linux-kernel - Re: [PATCH] vt_ioctl: make VT_RESIZEX behave like VT

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Date:   Tue, 29 Sep 2020 19:10:40 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Martin Hostettler <textshell@...uujin.de>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        jirislaby@...nel.org, Peilin Ye <yepeilin.cs@...il.com>,
        syzbot <syzbot+b308f5fd049fbbc6e74f@...kaller.appspotmail.com>,
        b.zolnierkie@...sung.com, deller@....de,
        syzkaller-bugs@...glegroups.com,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        dri-devel@...ts.freedesktop.org, linux-fbdev@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        George Kennedy <george.kennedy@...cle.com>
Subject: Re: [PATCH] vt_ioctl: make VT_RESIZEX behave like VT_RESIZE

On Tue, Sep 29, 2020 at 06:56:57PM +0200, Daniel Vetter wrote:
> On Tue, Sep 29, 2020 at 12:52:03PM +0200, Martin Hostettler wrote:
> > On Tue, Sep 29, 2020 at 10:12:46AM +0900, Tetsuo Handa wrote:
> > > On 2020/09/29 2:59, Martin Hostettler wrote:
> > > > On Sun, Sep 27, 2020 at 08:46:30PM +0900, Tetsuo Handa wrote:
> > > >> VT_RESIZEX was introduced in Linux 1.3.3, but it is unclear that what
> > > >> comes to the "+ more" part, and I couldn't find a user of VT_RESIZEX.
> > > >>
> > > > 
> > > > It seems this is/was used by "svgatextmode" which seems to be at
> > > > http://www.ibiblio.org/pub/Linux/utils/console/
> > > > 
> > > > Not sure if that kind of software still has a chance to work nowadays.
> > > > 
> > > 
> > > Thanks for the information.
> > > 
> > > It seems that v.v_vlin = curr_textmode->VDisplay / (MOFLG_ISSET(curr_textmode, ATTR_DOUBLESCAN) ? 2 : 1)
> > > and v.v_clin = curr_textmode->FontHeight . Thus, v.v_clin is font's height and seems to be non-zero.
> > > But according to https://bugs.gentoo.org/19485 , people are using kernel framebuffer instead.
> > > 
> > 
> > Yes, this seems to be from pre framebuffer times.
> > 
> > Back in the days "svga" was the wording you got for "pokes svga card
> > hardware registers from userspace drivers". And textmode means font
> > rendering is done via (fixed function in those times) hardware scanout
> > engine. Of course having only to update 2 bytes per character was a huge
> > saving early on. Likely this is also before vesa VBE was reliable.
> > 
> > So i guess the point where this all starts going wrong allowing the X parts
> > of the api to be combined with FB based rendering at all? Sounds the only
> > user didn't use that combination and so it was never tested?
> > 
> > Then again, this all relates to hardware from 20 years ago...
> 
> Imo userspace modesetting should be burned down anywhere we can. We've
> gotten away with this in drivers/gpu by just seamlessly transitioning to
> kernel drivers.
> 
> Since th only userspace we've found seems to be able to cope if this ioctl
> doesn't do anything, my vote goes towards ripping it out completely and
> doing nothing in there. Only question is whether we should error or fail
> with a silent success: Former is safer, latter can avoid a few regression
> reports since the userspace tools keep "working", and usually people don't
> notice for stuff this old. It worked in drivers/gpu :-)

This patch just ignores the ioctl and keeps on going, so userspace
"shouldn't" notice it :)

And it's in linux-next now, so all should be good.

thanks,

greg k-h