From: Yifeng Li tomli@tomli.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@linaro.org Fixes: a4199f5eb809 ("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
--- drivers/tty/vt/vt.c | 2 -- 1 file changed, 2 deletions(-)
--- a/drivers/tty/vt/vt.c +++ b/drivers/tty/vt/vt.c @@ -4152,8 +4152,6 @@ void do_blank_screen(int entering_gfx) return; }
- if (blank_state != blank_normal_wait) - return; blank_state = blank_off;
/* don't blank graphics */