Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Fri, 29 Mar 2019 10:43:41 +0300
From:   Dan Carpenter <dan.carpenter@oracle.com>
To:     Colin King <colin.king@canonical.com>
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Jiri Slaby <jslaby@suse.com>, kernel-janitors@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] n_tty: check for negative and zero space return from
 tty_write_room
Message-ID: <20190329074341.GW32590@kadam>
References: <20190328171005.5822-1-colin.king@canonical.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20190328171005.5822-1-colin.king@canonical.com>
User-Agent: Mutt/1.9.4 (2018-02-28)
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Thu, Mar 28, 2019 at 05:10:05PM +0000, Colin King wrote:
> From: Colin Ian King <colin.king@canonical.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@canonical.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