[RFC PATCH 0/4] Allow to use DRM fbdev emulation layer with CONFIG_FB disabled

[Daniel Vetter]

On Wed, Sep 01, 2021 at 11:08:10AM +0200, Javier Martinez Canillas wrote:
> On 8/31/21 2:35 PM, Daniel Vetter wrote:
> > On Sat, Aug 28, 2021 at 12:02:21AM +0200, Javier Martinez Canillas wrote:
> 
> [snip]
> 
> >>
> >> We talked about a drmcon with Peter Robinson as well but then decided that a
> >> way to disable CONFIG_FB but still having the DRM fbdev emulation could be a
> >> intermediary step, hence these RFC patches.
> >>
> >> But yes, I agree that a drmcon would be the proper approach for this, to not
> >> need any fbdev support at all. We will just keep the explicit disable for the
> >> fbdev drivers then in the meantime.
> > 
> > I think the only intermediate step would be to disable the fbdev uapi
> > (char node and anything in sysfs), while still registering against the
> > fbcon layer so you have a console.
> >
> 
> Right, $subject disabled the sysfs interface but left the fbdev chardev. I can
> try to do a v2 that also disables that interface but just keep the fbcon part.
>  
> > But looking at the things syzbot finds the really problematic code is all
> > in the fbcon and console layer in general, and /dev/fb0 seems pretty
> > solid.
> >
> 
> Yes, but still would be an improvement in the sense that no legacy fbdev uAPI
> will be exposed and so user-space would only depend on the DRM/KMS interface.
> 
> > I think for a substantial improvement here in robustness what you really
> > want is
> > - kmscon in userspace
> > - disable FB layer
> > - ideally also disable console/vt layer in the kernel
> 
> Earlier in the thread it was mentioned that an in-kernel drmcon could be used
> instead. My worry with kmscon is that moving something as critical as console
> output to user-space might make harder to troubleshoot early booting issues.
> 
> And also that will require user-space changes. An in-kernel drmcon could be a
> drop-in replacement though.

The drmcon wouldn't be a full console, but just an emergency log renderer.
See Sam's reply, he found the series again.

The real attack surface reduction is in getting rid of the console/vt uapi
implementation from the kernel.
-Daniel

> > - have a minimal emergency/boot-up log thing in drm, patches for that
> >   floated around a few times
> >
> 
> Interesting. Do you have any pointers for this? My search-fu failed me when
> trying to find these patches.
> 
> Best regards,
> -- 
> Javier Martinez Canillas
> Linux Engineering
> Red Hat
> 

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