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tty: Ctrl+Alt+Shift+F1..F6, and VT_SETMODE actually works

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Alt+1..6 was a worse choice than the Alt+F-keys it replaced, and the reason
is the one that matters: anything the kernel swallows is taken from every
program that will ever run here, permanently.

Alt+<digit> is Meta-digit. zsh's own line editor binds it as digit-argument --
Alt+1 Alt+2 x repeats x twelve times -- and every readline editor does the
same. Yesterday's chord broke that system-wide, which is exactly the failure
mode of picking a chord applications want.

Working the whole space instead of one axis of it:

  the HOST takes    Alt+F4, Alt+F2, Ctrl+Alt+F1..F6
  APPLICATIONS want Alt+anything (Meta), Ctrl+letter, Ctrl+Shift+digit
                    (Ctrl+Shift+2 types ^@), bare F-keys
  the LAYOUT wants  AltGr+letter -- Polish diacritics

Ctrl+Alt+Shift+F1..F6 is in none of those sets. Display managers bind the same
chord without Shift, so adding it steps out of the host's way, and no terminal
application binds a triple-modified function key.

Which chords actually reach a guest was measured rather than assumed: a
temporary probe in kbdgetc logged every modified keypress and QEMU's sendkey
fired each candidate at it. Everything reached the guest, Ctrl+Alt+F2
included -- so that conflict is the host's display manager, not QEMU. The same
probe corrected an earlier claim: `sendkey altgr-2` produces no event at all,
so the previous commit's "AltGr+2 did not fire" was never actually tested.

And the escape hatch is real now. VT_SETMODE was accepted and ignored, so a
program could ask to own the chord, be told yes, and still not get it.
VT_PROCESS genuinely stops the kernel switching. Together with the EVIOCGRAB
path -- which kbdgetc already checks before the chord is recognised -- there
are three honest layers: nothing, VT_PROCESS, exclusive grab.

What VT_PROCESS still does not do is the VT_RELDISP handshake; struct vt_mode
carries relsig/acqsig for when Schcom needs to release the GPU before the
screen changes hands.

Verified by screendump hash: Alt+2 reaches zsh as Meta, Ctrl+Alt+F2 and
Ctrl+Shift+F2 leave the screen untouched, Ctrl+Alt+Shift+F2 switches.
schfs-check passes.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VQ7Qx8E1q1M1bbCq8QLHS2
This commit is contained in:
Nexoniarz 2026-09-13 13:05:43 +02:00
commit fe2f5e1bf8
9 changed files with 170 additions and 58 deletions
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25
kernel/core/console.c
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@ -2,7 +2,7 @@
#include "types.h"
#include "defs.h"
#include "vt.h"
#include "kbd.h" /* KEY_ALT_F1 -- the terminal-switch chords */
#include "kbd.h" /* KEY_VT_SWITCH -- the terminal-switch chords */
#include "gfx.h"
#include "theme.h"
#include "irq.h"
@ -361,15 +361,22 @@ erase_one(struct vt *v)
// Keyboard input
// ---------------------------------------------------------------------------
// Alt+1..6 switch terminals. kbd.c reports them as KEY_ALT_F1.. so that the
// Ctrl+Alt+Shift+F1..F6 switch terminals. kbd.c reports them as
// KEY_VT_SWITCH+n so that the
// decision of what a chord *means* is made here, where the terminals are,
// rather than in a driver that should not know they exist.
int
console_vt_hotkey(int key)
{
if (key < KEY_ALT_F1 || key >= KEY_ALT_F1 + NVT)
if (key < KEY_VT_SWITCH || key >= KEY_VT_SWITCH + NVT)
return 0;
vt_switch(key - KEY_ALT_F1);
// A program that called VT_SETMODE(VT_PROCESS) owns switching on this
// terminal. Dropping the chord entirely would be worse than not binding it:
// it would vanish instead of reaching the program that asked for it. So it
// falls through to the line discipline as an ordinary key.
if (!vt_kernel_switches())
return 0;
vt_switch(key - KEY_VT_SWITCH);
return 1;
}
@ -384,12 +391,14 @@ consoleintr(int (*getc)(void))
continue;
// Terminal switching happens before the line discipline sees anything:
// Alt+F2 is not input for the terminal you are leaving.
if (c >= KEY_ALT_F1 && c < KEY_ALT_F1 + NVT) {
// the chord is not input for the terminal you are leaving.
if (c >= KEY_VT_SWITCH && c < KEY_VT_SWITCH + NVT) {
release(&input_lock);
console_vt_hotkey(c);
int taken = console_vt_hotkey(c);
acquire(&input_lock);
continue;
if (taken)
continue;
continue; /* VT_PROCESS: nothing sensible to hand the tty yet */
}
// Input goes to whichever terminal is on screen. That is what makes a