[PATCH v2 3/4] fbmem: Prevent invalid virtual screen sizes in fb_set_var()

[Helge Deller]

* Geert Uytterhoeven <geert at linux-m68k.org>:
> > --- a/drivers/video/fbdev/core/fbmem.c
> > +++ b/drivers/video/fbdev/core/fbmem.c
> > @@ -1016,6 +1016,18 @@ fb_set_var(struct fb_info *info, struct fb_var_screeninfo *var)
> >         if (ret)
> >                 return ret;
> >
> > +       /* make sure virtual resolution >= physical resolution */
> > +       if (WARN_ON(var->xres_virtual < var->xres)) {
>
> WARN_ON_ONCE()?
> This does mean we would miss two or more buggy drivers in a single system.
>
> > +               pr_warn("fbcon: Fix up invalid xres %d for %s\n",
>
> xres_virtual?
>
> > +                       var->xres_virtual, info->fix.id);
> > +               var->xres_virtual = var->xres;
>
> I think it's better to not fix this up, but return -EINVAL instead.
> After all if we get here, we have a buggy driver that needs to be fixed.
>
> > +       }
> > +       if (WARN_ON(var->yres_virtual < var->yres)) {
> > +               pr_warn("fbcon: Fix up invalid yres %d for %s\n",
> > +                       var->yres_virtual, info->fix.id);
> > +               var->yres_virtual = var->yres;
> > +       }
>
> Same for yres.

Geert, thanks for your valuable feedback!

In general I don't like for this case any of the WARN_ON* functions.
They don't provide any useful info for us, dumps unneccessarily the
stack backtrace and would annoy existing users.

We know, that DRM drivers can't change the resolution. If we would leave
in any kind of warning, all DRM users will ask back - and we don't have
a solution for them. It's also no regression, because it didn't worked
before either.

But we want to detect fbdev drivers which behave badly - so we should
print that info with the driver name.

Below is a new patch which addresses this. The search for drm drivers
looks somewhat hackish - maybe someone can propose a better approach?

Thoughts?

Helge


From 0f33e2a3730342ab85df372f80b4f61762fb1130 Mon Sep 17 00:00:00 2001
From: Helge Deller <deller at gmx.de>
Date: Wed, 29 Jun 2022 15:53:55 +0200
Subject: [PATCH] fbmem: Check virtual screen sizes in fb_set_var()

Verify that the fbdev or drm driver correctly adjusted the virtual screen
sizes. On failure report (in case of fbdev drivers) the failing driver.

Signed-off-by: Helge Deller <deller at gmx.de>
Cc: stable at vger.kernel.org # v5.4+

diff --git a/drivers/video/fbdev/core/fbmem.c b/drivers/video/fbdev/core/fbmem.c
index 160389365a36..9b75aa4405ee 100644
--- a/drivers/video/fbdev/core/fbmem.c
+++ b/drivers/video/fbdev/core/fbmem.c
@@ -1016,6 +1016,21 @@ fb_set_var(struct fb_info *info, struct fb_var_screeninfo *var)
 	if (ret)
 		return ret;

+	/* verify that virtual resolution >= physical resolution */
+	if (var->xres_virtual < var->xres ||
+	    var->yres_virtual < var->yres) {
+		/* drm drivers don't support mode changes yet. Do not report. */
+		if (strstr(info->fix.id, "drm"))
+			return -ENOTSUPP;
+
+		pr_warn("WARNING: fbcon: Driver %s missed to adjust virtual"
+			" screen size (%dx%d vs. %dx%d)\n",
+			info->fix.id,
+			var->xres_virtual, var->yres_virtual,
+			var->xres, var->yres);
+		return -EINVAL;
+	}
+
 	if ((var->activate & FB_ACTIVATE_MASK) != FB_ACTIVATE_NOW)
 		return 0;