releases/4.19.45/tty-vt.c-fix-tiocl_blankscreen-console-blanking-if-blankinterval-0.patch

   1 From 75ddbc1fb11efac87b611d48e9802f6fe2bb2163 Mon Sep 17 00:00:00 2001
   2 From: Yifeng Li <tomli@tomli.me>
   3 Date: Tue, 5 Mar 2019 07:02:49 +0800
   4 Subject: tty: vt.c: Fix TIOCL_BLANKSCREEN console blanking if blankinterval == 0
   5
   6 From: Yifeng Li <tomli@tomli.me>
   7
   8 commit 75ddbc1fb11efac87b611d48e9802f6fe2bb2163 upstream.
   9
  10 Previously, in the userspace, it was possible to use the "setterm" command
  11 from util-linux to blank the VT console by default, using the following
  12 command.
  13
  14 According to the man page,
  15
  16 > The force option keeps the screen blank even if a key is pressed.
  17
  18 It was implemented by calling TIOCL_BLANKSCREEN.
  19
  20         case BLANKSCREEN:
  21                 ioctlarg = TIOCL_BLANKSCREEN;
  22                 if (ioctl(STDIN_FILENO, TIOCLINUX, &ioctlarg))
  23                         warn(_("cannot force blank"));
  24                 break;
  25
  26 However, after Linux 4.12, this command ceased to work anymore, which is
  27 unexpected. By inspecting the kernel source, it shows that the issue was
  28 triggered by the side-effect from commit a4199f5eb809 ("tty: Disable
  29 default console blanking interval").
  30
  31 The console blanking is implemented by function do_blank_screen() in vt.c:
  32 "blank_state" will be initialized to "blank_normal_wait" in con_init() if
  33 AND ONLY IF ("blankinterval" > 0). If "blankinterval" is 0, "blank_state"
  34 will be "blank_off" (== 0), and a call to do_blank_screen() will always
  35 abort, even if a forced blanking is required from the user by calling
  36 TIOCL_BLANKSCREEN, the console won't be blanked.
  37
  38 This behavior is unexpected from a user's point-of-view, since it's not
  39 mentioned in any documentation. The setterm man page suggests it will
  40 always work, and the kernel comments in uapi/linux/tiocl.h says
  41
  42 > /* keep screen blank even if a key is pressed */
  43 > #define TIOCL_BLANKSCREEN 14
  44
  45 To fix it, we simply remove the "blank_state != blank_off" check, as
  46 pointed out by Nicolas Pitre, this check doesn't logically make sense
  47 and it's safe to remove.
  48
  49 Suggested-by: Nicolas Pitre <nicolas.pitre@linaro.org>
  50 Fixes: a4199f5eb809 ("tty: Disable default console blanking interval")
  51 Signed-off-by: Yifeng Li <tomli@tomli.me>
  52 Cc: stable <stable@vger.kernel.org>
  53 Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
  54
  55 ---
  56  drivers/tty/vt/vt.c |    2 --
  57  1 file changed, 2 deletions(-)
  58
  59 --- a/drivers/tty/vt/vt.c
  60 +++ b/drivers/tty/vt/vt.c
  61 @@ -4155,8 +4155,6 @@ void do_blank_screen(int entering_gfx)
  62                 return;
  63         }
  64
  65 -       if (blank_state != blank_normal_wait)
  66 -               return;
  67         blank_state = blank_off;
  68
  69         /* don't blank graphics */