[PATCH 4.14 38/63] tty: vt.c: Fix TIOCL_BLANKSCREEN console blanking if blankinterval == 0

From: Greg Kroah-Hartman
Date: Mon May 20 2019 - 08:22:05 EST

From: Yifeng Li <tomli@xxxxxxxx>

commit 75ddbc1fb11efac87b611d48e9802f6fe2bb2163 upstream.

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

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.

if (ioctl(STDIN_FILENO, TIOCLINUX, &ioctlarg))
warn(_("cannot force blank"));

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 a4199f5eb809 ("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 */

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@xxxxxxxxxx>
Fixes: a4199f5eb809 ("tty: Disable default console blanking interval")
Signed-off-by: Yifeng Li <tomli@xxxxxxxx>
Cc: stable <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

drivers/tty/vt/vt.c | 2 --
1 file changed, 2 deletions(-)

--- a/drivers/tty/vt/vt.c
+++ b/drivers/tty/vt/vt.c
@@ -3840,8 +3840,6 @@ void do_blank_screen(int entering_gfx)

- if (blank_state != blank_normal_wait)
- return;
blank_state = blank_off;

/* don't blank graphics */