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

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Message-ID: <20200929165657.GS438822@phenom.ffwll.local>
Date:   Tue, 29 Sep 2020 18:56:57 +0200
From:   Daniel Vetter <daniel@...ll.ch>
To:     Martin Hostettler <textshell@...uujin.de>
Cc:     Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        gregkh@...uxfoundation.org, jirislaby@...nel.org,
        Peilin Ye <yepeilin.cs@...il.com>,
        syzbot <syzbot+b308f5fd049fbbc6e74f@...kaller.appspotmail.com>,
        b.zolnierkie@...sung.com, daniel.vetter@...ll.ch, 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 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 :-)

Cheers, Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch