linux-kernel - Re: [PATCH 1/3] vt: avoid a VLA in the unicode screen scroll function

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Date:   Wed, 18 Jul 2018 03:48:13 +0200
From:   Adam Borowski <kilobyte@...band.pl>
To:     Nicolas Pitre <nicolas.pitre@...aro.org>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Kees Cook <keescook@...omium.org>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Dave Mielke <Dave@...lke.cc>,
        Samuel Thibault <samuel.thibault@...-lyon.org>,
        linux-kernel@...r.kernel.org, linux-console@...r.kernel.org
Subject: Re: [PATCH 1/3] vt: avoid a VLA in the unicode screen scroll function

On Tue, Jul 17, 2018 at 09:02:40PM -0400, Nicolas Pitre wrote:
> The nr argument is typically small: most often nr == 1. However this
> could be abused with a very large explicit scroll in a resized screen.
> Make the code scroll lines one at a time in all cases to avoid the VLA.
> Anything smarter is most likely not warranted here.

Even though nr can be 32767 at most, your new version is O(nr*nr) for no
reason.  Instead of O(n) memory or O(n²) time, a variant of the original
that copies values one at a time would be shorter and faster.

> Requested-by: Kees Cook <keescook@...omium.org>
> Signed-off-by: Nicolas Pitre <nico@...aro.org>
> ---
>  drivers/tty/vt/vt.c | 18 ++++++++++--------
>  1 file changed, 10 insertions(+), 8 deletions(-)
> 
> diff --git a/drivers/tty/vt/vt.c b/drivers/tty/vt/vt.c
> index 2d14bb195d..03e79f7787 100644
> --- a/drivers/tty/vt/vt.c
> +++ b/drivers/tty/vt/vt.c
> @@ -433,20 +433,22 @@ static void vc_uniscr_scroll(struct vc_data *vc, unsigned int t, unsigned int b,
>  
>  	if (uniscr) {
>  		unsigned int s, d, rescue, clear;
> -		char32_t *save[nr];
>  
>  		s = clear = t;
> -		d = t + nr;
> -		rescue = b - nr;
> +		d = t + 1;
> +		rescue = b - 1;
>  		if (dir == SM_UP) {
>  			swap(s, d);
>  			swap(clear, rescue);
>  		}
> -		memcpy(save, uniscr->lines + rescue, nr * sizeof(*save));
> -		memmove(uniscr->lines + d, uniscr->lines + s,
> -			(b - t - nr) * sizeof(*uniscr->lines));
> -		memcpy(uniscr->lines + clear, save, nr * sizeof(*save));
> -		vc_uniscr_clear_lines(vc, clear, nr);
> +		while (nr--) {
> +			char32_t *tmp;
> +			tmp = uniscr->lines[rescue];
> +			memmove(uniscr->lines + d, uniscr->lines + s,
> +				(b - t - 1) * sizeof(*uniscr->lines));
> +			uniscr->lines[clear] = tmp;
> +			vc_uniscr_clear_lines(vc, clear, 1);
> +		}
>  	}
>  }

What the function does is rotating an array (slice [t..b) here), by nr if
SM_DOWN or by -nr ie (b - t - nr) if SM_UP.  A nice problem that almost every
"code interview questions" book includes :)

Please say if you don't have time for such games, I've just refreshed what's
a good answer. :þ


Meow.
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