc/sysrq-trigger
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....
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Implemented in
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Implemented in:
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....
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drivers/tty/sysrq.c
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....
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On your host, on modern systems that don't have the `SysRq` key you can do:
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....
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Alt-PrtSc-space
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....
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which prints a message to `dmesg` of type:
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....
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sysrq: SysRq : HELP : loglevel(0-9) reboot(b) crash(c) terminate-all-tasks(e) memory-full-oom-kill(f) kill-all-tasks(i) thaw-filesystems(j) sak(k) show-backtrace-all-active-cpus(l) show-memory-usage(m) nice-all-RT-tasks(n) poweroff(o) show-registers(p) show-all-timers(q) unraw(r) sync(s) show-task-states(t) unmount(u) show-blocked-tasks(w) dump-ftrace-buffer(z)
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....
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Individual SysRq can be enabled or disabled with the bitmask:
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....
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/proc/sys/kernel/sysrq
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....
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The bitmask is documented at:
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....
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less linux/Documentation/admin-guide/sysrq.rst
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....
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===== Multiple TTYs
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Switch between TTYs with:

‎kernel_module/user/ctrl_alt_del.c

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#define _XOPEN_SOURCE 700
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#include <signal.h>
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#include <stdio.h>
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#include <stdlib.h>
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#include <sys/reboot.h>
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#include <unistd.h>
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void signal_handler(int sig) {
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write(STDOUT_FILENO, "cad\n", 4);
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signal(sig, signal_handler);
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}
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int main(void) {
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int i = 0;
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/* Disable the forced reboot, enable sending SIGINT to init. */
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reboot(RB_DISABLE_CAD);
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signal(SIGINT, signal_handler);
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while (1) {
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sleep(1);
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printf("%d\n", i);
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i++;
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}
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return EXIT_SUCCESS;
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}

‎kernel_module/user/poweroff.c

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@@ -2,6 +2,7 @@
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* https://stackoverflow.com/questions/28812514/how-to-shutdown-linux-using-c-or-qt-without-call-to-system
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**/
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#define _XOPEN_SOURCE 700
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#include <stdio.h>
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#include <sys/reboot.h>
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#include <unistd.h>

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