linux/drivers/tty
Yifeng Li 75ddbc1fb1 tty: vt.c: Fix TIOCL_BLANKSCREEN console blanking if blankinterval == 0
Previously, in the userspace, it was possible to use the "setterm" command
from util-linux to blank the VT console by default, using the following
command.

According to the man page,

> The force option keeps the screen blank even if a key is pressed.

It was implemented by calling TIOCL_BLANKSCREEN.

	case BLANKSCREEN:
		ioctlarg = TIOCL_BLANKSCREEN;
		if (ioctl(STDIN_FILENO, TIOCLINUX, &ioctlarg))
			warn(_("cannot force blank"));
		break;

However, after Linux 4.12, this command ceased to work anymore, which is
unexpected. By inspecting the kernel source, it shows that the issue was
triggered by the side-effect from commit a4199f5eb8 ("tty: Disable
default console blanking interval").

The console blanking is implemented by function do_blank_screen() in vt.c:
"blank_state" will be initialized to "blank_normal_wait" in con_init() if
AND ONLY IF ("blankinterval" > 0). If "blankinterval" is 0, "blank_state"
will be "blank_off" (== 0), and a call to do_blank_screen() will always
abort, even if a forced blanking is required from the user by calling
TIOCL_BLANKSCREEN, the console won't be blanked.

This behavior is unexpected from a user's point-of-view, since it's not
mentioned in any documentation. The setterm man page suggests it will
always work, and the kernel comments in uapi/linux/tiocl.h says

> /* keep screen blank even if a key is pressed */
> #define TIOCL_BLANKSCREEN 14

To fix it, we simply remove the "blank_state != blank_off" check, as
pointed out by Nicolas Pitre, this check doesn't logically make sense
and it's safe to remove.

Suggested-by: Nicolas Pitre <nicolas.pitre@linaro.org>
Fixes: a4199f5eb8 ("tty: Disable default console blanking interval")
Signed-off-by: Yifeng Li <tomli@tomli.me>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
2019-03-28 01:28:23 +09:00
..
hvc tty: hvc_xen: Mark expected switch fall-through 2019-02-26 12:30:44 +01:00
ipwireless tty: ipwireless: fix missing checks for ioremap 2019-03-28 01:28:23 +09:00
serdev serdev: ttyport: call tiocmget and tiocmset ops directly 2019-01-30 11:48:08 +01:00
serial serial: sc16is7xx: Drop of_match_ptr() use 2019-03-28 00:30:51 +09:00
vt tty: vt.c: Fix TIOCL_BLANKSCREEN console blanking if blankinterval == 0 2019-03-28 01:28:23 +09:00
amiserial.c
cyclades.c
ehv_bytechan.c
goldfish.c
isicom.c
Kconfig tty: ldisc: add sysctl to prevent autoloading of ldiscs 2019-01-30 09:31:30 +01:00
Makefile
mips_ejtag_fdc.c
moxa.c
moxa.h
mxser.c
mxser.h
n_gsm.c tty: n_gsm: Mark expected switch fall-throughs 2019-02-26 12:30:44 +01:00
n_hdlc.c tty/n_hdlc: mark expected switch fall-through 2019-02-14 10:49:19 +01:00
n_null.c
n_r3964.c tty: Don't block on IO when ldisc change is pending 2018-12-05 12:16:33 +01:00
n_tracerouter.c
n_tracesink.c
n_tracesink.h
n_tty.c n_tty: update comment for WAKEUP_CHARS define 2019-01-18 13:25:11 +01:00
nozomi.c tty/nozomi: use pci_iomap instead of ioremap_nocache 2019-02-12 10:26:45 +01:00
pty.c
rocket_int.h
rocket.c
rocket.h
synclink_gt.c
synclink.c tty/synclink: remove ISA support 2019-02-12 08:51:33 +01:00
synclinkmp.c
sysrq.c tty/sysrq: Convert show_lock to raw_spinlock_t 2019-03-28 01:24:55 +09:00
tty_audit.c audit: join tty records to their syscall 2019-02-07 21:33:54 -05:00
tty_baudrate.c termios, tty/tty_baudrate.c: fix buffer overrun 2018-11-08 03:36:45 -08:00
tty_buffer.c tty: increase the default flip buffer limit to 2*640K 2019-01-30 12:03:58 +01:00
tty_io.c tty: ldisc: add sysctl to prevent autoloading of ldiscs 2019-01-30 09:31:30 +01:00
tty_ioctl.c
tty_jobctrl.c tty: fix read of tty->pgrp outside of ctrl_lock 2019-03-28 01:28:23 +09:00
tty_ldisc.c tty: ldisc: add sysctl to prevent autoloading of ldiscs 2019-01-30 09:31:30 +01:00
tty_ldsem.c tty/ldsem: Decrement wait_readers on timeouted down_read() 2018-12-05 12:16:33 +01:00
tty_mutex.c
tty_port.c tty: do not set TTY_IO_ERROR flag if console port 2018-11-27 09:58:11 +01:00
vcc.c