On Thu, Jan 19, 2017 at 05:12:15PM +0100, Manuel Schölling wrote:

On Thu, 2017-01-19 at 14:23 +0100, Greg KH wrote:

quoted

On Fri, Jan 13, 2017 at 09:07:57PM +0100, Manuel Schölling wrote:

quoted

Add a scrollback buffers for each VGA console. The benefit is that
the scrollback history is not flushed when switching between
consoles
but is persistent.
The buffers are allocated on demand when a new console is opened.

This breaks tools like clear_console that rely on flushing the
scrollback history by switching back and forth between consoles
which is why this feature is disabled by default.
Use the escape sequence \e[3J instead for flushing the buffer.

Signed-off-by: Manuel Schölling <redacted>
Reviewed-by: Andrey Utkin <redacted>
Tested-by: Andrey Utkin <redacted>
Tested-by: Adam Borowski <redacted>
---
 drivers/video/console/Kconfig  |  25 +++++++-
 drivers/video/console/vgacon.c | 142 ++++++++++++++++++++++++++---
------------
 2 files changed, 111 insertions(+), 56 deletions(-)

diff --git a/drivers/video/console/Kconfig

b/drivers/video/console/Kconfig
index c3f1fb9ee820..f500e58f7636 100644

--- a/drivers/video/console/Kconfig
+++ b/drivers/video/console/Kconfig

@@ -43,9 +43,28 @@ config VGACON_SOFT_SCROLLBACK_SIZE

        range 1 1024
        default "64"
        help
-         Enter the amount of System RAM to allocate for the
scrollback
-	 buffer.  Each 64KB will give you approximately 16 80x25
-	 screenfuls of scrollback buffer
+	  Enter the amount of System RAM to allocate for
scrollback
+	  buffers of VGA consoles. Each 64KB will give you
approximately
+	  16 80x25 screenfuls of scrollback buffer.
+
+config VGACON_SOFT_SCROLLBACK_PERSISTENT
+	bool "Persistent Scrollback History for each console"
+	depends on VGACON_SOFT_SCROLLBACK
+	default n
+	help
+	  Say Y here if the scrollback history should persist when
switching
+	  between consoles. Otherwise, the scrollback history will
be flushed
+	  each time the console is switched.
+
+	  This feature might break your tool of choice to flush
the scrollback
+	  buffer, e.g. clear(1) will work fine but Debian's
clear_console(1)
+	  will be broken, which might cause security issues.
+	  You can use the escape sequence \e[3J instead if this
feature is
+	  activated.

This issue is the one that makes me the most worried.  Why doesn't
clear_console() work anymore?  Why doesn't it use \e[3J ?

Well, clear_console() just switches from one console to another and
back again. It just assumes that the scrollback buffer is flushed when
switching.
My plan is to make a patch for clear_console() as soon as these patches
are in the kernel - it's chicken-and-egg problem.

I'd recommend that patch get to clear_console() first, having it use the
new escape sequence, if it isn't supported, shouldn't cause any
problems, right?

thanks,

greg k-h