linux-kernel - Re: [PATCH] n_tty: check for negative and zero space return from tty_write

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Date:   Fri, 29 Mar 2019 10:43:41 +0300
From:   Dan Carpenter <dan.carpenter@...cle.com>
To:     Colin King <colin.king@...onical.com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jslaby@...e.com>, kernel-janitors@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] n_tty: check for negative and zero space return from
 tty_write_room

On Thu, Mar 28, 2019 at 05:10:05PM +0000, Colin King wrote:
> From: Colin Ian King <colin.king@...onical.com>
> 
> The return from tty_write_room could potentially be negative if
> a tty write_room driver returns an error number (not that any seem
> to do). Rather than just check for a zero return, also check for
> a -ve return. This avoids the unsigned nr being set to a large unsigned
> value on the assignment from variable space and can lead to overflowing
> the buffer buf.  Better to be safe than assume all write_room
> implementations in tty drivers are going to do the right thing.
> 
> Signed-off-by: Colin Ian King <colin.king@...onical.com>
> ---
>  drivers/tty/n_tty.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/tty/n_tty.c b/drivers/tty/n_tty.c
> index 9cdb0fa3c4bf..66630787fbf9 100644
> --- a/drivers/tty/n_tty.c
> +++ b/drivers/tty/n_tty.c
> @@ -550,7 +550,7 @@ static ssize_t process_output_block(struct tty_struct *tty,
>  	mutex_lock(&ldata->output_lock);
>  
>  	space = tty_write_room(tty);
> -	if (!space) {
> +	if (space <= 0) {
>  		mutex_unlock(&ldata->output_lock);
>  		return 0;

Instead of zero we should probably return "space" to preserve the error
code.

regards,
dan carpenter