Re: [PATCH] vgacon: Propagate console boot parameters before calling `vc_resize'

From: wim <wim@djo.tudelft.nl>
To: "Maciej W. Rozycki" <macro@orcam.me.uk>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	"Pavel V. Panteleev" <panteleev_p@mcst.ru>,
	dri-devel@lists.freedesktop.org, linux-fbdev@vger.kernel.org,
	linux-kernel@vger.kernel.org, wim <wim@djo.tudelft.nl>
Subject: Re: [PATCH] vgacon: Propagate console boot parameters before calling `vc_resize'
Date: Wed, 27 Oct 2021 02:51:10 +0200	[thread overview]
Message-ID: <20211027005110.GA26354@djo.tudelft.nl> (raw)
In-Reply-To: <alpine.DEB.2.21.2110252317110.58149@angie.orcam.me.uk>

On Tue, Oct 26, 2021 at 12:26:22AM +0200, Maciej W. Rozycki wrote:
> Fix a division by zero in `vgacon_resize' with a backtrace like:
> ...
> linux-vt-vgacon-init-cell-height-fix.diff
> Index: linux-macro-ide-tty/drivers/video/console/vgacon.c
> ===================================================================
> --- linux-macro-ide-tty.orig/drivers/video/console/vgacon.c
> +++ linux-macro-ide-tty/drivers/video/console/vgacon.c
> @@ -366,11 +366,17 @@ static void vgacon_init(struct vc_data *
>  	struct uni_pagedir *p;
>  
>  	/*
> -	 * We cannot be loaded as a module, therefore init is always 1,
> -	 * but vgacon_init can be called more than once, and init will
> -	 * not be 1.
> +	 * We cannot be loaded as a module, therefore init will be 1
> +	 * if we are the default console, however if we are a fallback
> +	 * console, for example if fbcon has failed registration, then
> +	 * init will be 0, so we need to make sure our boot parameters
> +	 * have been copied to the console structure for vgacon_resize
> +	 * ultimately called by vc_resize.  Any subsequent calls to
> +	 * vgacon_init init will have init set to 0 too.
>  	 */
>  	c->vc_can_do_color = vga_can_do_color;
> +	c->vc_scan_lines = vga_scan_lines;
> +	c->vc_font.height = c->vc_cell_height = vga_video_font_height;
>  
>  	/* set dimensions manually if init != 0 since vc_resize() will fail */
>  	if (init) {
> @@ -379,8 +385,6 @@ static void vgacon_init(struct vc_data *
>  	} else
>  		vc_resize(c, vga_video_num_columns, vga_video_num_lines);
>  
> -	c->vc_scan_lines = vga_scan_lines;
> -	c->vc_font.height = c->vc_cell_height = vga_video_font_height;
>  	c->vc_complement_mask = 0x7700;
>  	if (vga_512_chars)
>  		c->vc_hi_font_mask = 0x0800;

Just to let you know that the above patch fixes the crashes I experienced.
Tried in kernel 4.9.287.
The git tree is broken in the sense that it doesn't know the tags v4.9.284,
v4.9.285, v4.9.286 and v2.4.287.

Wim.