[PATCH 3/3] drm/fb-helper: fix input validation gaps in check_var

[Javier Martinez Canillas]

Daniel Vetter <daniel at ffwll.ch> writes:

> On Wed, Apr 05, 2023 at 06:27:17PM +0200, Javier Martinez Canillas wrote:
>> Daniel Vetter <daniel at ffwll.ch> writes:

[...]

>> >
>> > The __fill_var is after this. I'm honestly not sure what the exact
>> 
>> Ah, your patch adds it after that indeed. Please ignore my comment then.
>
> So rb: you?
>

Yes, I already provided it in my previous email and has been picked by
patchwork. I could do again but probably will confuse dim when applying.

The only patch from your series that is missing an {r,a}b is #1 right now:

https://patchwork.kernel.org/project/dri-devel/list/?series=736966&archived=both

[...]

>> > What I'm wondering now is whether too small x/yres won't lead to problems
>> > of some sorts ... For multi-screen we set the virtual size to be big
>> > enough for all crtc, and then just set x/yres to be the smallest output.
>> > That way fbcon knows to only draw as much as is visible on all screens.
>> > But if you then pan that too much, the bigger screens might not have a big
>> > enough buffer anymore and things fail (but shouldn't).
>> >
>> > Not sure how to fix that tbh.
>> 
>> Would this be a problem in practice?
>
> I'm frankly not sure. You'd get a black screen for fbcon/fbdev across all
> outputs, but only if you have userspace doing this intentionally.
>
> In a way it's just another artifact of the drm fbdev emulation not using
> ATOMIC_TEST_ONLY in the various places where it should, and so doesn't
> really know whether a configuration change will work out.
>
> We already have this in obscure mulit-monitor cases where adding another
> screen kills fbcon, because the display hw is running out of fifo or
> clocks or whatever, and because the drm fbdev code doesn't check but just
> blindly commits the entire thing as an atomic commit, the overall commit
> fails.
>
> This worked "better" with legacy kms because there we commit per-crtc, so
> if any specific crtc runs into a limit check, only that one fails to light
> up.
>
> Imo given that no one cared enough yet to write up atomic TEST_ONLY
> support for fbdev emulation I think we can continue to just ignore this
> problem.
>

Agreed. If that ends being a problem for people in practice then I guess
someone can type atomic TEST_ONLY support for the fbdev emulation layer.

> What should not happen is that fbcon code blows up drawing out of bounds
> or something like that, resulting in a kernel crash. So from that pov I
> think it's "safe" :-)

Great. Thanks a lot for your explanations.

> -Daniel

-- 
Best regards,

Javier Martinez Canillas
Core Platforms
Red Hat