[PATCH] fbdev: Don't spam dmesg on bad userspace ioctl input

[Daniel Vetter]

There's a few reasons the kernel should not spam dmesg on bad
userspace ioctl input:
- at warning level it results in CI false positives
- it allows userspace to drown dmesg output, potentially hiding real
  issues.

None of the other generic EINVAL checks report in the
FBIOPUT_VSCREENINFO ioctl do this, so it's also inconsistent.

I guess the intent of the patch which introduced this warning was that
the drivers ->fb_check_var routine should fail in that case. Reality
is that there's too many fbdev drivers and not enough people
maintaining them by far, and so over the past few years we've simply
handled all these validation gaps by tighning the checks in the core,
because that's realistically really all that will ever happen.

Reported-by: syzbot+20dcf81733d43ddff661 at syzkaller.appspotmail.com
Link: https://syzkaller.appspot.com/bug?id=c5faf983bfa4a607de530cd3bb008888bf06cefc
Fixes: 6c11df58fd1a ("fbmem: Check virtual screen sizes in fb_set_var()")
Cc: Helge Deller <deller at gmx.de>
Cc: Geert Uytterhoeven <geert at linux-m68k.org>
Cc: stable at vger.kernel.org # v5.4+
Cc: Daniel Vetter <daniel at ffwll.ch>
Cc: Javier Martinez Canillas <javierm at redhat.com>
Cc: Thomas Zimmermann <tzimmermann at suse.de>
Signed-off-by: Daniel Vetter <daniel.vetter at intel.com>
---
 drivers/video/fbdev/core/fbmem.c | 4 ----
 1 file changed, 4 deletions(-)

diff --git a/drivers/video/fbdev/core/fbmem.c b/drivers/video/fbdev/core/fbmem.c
index 875541ff185b..9757f4bcdf57 100644
--- a/drivers/video/fbdev/core/fbmem.c
+++ b/drivers/video/fbdev/core/fbmem.c
@@ -1021,10 +1021,6 @@ fb_set_var(struct fb_info *info, struct fb_var_screeninfo *var)
 	/* verify that virtual resolution >= physical resolution */
 	if (var->xres_virtual < var->xres ||
 	    var->yres_virtual < var->yres) {
-		pr_warn("WARNING: fbcon: Driver '%s' missed to adjust virtual screen size (%ux%u vs. %ux%u)\n",
-			info->fix.id,
-			var->xres_virtual, var->yres_virtual,
-			var->xres, var->yres);
 		return -EINVAL;
 	}
 
-- 
2.40.0