linux-kernel - [PATCH 4.19 063/105] tty: vt.c: Fix TIOCL_BLANKSCREEN console blanking if blankinterval == 0

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Message-Id: <20190520115251.512795386@linuxfoundation.org>
Date:   Mon, 20 May 2019 14:14:09 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Nicolas Pitre <nicolas.pitre@...aro.org>,
        Yifeng Li <tomli@...li.me>
Subject: [PATCH 4.19 063/105] tty: vt.c: Fix TIOCL_BLANKSCREEN console blanking if blankinterval == 0

From: Yifeng Li <tomli@...li.me>

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
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 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 */
> #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@...aro.org>
Fixes: a4199f5eb809 ("tty: Disable default console blanking interval")
Signed-off-by: Yifeng Li <tomli@...li.me>
Cc: stable <stable@...r.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

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

--- a/drivers/tty/vt/vt.c
+++ b/drivers/tty/vt/vt.c
@@ -4155,8 +4155,6 @@ void do_blank_screen(int entering_gfx)
 		return;
 	}

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

 	/* don't blank graphics */