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readme: ttys are starting to make sense!
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Ciro Santilli
156
README.adoc
156
README.adoc
@@ -749,6 +749,36 @@ Double quotes can be used to escape spaces as in `opt="a b"`, but double quotes
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This even lead us to use base64 encoding with `-E`!
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==== Kernel command line parameters definition points
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There are two methods:
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* `__setup` as in:
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+
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....
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__setup("console=", console_setup);
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....
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* `core_param` as in:
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+
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....
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core_param(panic, panic_timeout, int, 0644);
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....
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`core_param` suggests how they are different:
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....
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/**
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* core_param - define a historical core kernel parameter.
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...
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* core_param is just like module_param(), but cannot be modular and
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* doesn't add a prefix (such as "printk."). This is for compatibility
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* with __setup(), and it makes sense as truly core parameters aren't
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* tied to the particular file they're in.
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*/
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....
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=== insmod alternatives
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==== modprobe
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@@ -3383,33 +3413,123 @@ The bitmask is documented at:
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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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==== TTY
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Switch between TTYs with:
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In order to play with TTYs, do this:
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....
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sendkey alt-left
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sendkey alt-right
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sendkey alt-f1
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sendkey alt-f2
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printf '
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tty2::respawn:/sbin/getty -n -L -l /loginroot.sh tty2 0 vt100
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tty3::respawn:-/bin/sh
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tty4::respawn:/sbin/getty 0 tty4
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tty63::respawn:-/bin/sh
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::respawn:/sbin/getty -L ttyS0 0 vt100
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::respawn:/sbin/getty -L ttyS1 0 vt100
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::respawn:/sbin/getty -L ttyS2 0 vt100
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# Leave one serial empty.
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#::respawn:/sbin/getty -L ttyS3 0 vt100
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' >> rootfs_overlay/etc/inittab
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./build
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./run -x -- \
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-serial telnet::1235,server,nowait \
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-serial telnet::1236,server,nowait \
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-serial telnet::1237,server,nowait \
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;
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....
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TODO: only works after I do a `chvt 1`, but then how to put a terminal on `alt-f2`? I just get a blank screen. One day, one day:
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and on a second shell:
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....
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telnet localhost 1235
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....
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We don't add more TTYs by default because it would spawn more processes, even if we use `askfirst` instead of `respawn`.
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On the GUI, switch TTYs with:
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* `Alt-Left` or `Alt-Right:` go to previous / next populated TTY. Skips over empty TTYs.
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* `Alt-Fn`: go to the nth TTY. If it is not populated, don't go there.
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* `chvt <n>`: go to the n-th virtual TTY, even if it is empty: https://superuser.com/questions/33065/console-commands-to-change-virtual-ttys-in-linux-and-openbsd
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You can also test this on most hosts such as Ubuntu 18.04, except that when in the GUI, you must use `Ctrl-Alt-Fx` to switch to another terminal.
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Then, go to the text shells we opened previously, and press enter, and you will see that we have:
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* one shell on the shell that was used to run QEMU
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* one shell on the second shell running `telnet`
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although we cannot change between terminals from there.
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Each populated TTY contains a "shell":
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* `-/bin/sh`: goes directly into an `sh` without a login prompt. Don't forget the dash `-`: https://askubuntu.com/questions/902998/how-to-check-which-tty-am-i-using
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+
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TODO: does not work for the `ttyS*` terminals. Why?
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* `/sbin/getty` asks for password, and then gives you an `sh`
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+
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We can overcome the password prompt with the `-l /loginroot.sh` technique explained at: https://askubuntu.com/questions/902998/how-to-check-which-tty-am-i-using but I don't see any advantage over `-/bin/sh` currently.
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Identify the current TTY with the command:
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....
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tty
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....
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* https://unix.stackexchange.com/questions/270272/how-to-get-the-tty-in-which-bash-is-running/270372
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* https://unix.stackexchange.com/questions/187319/how-to-get-the-real-name-of-the-controlling-terminal
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* https://unix.stackexchange.com/questions/77796/how-to-get-the-current-terminal-name
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* https://askubuntu.com/questions/902998/how-to-check-which-tty-am-i-using
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This outputs:
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* `/dev/console` for the initial GUI terminal. But I think it is the same as `/dev/tty1`, because if I try to do
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+
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....
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tty1::respawn:-/bin/sh
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....
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+
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it makes the terminal go crazy, as if multiple processes are randomly eating up the characters.
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* `/dev/ttyN` for the other graphic TTYs. Note that there are only 63 available ones, from `/dev/tty1` to `/dev/tty63` (`/dev/tty0` is the current one): link:https://superuser.com/questions/449781/why-is-there-so-many-linux-dev-tty[]. I think this is determined by:
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+
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....
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#define MAX_NR_CONSOLES 63
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....
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+
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in `linux/include/uapi/linux/vt.h`.
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* `/dev/ttySN` for the text shells.
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+
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These are Serial ports, see this to understand what those represent physically: https://unix.stackexchange.com/questions/307390/what-is-the-difference-between-ttys0-ttyusb0-and-ttyama0-in-linux/367882#367882
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+
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There are only 4 serial ports, I think this is determined by QEMU. TODO check.
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+
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See also: https://stackoverflow.com/questions/16706423/two-instances-of-busybox-on-separate-serial-lines-ttysn
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Next try:
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....
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insmod /kthread.ko
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....
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and switch between virtual terminals, to understand that the dmesg goes to whatever current virtual terminal you are on, but not the others, and not to the serial terminals.
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See also:
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* https://stackoverflow.com/questions/16706423/two-instances-of-busybox-on-separate-serial-lines-ttysn
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* https://serverfault.com/questions/119736/how-to-enable-multiple-virtual-consoles-on-linux
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* https://superuser.com/questions/33065/console-commands-to-change-virtual-ttys-in-linux-and-openbsd
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* https://stackoverflow.com/questions/39373236/redirect-multiple-uarts-in-qemu
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* https://github.com/mirror/busybox/blob/1_28_3/examples/inittab#L60
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* http://web.archive.org/web/20180117124612/http://nairobi-embedded.org/qemu_serial_port_system_console.html
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Also tried to add some extra lines to `/etc/inittab` of type:
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===== console kernel boot parameter
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....
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console::respawn:/sbin/getty -n -L -l /loginroot.sh ttyS1 0 vt100
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....
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Take the command described at <<tty>> and try adding the following:
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but nothing changed.
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Note that on Ubuntu 17.10, to get to the text terminal from the GUI we first need `Ctrl-Alt-Fx`, and once in the text terminals, `Alt-Fn` works without `Ctrl`.
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* `-e 'console=tty7'`: boot messages still show on `/dev/tty1` (TODO how to change that?), but we don't get a shell at the end of boot there.
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+
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Instead, the shell appears on `/dev/tty7`.
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* `-e 'console=tty2'` like `/dev/tty7`, but `/dev/tty2` is broken, because we have two shells there:
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** one due to the `::respawn:-/bin/sh` entry which uses whatever `console` points to
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** another one due to the `tty2::respawn:/sbin/getty` entry we added
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* `-e 'console=ttyS0'` much like `tty2`, but messages show only on serial, and the terminal is broken due to having multiple shells on it
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* `-e 'console=tty1 console=ttyS0'`: boot messages show on both `tty1` and `ttyS0`, but only `S0` gets a shell because it came last
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==== CONFIG_LOGO
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@@ -5262,7 +5382,7 @@ Then copy and paste the diff additions to link:br2[] to make them permanent.
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=== Change user
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At startup, we login automatically as the `root` user as explained at: https://unix.stackexchange.com/questions/299408/how-to-login-automatically-without-typing-root-in-buildroot-x86-64-qemu/300152#300152
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At startup, we login automatically as the `root` user.
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If you want to switch to another user to test some permissions, we have already created an `user0` user through the link:user_table[] file, and you can just login as that user with:
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