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    SubjectRe: [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@gmx.de>
    > > > Reviewed-by: Andrey Utkin <andrey_utkin@fastmail.com>
    > > > Tested-by: Andrey Utkin <andrey_utkin@fastmail.com>
    > > > Tested-by: Adam Borowski <kilobyte@angband.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

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