linux-kernel - Re: [PATCH v10 3/4] console: Add persistent scrollback buffers for all VGA consoles

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Date:   Thu, 19 Jan 2017 17:33:14 +0100
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Manuel Schölling <manuel.schoelling@....de>
Cc:     jslaby@...e.com, kilobyte@...band.pl, lkml14@...tdoyle.com,
        rdunlap@...radead.org, shorne@...il.com, andrey_utkin@...tmail.com,
        akpm@...ux-foundation.org, paul.burton@...tec.com,
        daniel.vetter@...ll.ch, tj@...nel.org, hdegoede@...hat.com,
        linux-kernel@...r.kernel.org, linux-fbdev@...r.kernel.org
Subject: Re: [PATCH v10 3/4] console: Add persistent scrollback buffers for
 all VGA consoles

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:
> > On Fri, Jan 13, 2017 at 09:07:57PM +0100, Manuel Schölling wrote:
> > > 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 <manuel.schoelling@....de>
> > > Reviewed-by: Andrey Utkin <andrey_utkin@...tmail.com>
> > > Tested-by: Andrey Utkin <andrey_utkin@...tmail.com>
> > > Tested-by: Adam Borowski <kilobyte@...band.pl>
> > > ---
> > >  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