linux-kernel - Re: [PATCH 2/3] tty: n_gsm: fix mux activation issues in gsm

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Message-ID: <257e1972-a282-f5c9-40c4-a532fdfaf935@kernel.org>
Date:   Mon, 9 May 2022 12:28:33 +0200
From:   Jiri Slaby <jirislaby@...nel.org>
To:     "D. Starke" <daniel.starke@...mens.com>,
        linux-serial@...r.kernel.org, gregkh@...uxfoundation.org
Cc:     linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3] tty: n_gsm: fix mux activation issues in gsm_config()

On 04. 05. 22, 10:17, D. Starke wrote:
> From: Daniel Starke <daniel.starke@...mens.com>
> 
> The current implementation activates the mux if it was restarted and opens
> the control channel if the mux was previously closed and we are now acting
> as initiator instead of responder, which is the default setting.
> This has two issues.
> 1) No mux is activated if we keep all default values and only switch to
> initiator. The control channel is not allocated but will be opened next
> which results in a NULL pointer dereference.
> 2) Switching the configuration after it was once configured while keeping
> the initiator value the same will not reopen the control channel if it was
> closed due to parameter incompatibilities. The mux remains dead.
> 
> Fix 1) by always activating the mux if it is dead after configuration.
> Fix 2) by always opening the control channel after mux activation.
> 
> Fixes: e1eaea46bb40 ("tty: n_gsm line discipline")
> Cc: stable@...r.kernel.org
> Signed-off-by: Daniel Starke <daniel.starke@...mens.com>
> ---
>   drivers/tty/n_gsm.c | 12 ++++++++----
>   1 file changed, 8 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/tty/n_gsm.c b/drivers/tty/n_gsm.c
> index 9b0b435cf26e..bcb714031d69 100644
> --- a/drivers/tty/n_gsm.c
> +++ b/drivers/tty/n_gsm.c
> @@ -2352,6 +2352,7 @@ static void gsm_copy_config_values(struct gsm_mux *gsm,
>   
>   static int gsm_config(struct gsm_mux *gsm, struct gsm_config *c)
>   {
> +	int ret = 0;

Why is the initialization needed? You can as well declare the variable 
only inside the if below.

>   	int need_close = 0;
>   	int need_restart = 0;
>   
> @@ -2419,10 +2420,13 @@ static int gsm_config(struct gsm_mux *gsm, struct gsm_config *c)
>   	 * FIXME: We need to separate activation/deactivation from adding
>   	 * and removing from the mux array
>   	 */
> -	if (need_restart)
> -		gsm_activate_mux(gsm);
> -	if (gsm->initiator && need_close)
> -		gsm_dlci_begin_open(gsm->dlci[0]);
> +	if (gsm->dead) {
> +		ret = gsm_activate_mux(gsm);
> +		if (ret)
> +			return ret;
> +		if (gsm->initiator)
> +			gsm_dlci_begin_open(gsm->dlci[0]);
> +	}
>   	return 0;
>   }
>   


-- 
js
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