drm/panic: Fix qr_code, ensure vmargin is positive

Depending on qr_code size and screen size, the vertical margin can
be negative, that means there is not enough room to draw the qr_code.

So abort early, to avoid a segfault by trying to draw at negative
coordinates.

Fixes: cb5164a ("drm/panic: Add a QR code panic screen")
Reviewed-by: Javier Martinez Canillas <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Jocelyn Falempe <[email protected]>

Author: Jocelyn Falempe

drivers/gpu/drm/drm_panic.c

@@ -736,7 +736,10 @@ static int _draw_panic_static_qr_code(struct drm_scanout_buffer *sb)
 	pr_debug("QR width %d and scale %d\n", qr_width, scale);
 	r_qr_canvas = DRM_RECT_INIT(0, 0, qr_canvas_width * scale, qr_canvas_width * scale);
 
-	v_margin = (sb->height - drm_rect_height(&r_qr_canvas) - drm_rect_height(&r_msg)) / 5;
+	v_margin = sb->height - drm_rect_height(&r_qr_canvas) - drm_rect_height(&r_msg);
+	if (v_margin < 0)
+		return -ENOSPC;
+	v_margin /= 5;
 
 	drm_rect_translate(&r_qr_canvas, (sb->width - r_qr_canvas.x2) / 2, 2 * v_margin);
 	r_qr = DRM_RECT_INIT(r_qr_canvas.x1 + QR_MARGIN * scale, r_qr_canvas.y1 + QR_MARGIN * scale,