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ctrl-alt-del: investigate a bit further, clarify
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Ciro Santilli 六四事件 法轮功
140
README.adoc
140
README.adoc
@@ -8161,54 +8161,33 @@ sendkey shift-pgdown
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===== Ctrl Alt Del
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Run `/sbin/reboot` on guest:
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If you run in <<qemu-graphic-mode>>:
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....
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./run --graphic
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....
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and then from the graphic window you enter the keys:
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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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then this runs the following command on the guest:
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....
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/sbin/reboot
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....
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This is 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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Linux tries to reboot, and QEMU shutdowns due to the `-no-reboot` option which we set by default for, see: xref:exit-emulator-on-panic[xrefstyle=full].
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This leads Linux to try to reboot, and QEMU shutdowns due to the `-no-reboot` option which we set by default for, see: xref:exit-emulator-on-panic[xrefstyle=full].
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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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Here is a minimal example of Ctrl Alt Del:
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....
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./run --kernel-cli 'init=/lkmc/linux/ctrl_alt_del.out' --graphic
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@@ -8249,6 +8228,93 @@ 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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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` system calls can set either of the these behaviours for `Ctrl-Alt-Del`:
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* do a hard shutdown syscall. Set in uClibc 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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+
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Done by BusyBox' `reboot -f`.
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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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+
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Done by BusyBox' `reboot`.
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When a BusyBox init is with the signal, it prints the following lines:
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....
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The system is going down NOW!
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Sent SIGTERM to all processes
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Sent SIGKILL to all processes
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Requesting system reboot
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....
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On busybox-1.29.2's init at init/init.c we see how the kill signals are sent:
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....
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static void run_shutdown_and_kill_processes(void)
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{
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/* Run everything to be run at "shutdown". This is done _prior_
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* to killing everything, in case people wish to use scripts to
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* shut things down gracefully... */
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run_actions(SHUTDOWN);
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message(L_CONSOLE | L_LOG, "The system is going down NOW!");
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/* Send signals to every process _except_ pid 1 */
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kill(-1, SIGTERM);
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message(L_CONSOLE, "Sent SIG%s to all processes", "TERM");
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sync();
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sleep(1);
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kill(-1, SIGKILL);
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message(L_CONSOLE, "Sent SIG%s to all processes", "KILL");
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sync();
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/*sleep(1); - callers take care about making a pause */
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}
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....
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and `run_shutdown_and_kill_processes` is called from:
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....
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/* The SIGPWR/SIGUSR[12]/SIGTERM handler */
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static void halt_reboot_pwoff(int sig) NORETURN;
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static void halt_reboot_pwoff(int sig)
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....
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which also prints the final line:
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....
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message(L_CONSOLE, "Requesting system %s", m);
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....
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which is set as the signal handler via TODO.
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Bibliography:
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* https://superuser.com/questions/193652/does-linux-have-a-ctrlaltdel-equivalent/1324415#1324415
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