[PATCH v2] Bluetooth: rfcomm: fix modem control handling

[PATCH v2] Bluetooth: rfcomm: fix modem control handling

[Johan Hovold]

The RFCOMM driver confuses the local and remote modem control signals,
which specifically means that the reported DTR and RTS state will
instead reflect the remote end (i.e. DSR and CTS).

This issue dates back to the original driver (and a follow-on update)
merged in 2002, which resulted in a non-standard implementation of
TIOCMSET that allowed controlling also the TS07.10 IC and DV signals by
mapping them to the RI and DCD input flags, while TIOCMGET failed to
return the actual state of DTR and RTS.

Note that the bogus control of input signals in tiocmset() is just
dead code as those flags will have been masked out by the tty layer
since 2003.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Cc: stable@vger.kernel.org
Signed-off-by: Johan Hovold <johan@kernel.org>
---

Changes in v2
 - fix a compilation issue discovered before sending v1 but never folded
   into the actual patch...


 net/bluetooth/rfcomm/tty.c | 26 +++++++++++---------------
 1 file changed, 11 insertions(+), 15 deletions(-)

diff --git a/net/bluetooth/rfcomm/tty.c b/net/bluetooth/rfcomm/tty.c
index 376ce6de84be..b783526ab588 100644
--- a/net/bluetooth/rfcomm/tty.c
+++ b/net/bluetooth/rfcomm/tty.c
@@ -643,8 +643,8 @@ static void rfcomm_dev_modem_status(struct rfcomm_dlc *dlc, u8 v24_sig)
 		tty_port_tty_hangup(&dev->port, true);
 
 	dev->modem_status =
-		((v24_sig & RFCOMM_V24_RTC) ? (TIOCM_DSR | TIOCM_DTR) : 0) |
-		((v24_sig & RFCOMM_V24_RTR) ? (TIOCM_RTS | TIOCM_CTS) : 0) |
+		((v24_sig & RFCOMM_V24_RTC) ? TIOCM_DSR : 0) |
+		((v24_sig & RFCOMM_V24_RTR) ? TIOCM_CTS : 0) |
 		((v24_sig & RFCOMM_V24_IC)  ? TIOCM_RI : 0) |
 		((v24_sig & RFCOMM_V24_DV)  ? TIOCM_CD : 0);
 }
@@ -1055,10 +1055,14 @@ static void rfcomm_tty_hangup(struct tty_struct *tty)
 static int rfcomm_tty_tiocmget(struct tty_struct *tty)
 {
 	struct rfcomm_dev *dev = tty->driver_data;
+	struct rfcomm_dlc *dlc = dev->dlc;
+	u8 v24_sig;
 
 	BT_DBG("tty %p dev %p", tty, dev);
 
-	return dev->modem_status;
+	rfcomm_dlc_get_modem_status(dlc, &v24_sig);
+
+	return (v24_sig & (TIOCM_DTR | TIOCM_RTS)) | dev->modem_status;
 }
 
 static int rfcomm_tty_tiocmset(struct tty_struct *tty, unsigned int set, unsigned int clear)
@@ -1071,23 +1075,15 @@ static int rfcomm_tty_tiocmset(struct tty_struct *tty, unsigned int set, unsigne
 
 	rfcomm_dlc_get_modem_status(dlc, &v24_sig);
 
-	if (set & TIOCM_DSR || set & TIOCM_DTR)
+	if (set & TIOCM_DTR)
 		v24_sig |= RFCOMM_V24_RTC;
-	if (set & TIOCM_RTS || set & TIOCM_CTS)
+	if (set & TIOCM_RTS)
 		v24_sig |= RFCOMM_V24_RTR;
-	if (set & TIOCM_RI)
-		v24_sig |= RFCOMM_V24_IC;
-	if (set & TIOCM_CD)
-		v24_sig |= RFCOMM_V24_DV;
 
-	if (clear & TIOCM_DSR || clear & TIOCM_DTR)
+	if (clear & TIOCM_DTR)
 		v24_sig &= ~RFCOMM_V24_RTC;
-	if (clear & TIOCM_RTS || clear & TIOCM_CTS)
+	if (clear & TIOCM_RTS)
 		v24_sig &= ~RFCOMM_V24_RTR;
-	if (clear & TIOCM_RI)
-		v24_sig &= ~RFCOMM_V24_IC;
-	if (clear & TIOCM_CD)
-		v24_sig &= ~RFCOMM_V24_DV;
 
 	rfcomm_dlc_set_modem_status(dlc, v24_sig);
 
-- 
2.49.1

Re: [PATCH v2] Bluetooth: rfcomm: fix modem control handling

[patchwork-bot+bluetooth]

Hello:

This patch was applied to bluetooth/bluetooth-next.git (master)
by Luiz Augusto von Dentz <luiz.von.dentz@intel.com>:

On Thu, 23 Oct 2025 14:05:30 +0200 you wrote:
> The RFCOMM driver confuses the local and remote modem control signals,
> which specifically means that the reported DTR and RTS state will
> instead reflect the remote end (i.e. DSR and CTS).
> 
> This issue dates back to the original driver (and a follow-on update)
> merged in 2002, which resulted in a non-standard implementation of
> TIOCMSET that allowed controlling also the TS07.10 IC and DV signals by
> mapping them to the RI and DCD input flags, while TIOCMGET failed to
> return the actual state of DTR and RTS.
> 
> [...]

Here is the summary with links:
  - [v2] Bluetooth: rfcomm: fix modem control handling
    https://git.kernel.org/bluetooth/bluetooth-next/c/8d2c47aeb078

You are awesome, thank you!
-- 
Deet-doot-dot, I am a bot.
https://korg.docs.kernel.org/patchwork/pwbot.html

Re: [PATCH v2] Bluetooth: rfcomm: fix modem control handling

[Paul Menzel]

Dear Johan,


Thank you for your patch.


Am 23.10.25 um 14:05 schrieb Johan Hovold:
> The RFCOMM driver confuses the local and remote modem control signals,
> which specifically means that the reported DTR and RTS state will
> instead reflect the remote end (i.e. DSR and CTS).
> 
> This issue dates back to the original driver (and a follow-on update)
> merged in 2002, which resulted in a non-standard implementation of
> TIOCMSET that allowed controlling also the TS07.10 IC and DV signals by
> mapping them to the RI and DCD input flags, while TIOCMGET failed to
> return the actual state of DTR and RTS.
> 
> Note that the bogus control of input signals in tiocmset() is just
> dead code as those flags will have been masked out by the tty layer
> since 2003.
> 
> Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")

There is a linux-history git archive [1], if somebody wants dig further. 
But not relevant for the tag used by the stable folks.

Is there any way to test your change, to read DTR and RTS state?

> Cc: stable@vger.kernel.org
> Signed-off-by: Johan Hovold <johan@kernel.org>
> ---
> 
> Changes in v2
>   - fix a compilation issue discovered before sending v1 but never folded
>     into the actual patch...
> 
> 
>   net/bluetooth/rfcomm/tty.c | 26 +++++++++++---------------
>   1 file changed, 11 insertions(+), 15 deletions(-)
> 
> diff --git a/net/bluetooth/rfcomm/tty.c b/net/bluetooth/rfcomm/tty.c
> index 376ce6de84be..b783526ab588 100644
> --- a/net/bluetooth/rfcomm/tty.c
> +++ b/net/bluetooth/rfcomm/tty.c
> @@ -643,8 +643,8 @@ static void rfcomm_dev_modem_status(struct rfcomm_dlc *dlc, u8 v24_sig)
>   		tty_port_tty_hangup(&dev->port, true);
>   
>   	dev->modem_status =
> -		((v24_sig & RFCOMM_V24_RTC) ? (TIOCM_DSR | TIOCM_DTR) : 0) |
> -		((v24_sig & RFCOMM_V24_RTR) ? (TIOCM_RTS | TIOCM_CTS) : 0) |
> +		((v24_sig & RFCOMM_V24_RTC) ? TIOCM_DSR : 0) |
> +		((v24_sig & RFCOMM_V24_RTR) ? TIOCM_CTS : 0) |
>   		((v24_sig & RFCOMM_V24_IC)  ? TIOCM_RI : 0) |
>   		((v24_sig & RFCOMM_V24_DV)  ? TIOCM_CD : 0);
>   }
> @@ -1055,10 +1055,14 @@ static void rfcomm_tty_hangup(struct tty_struct *tty)
>   static int rfcomm_tty_tiocmget(struct tty_struct *tty)
>   {
>   	struct rfcomm_dev *dev = tty->driver_data;
> +	struct rfcomm_dlc *dlc = dev->dlc;
> +	u8 v24_sig;
>   
>   	BT_DBG("tty %p dev %p", tty, dev);
>   
> -	return dev->modem_status;
> +	rfcomm_dlc_get_modem_status(dlc, &v24_sig);
> +
> +	return (v24_sig & (TIOCM_DTR | TIOCM_RTS)) | dev->modem_status;
>   }
>   
>   static int rfcomm_tty_tiocmset(struct tty_struct *tty, unsigned int set, unsigned int clear)
> @@ -1071,23 +1075,15 @@ static int rfcomm_tty_tiocmset(struct tty_struct *tty, unsigned int set, unsigne
>   
>   	rfcomm_dlc_get_modem_status(dlc, &v24_sig);
>   
> -	if (set & TIOCM_DSR || set & TIOCM_DTR)
> +	if (set & TIOCM_DTR)
>   		v24_sig |= RFCOMM_V24_RTC;
> -	if (set & TIOCM_RTS || set & TIOCM_CTS)
> +	if (set & TIOCM_RTS)
>   		v24_sig |= RFCOMM_V24_RTR;
> -	if (set & TIOCM_RI)
> -		v24_sig |= RFCOMM_V24_IC;
> -	if (set & TIOCM_CD)
> -		v24_sig |= RFCOMM_V24_DV;
>   
> -	if (clear & TIOCM_DSR || clear & TIOCM_DTR)
> +	if (clear & TIOCM_DTR)
>   		v24_sig &= ~RFCOMM_V24_RTC;
> -	if (clear & TIOCM_RTS || clear & TIOCM_CTS)
> +	if (clear & TIOCM_RTS)
>   		v24_sig &= ~RFCOMM_V24_RTR;
> -	if (clear & TIOCM_RI)
> -		v24_sig &= ~RFCOMM_V24_IC;
> -	if (clear & TIOCM_CD)
> -		v24_sig &= ~RFCOMM_V24_DV;
>   
>   	rfcomm_dlc_set_modem_status(dlc, v24_sig);
>   


Kind regards,

Paul


[1]: 
https://web.git.kernel.org/pub/scm/linux/kernel/git/history/history.git/

Re: [PATCH v2] Bluetooth: rfcomm: fix modem control handling

[Johan Hovold]

On Wed, Oct 29, 2025 at 09:15:28PM +0100, Paul Menzel wrote:

> Am 23.10.25 um 14:05 schrieb Johan Hovold:
> > The RFCOMM driver confuses the local and remote modem control signals,
> > which specifically means that the reported DTR and RTS state will
> > instead reflect the remote end (i.e. DSR and CTS).
> > 
> > This issue dates back to the original driver (and a follow-on update)
> > merged in 2002, which resulted in a non-standard implementation of
> > TIOCMSET that allowed controlling also the TS07.10 IC and DV signals by
> > mapping them to the RI and DCD input flags, while TIOCMGET failed to
> > return the actual state of DTR and RTS.
> > 
> > Note that the bogus control of input signals in tiocmset() is just
> > dead code as those flags will have been masked out by the tty layer
> > since 2003.
> > 
> > Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> 
> There is a linux-history git archive [1], if somebody wants dig further. 
> But not relevant for the tag used by the stable folks.

Yeah, that's tree I use.

> Is there any way to test your change, to read DTR and RTS state?

This can be tested using the TIOCMGET and TIOCMSET (or
TIOCMBIS/TIOCMBIC) ioctls. I use a custom c application, and I'm not
aware of any particular application you can you use for testing if
that's what you were after.

Johan