[PATCH 1/1] pty: BREAK for pseudoterminals

[PATCH 1/1] pty: BREAK for pseudoterminals

[Nan Li]

This will greatly enhance the usefulness of QEMU virtual serial ports, because the Linux kernel interprets a break on the serial console as a SysRq, but there is currently no way to pass this signal over a pseudo-terminal. This patch will work for transmitting BREAK from master to slave through pseudo-terminal.

Signed-off-by: Nan Li <nli@suse.com>
---
 drivers/tty/pty.c | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/drivers/tty/pty.c b/drivers/tty/pty.c
index e72ee62..ac8893a 100644
--- a/drivers/tty/pty.c
+++ b/drivers/tty/pty.c
@@ -31,6 +31,7 @@
 static struct tty_driver *ptm_driver;
 static struct tty_driver *pts_driver;
 static DEFINE_MUTEX(devpts_mutex);
+#define BREAK_STRING '\0'
 #endif
 
 static void pty_close(struct tty_struct *tty, struct file *filp)
@@ -674,6 +675,23 @@ static void pty_unix98_shutdown(struct tty_struct *tty)
 	devpts_kill_index(tty->driver_data, tty->index);
 }
 
+static int pty_unix98_break_ctl(struct tty_struct *tty, int break_state)
+{
+	int c;
+	struct tty_struct *to = tty->link;
+
+	if (break_state) {
+		/* Stuff the break data into the input queue of the other end */
+		c = tty_insert_flip_char(to->port, BREAK_STRING, TTY_BREAK);
+		/* And shovel */
+		if (c)
+			tty_flip_buffer_push(to->port);
+		else
+			return -ENOMEM;
+	}
+	return 0;
+}
+
 static const struct tty_operations ptm_unix98_ops = {
 	.lookup = ptm_unix98_lookup,
 	.install = pty_unix98_install,
@@ -686,6 +704,7 @@ static const struct tty_operations ptm_unix98_ops = {
 	.chars_in_buffer = pty_chars_in_buffer,
 	.unthrottle = pty_unthrottle,
 	.ioctl = pty_unix98_ioctl,
+	.break_ctl = pty_unix98_break_ctl,
 	.resize = pty_resize,
 	.shutdown = pty_unix98_shutdown,
 	.cleanup = pty_cleanup
-- 
1.7.12.4

Re: [PATCH 1/1] pty: BREAK for pseudoterminals

[Peter Hurley]

On 02/05/2015 02:11 PM, Nan Li wrote:
> This will greatly enhance the usefulness of QEMU virtual serial ports, because the Linux kernel interprets a break on the serial console as a SysRq, but there is currently no way to pass this signal over a pseudo-terminal. This patch will work for transmitting BREAK from master to slave through pseudo-terminal.

pty is an IPC mechanism, not a virtualization driver.

Those programs that know they're consuming from a slave pty may not
have bothered to set termios for BREAKs, and may not be prepared
for NULLs in the input stream.

Regards,
Peter Hurley


> Signed-off-by: Nan Li <nli@suse.com>
> ---
>  drivers/tty/pty.c | 19 +++++++++++++++++++
>  1 file changed, 19 insertions(+)
> 
> diff --git a/drivers/tty/pty.c b/drivers/tty/pty.c
> index e72ee62..ac8893a 100644
> --- a/drivers/tty/pty.c
> +++ b/drivers/tty/pty.c
> @@ -31,6 +31,7 @@
>  static struct tty_driver *ptm_driver;
>  static struct tty_driver *pts_driver;
>  static DEFINE_MUTEX(devpts_mutex);
> +#define BREAK_STRING '\0'
>  #endif
>  
>  static void pty_close(struct tty_struct *tty, struct file *filp)
> @@ -674,6 +675,23 @@ static void pty_unix98_shutdown(struct tty_struct *tty)
>  	devpts_kill_index(tty->driver_data, tty->index);
>  }
>  
> +static int pty_unix98_break_ctl(struct tty_struct *tty, int break_state)
> +{
> +	int c;
> +	struct tty_struct *to = tty->link;
> +
> +	if (break_state) {
> +		/* Stuff the break data into the input queue of the other end */
> +		c = tty_insert_flip_char(to->port, BREAK_STRING, TTY_BREAK);
> +		/* And shovel */
> +		if (c)
> +			tty_flip_buffer_push(to->port);
> +		else
> +			return -ENOMEM;
> +	}
> +	return 0;
> +}
> +
>  static const struct tty_operations ptm_unix98_ops = {
>  	.lookup = ptm_unix98_lookup,
>  	.install = pty_unix98_install,
> @@ -686,6 +704,7 @@ static const struct tty_operations ptm_unix98_ops = {
>  	.chars_in_buffer = pty_chars_in_buffer,
>  	.unthrottle = pty_unthrottle,
>  	.ioctl = pty_unix98_ioctl,
> +	.break_ctl = pty_unix98_break_ctl,
>  	.resize = pty_resize,
>  	.shutdown = pty_unix98_shutdown,
>  	.cleanup = pty_cleanup
> 

Re: [PATCH 1/1] pty: BREAK for pseudoterminals

[Petr Tesarik]

On Mon, 16 Feb 2015 08:04:02 -0500
Peter Hurley <peter@hurleysoftware.com> wrote:

> On 02/05/2015 02:11 PM, Nan Li wrote:
> > This will greatly enhance the usefulness of QEMU virtual serial ports, because the Linux kernel interprets a break on the serial console as a SysRq, but there is currently no way to pass this signal over a pseudo-terminal. This patch will work for transmitting BREAK from master to slave through pseudo-terminal.
> 
> pty is an IPC mechanism, not a virtualization driver.

No, but it can be used as a TTY. Teletypes have always had the capacity
to send and receive BREAK.

> Those programs that know they're consuming from a slave pty may not
> have bothered to set termios for BREAKs, and may not be prepared
> for NULLs in the input stream.

Well, the default termios includes IGNBRK, so unless they bothered to
do anything about BREAKs, they won't see any change.

Anyway, the current kernel behaviour is clearly suboptimal. Calling
tcsendbreak() on a pty descriptor does nothing but reports success.
There are obviously two ways to fix it: either report an error, or
deliver the BREAK for real.

This patch implements the latter, adding at least one valid use case
to explain why it is better than the former.

Regards,
Petr Tesarik

> > Signed-off-by: Nan Li <nli@suse.com>
> > ---
> >  drivers/tty/pty.c | 19 +++++++++++++++++++
> >  1 file changed, 19 insertions(+)
> > 
> > diff --git a/drivers/tty/pty.c b/drivers/tty/pty.c
> > index e72ee62..ac8893a 100644
> > --- a/drivers/tty/pty.c
> > +++ b/drivers/tty/pty.c
> > @@ -31,6 +31,7 @@
> >  static struct tty_driver *ptm_driver;
> >  static struct tty_driver *pts_driver;
> >  static DEFINE_MUTEX(devpts_mutex);
> > +#define BREAK_STRING '\0'
> >  #endif
> >  
> >  static void pty_close(struct tty_struct *tty, struct file *filp)
> > @@ -674,6 +675,23 @@ static void pty_unix98_shutdown(struct
> > tty_struct *tty) devpts_kill_index(tty->driver_data, tty->index);
> >  }
> >  
> > +static int pty_unix98_break_ctl(struct tty_struct *tty, int
> > break_state) +{
> > +	int c;
> > +	struct tty_struct *to = tty->link;
> > +
> > +	if (break_state) {
> > +		/* Stuff the break data into the input queue of
> > the other end */
> > +		c = tty_insert_flip_char(to->port, BREAK_STRING,
> > TTY_BREAK);
> > +		/* And shovel */
> > +		if (c)
> > +			tty_flip_buffer_push(to->port);
> > +		else
> > +			return -ENOMEM;
> > +	}
> > +	return 0;
> > +}
> > +
> >  static const struct tty_operations ptm_unix98_ops = {
> >  	.lookup = ptm_unix98_lookup,
> >  	.install = pty_unix98_install,
> > @@ -686,6 +704,7 @@ static const struct tty_operations
> > ptm_unix98_ops = { .chars_in_buffer = pty_chars_in_buffer,
> >  	.unthrottle = pty_unthrottle,
> >  	.ioctl = pty_unix98_ioctl,
> > +	.break_ctl = pty_unix98_break_ctl,
> >  	.resize = pty_resize,
> >  	.shutdown = pty_unix98_shutdown,
> >  	.cleanup = pty_cleanup
> > 
> 

Re: [PATCH 1/1] pty: BREAK for pseudoterminals

[Peter Hurley]

Hi Petr,

On 02/16/2015 08:22 AM, Petr Tesarik wrote:
> On Mon, 16 Feb 2015 08:04:02 -0500
> Peter Hurley <peter@hurleysoftware.com> wrote:
> 
>> On 02/05/2015 02:11 PM, Nan Li wrote:
>>> This will greatly enhance the usefulness of QEMU virtual serial ports, because the Linux kernel interprets a break on the serial console as a SysRq, but there is currently no way to pass this signal over a pseudo-terminal. This patch will work for transmitting BREAK from master to slave through pseudo-terminal.
>>
>> pty is an IPC mechanism, not a virtualization driver.
> 
> No, but it can be used as a TTY. Teletypes have always had the capacity
> to send and receive BREAK.

In some general-purpose but restricted capacity, the *slave* end _mimics_
a tty. That doesn't mean that it is suitable for every conceivable
use as a tty, nor should it.

If BREAK was actually useful for real terminal i/o, the pty driver would
already support this. Note the pty driver doesn't emulate modem signals
either, for the same reason.

>> Those programs that know they're consuming from a slave pty may not
>> have bothered to set termios for BREAKs, and may not be prepared
>> for NULLs in the input stream.
> 
> Well, the default termios includes IGNBRK, so unless they bothered to
> do anything about BREAKs, they won't see any change.

Userspace programs are sloppy, especially with terminal i/o and settings.
Unlikely is not the same as not possible.

> Anyway, the current kernel behaviour is clearly suboptimal. Calling
> tcsendbreak() on a pty descriptor does nothing but reports success.
> There are obviously two ways to fix it: either report an error, or
> deliver the BREAK for real.

The pty master end is even less of a tty than the slave end, but this
isn't really about errno. This patch doesn't address either of your
points wrt tcsendbreak() on the slave descriptor which is the actual
terminal end.

> This patch implements the latter, adding at least one valid use case
> to explain why it is better than the former.

I disagree that this is a valid use case for the _pty driver_.

AFAICT this is simply for convenience, as sysrq functionality is
already available via sendkey.

Regards,
Peter Hurley


>>> Signed-off-by: Nan Li <nli@suse.com>
>>> ---
>>>  drivers/tty/pty.c | 19 +++++++++++++++++++
>>>  1 file changed, 19 insertions(+)
>>>
>>> diff --git a/drivers/tty/pty.c b/drivers/tty/pty.c
>>> index e72ee62..ac8893a 100644
>>> --- a/drivers/tty/pty.c
>>> +++ b/drivers/tty/pty.c
>>> @@ -31,6 +31,7 @@
>>>  static struct tty_driver *ptm_driver;
>>>  static struct tty_driver *pts_driver;
>>>  static DEFINE_MUTEX(devpts_mutex);
>>> +#define BREAK_STRING '\0'
>>>  #endif
>>>  
>>>  static void pty_close(struct tty_struct *tty, struct file *filp)
>>> @@ -674,6 +675,23 @@ static void pty_unix98_shutdown(struct
>>> tty_struct *tty) devpts_kill_index(tty->driver_data, tty->index);
>>>  }
>>>  
>>> +static int pty_unix98_break_ctl(struct tty_struct *tty, int
>>> break_state) +{
>>> +	int c;
>>> +	struct tty_struct *to = tty->link;
>>> +
>>> +	if (break_state) {
>>> +		/* Stuff the break data into the input queue of
>>> the other end */
>>> +		c = tty_insert_flip_char(to->port, BREAK_STRING,
>>> TTY_BREAK);
>>> +		/* And shovel */
>>> +		if (c)
>>> +			tty_flip_buffer_push(to->port);
>>> +		else
>>> +			return -ENOMEM;
>>> +	}
>>> +	return 0;
>>> +}
>>> +
>>>  static const struct tty_operations ptm_unix98_ops = {
>>>  	.lookup = ptm_unix98_lookup,
>>>  	.install = pty_unix98_install,
>>> @@ -686,6 +704,7 @@ static const struct tty_operations
>>> ptm_unix98_ops = { .chars_in_buffer = pty_chars_in_buffer,
>>>  	.unthrottle = pty_unthrottle,
>>>  	.ioctl = pty_unix98_ioctl,
>>> +	.break_ctl = pty_unix98_break_ctl,
>>>  	.resize = pty_resize,
>>>  	.shutdown = pty_unix98_shutdown,
>>>  	.cleanup = pty_cleanup
>>>
>>
> 

Re: [PATCH 1/1] pty: BREAK for pseudoterminals

[Petr Tesarik]

On Mon, 16 Feb 2015 11:24:16 -0500
Peter Hurley <peter@hurleysoftware.com> wrote:

> Hi Petr,
> 
> On 02/16/2015 08:22 AM, Petr Tesarik wrote:
> > On Mon, 16 Feb 2015 08:04:02 -0500
> > Peter Hurley <peter@hurleysoftware.com> wrote:
> > 
> >> On 02/05/2015 02:11 PM, Nan Li wrote:
> >>> This will greatly enhance the usefulness of QEMU virtual serial ports, because the Linux kernel interprets a break on the serial console as a SysRq, but there is currently no way to pass this signal over a pseudo-terminal. This patch will work for transmitting BREAK from master to slave through pseudo-terminal.
> >>
> >> pty is an IPC mechanism, not a virtualization driver.
> > 
> > No, but it can be used as a TTY. Teletypes have always had the capacity
> > to send and receive BREAK.
> 
> In some general-purpose but restricted capacity, the *slave* end _mimics_
> a tty. That doesn't mean that it is suitable for every conceivable
> use as a tty, nor should it.

Unless there's some specification of what should and what should not be
implemented, this question is open for discussion, methinks.

> If BREAK was actually useful for real terminal i/o, the pty driver
> would already support this.

If I strictly followed this statement, no improvement would ever be
possible. Or did I miss something? Are Linux PTYs a legacy subsystem
that never gets any new features?

>[...]
> > Well, the default termios includes IGNBRK, so unless they bothered
> > to do anything about BREAKs, they won't see any change.
> 
> Userspace programs are sloppy, especially with terminal i/o and
> settings. Unlikely is not the same as not possible.

Sure. New features may break sloppy programs. OTOH, the obvious
workaround is not using such programs together with new programs that
actually use tcsendbreak() for something... until those sloppy programs
are fixed. It's not like the whole system stops working once this patch
is applied.

> > Anyway, the current kernel behaviour is clearly suboptimal. Calling
> > tcsendbreak() on a pty descriptor does nothing but reports success.
> > There are obviously two ways to fix it: either report an error, or
> > deliver the BREAK for real.
> 
> The pty master end is even less of a tty than the slave end, but this
> isn't really about errno. This patch doesn't address either of your
> points wrt tcsendbreak() on the slave descriptor which is the actual
> terminal end.

That's a valid point. And, indeed, the terminal end actually needs the
handling of BREAK to make it useful.

> > This patch implements the latter, adding at least one valid use case
> > to explain why it is better than the former.
> 
> I disagree that this is a valid use case for the _pty driver_.
> 
> AFAICT this is simply for convenience, as sysrq functionality is
> already available via sendkey.

That's a completely different story. This patch (after fixing it to
work with the terminal end) would allow me to set up a QEMU emulated
serial port using a pty (i.e. "-chardev pty") and send a BREAK signal
to it, no matter what is running in the guest.

I mean, I can run an emulated MIPS64 as a QEMU guest on an x86_64 host,
and still somehow pass SysRq to it. IIUC this will never be possible
with KVP.

Another use case: In my job, I'm struggling with different serial
consoles (some using ipmi SoL, some using telnet to a service
processor, some connected with a real RS-232 link). If I could send
BREAK over a pty, I could extend ipmiconsole to translate it to the SOL
message, telnet to translate it to the telnet escape, amtterm to send a
corresponding message... Then I could send a BREAK to any of my systems
simply by pressing 'C-A b' in screen(1) without having to think how is
this particular machine connected and what the correct sequence is for
that protocol.

Just my two cents,
Petr Tesarik

Re: [PATCH 1/1] pty: BREAK for pseudoterminals

[Peter Hurley]

On 02/16/2015 12:16 PM, Petr Tesarik wrote:
> On Mon, 16 Feb 2015 11:24:16 -0500
> Peter Hurley <peter@hurleysoftware.com> wrote:
> 
>> Hi Petr,
>>
>> On 02/16/2015 08:22 AM, Petr Tesarik wrote:
>>> On Mon, 16 Feb 2015 08:04:02 -0500
>>> Peter Hurley <peter@hurleysoftware.com> wrote:
>>>
>>>> On 02/05/2015 02:11 PM, Nan Li wrote:
>>>>> This will greatly enhance the usefulness of QEMU virtual serial ports, because the Linux kernel interprets a break on the serial console as a SysRq, but there is currently no way to pass this signal over a pseudo-terminal. This patch will work for transmitting BREAK from master to slave through pseudo-terminal.
>>>>
>>>> pty is an IPC mechanism, not a virtualization driver.
>>>
>>> No, but it can be used as a TTY. Teletypes have always had the capacity
>>> to send and receive BREAK.
>>
>> In some general-purpose but restricted capacity, the *slave* end _mimics_
>> a tty. That doesn't mean that it is suitable for every conceivable
>> use as a tty, nor should it.
> 
> Unless there's some specification of what should and what should not be
> implemented, this question is open for discussion, methinks.

This question is open for discussion regardless of specifications.
I thought that's what these emails were. :)

FWIW, here's the relevant excerpt from SUSv4 regarding tcsendbreak():

"If the terminal is not using asynchronous serial data transmission,
it is implementation-defined whether tcsendbreak() sends data to
generate a break condition or returns without taking any action."


>> If BREAK was actually useful for real terminal i/o, the pty driver
>> would already support this.
> 
> If I strictly followed this statement, no improvement would ever be
> possible. Or did I miss something? Are Linux PTYs a legacy subsystem
> that never gets any new features?

I'm not opposed to new features, but I do think that new kernel features
should only address those requirements which cannot be met in userspace
(whether that's functionality or performance or whatever the requirements).


>> [...]
>>> Well, the default termios includes IGNBRK, so unless they bothered
>>> to do anything about BREAKs, they won't see any change.
>>
>> Userspace programs are sloppy, especially with terminal i/o and
>> settings. Unlikely is not the same as not possible.
> 
> Sure. New features may break sloppy programs. OTOH, the obvious
> workaround is not using such programs together with new programs that
> actually use tcsendbreak() for something... until those sloppy programs
> are fixed. It's not like the whole system stops working once this patch
> is applied.

Userspace breakage is not an acceptable outcome, even if the program is
provably buggy (other than for security-related issues).


>>> Anyway, the current kernel behaviour is clearly suboptimal. Calling
>>> tcsendbreak() on a pty descriptor does nothing but reports success.
>>> There are obviously two ways to fix it: either report an error, or
>>> deliver the BREAK for real.
>>
>> The pty master end is even less of a tty than the slave end, but this
>> isn't really about errno. This patch doesn't address either of your
>> points wrt tcsendbreak() on the slave descriptor which is the actual
>> terminal end.
> 
> That's a valid point. And, indeed, the terminal end actually needs the
> handling of BREAK to make it useful.

There's two problems with adding this to the slave end:

1. The master pty termios is not programmable, so it can't set IGNBRK.
2. It creates a security maintenance burden because the unprivileged slave
   pty end must not be allowed to terminate the privileged master end,
   such as accidentally via BRKINT.


>>> This patch implements the latter, adding at least one valid use case
>>> to explain why it is better than the former.
>>
>> I disagree that this is a valid use case for the _pty driver_.
>>
>> AFAICT this is simply for convenience, as sysrq functionality is
>> already available via sendkey.
> 
> That's a completely different story. This patch (after fixing it to
> work with the terminal end) would allow me to set up a QEMU emulated
> serial port using a pty (i.e. "-chardev pty") and send a BREAK signal
> to it, no matter what is running in the guest.


> I mean, I can run an emulated MIPS64 as a QEMU guest on an x86_64 host,
> and still somehow pass SysRq to it. IIUC this will never be possible
> with KVP.



> Another use case: In my job, I'm struggling with different serial
> consoles (some using ipmi SoL, some using telnet to a service
> processor, some connected with a real RS-232 link). If I could send
> BREAK over a pty, I could extend ipmiconsole to translate it to the SOL
> message, telnet to translate it to the telnet escape, amtterm to send a
> corresponding message... Then I could send a BREAK to any of my systems
> simply by pressing 'C-A b' in screen(1) without having to think how is
> this particular machine connected and what the correct sequence is for
> that protocol.
> 
> Just my two cents,
> Petr Tesarik
> 

Re: [PATCH 1/1] pty: BREAK for pseudoterminals

[Peter Hurley]

On 02/16/2015 01:30 PM, Peter Hurley wrote:
> On 02/16/2015 12:16 PM, Petr Tesarik wrote:
>> On Mon, 16 Feb 2015 11:24:16 -0500
>> Peter Hurley <peter@hurleysoftware.com> wrote:
>>> On 02/16/2015 08:22 AM, Petr Tesarik wrote:
>>>> On Mon, 16 Feb 2015 08:04:02 -0500
>>>> Peter Hurley <peter@hurleysoftware.com> wrote:
>>>>> On 02/05/2015 02:11 PM, Nan Li wrote:

[...]

>>> AFAICT this is simply for convenience, as sysrq functionality is
>>> already available via sendkey.
>>
>> That's a completely different story. This patch (after fixing it to
>> work with the terminal end) would allow me to set up a QEMU emulated
>> serial port using a pty (i.e. "-chardev pty") and send a BREAK signal
>> to it, no matter what is running in the guest.
> 
> 
>> I mean, I can run an emulated MIPS64 as a QEMU guest on an x86_64 host,
>> and still somehow pass SysRq to it. IIUC this will never be possible
>> with KVP.

Sorry about that; accidentally pressed send :/

I see this as a shortcoming of the emulation, not of the underlying
IPC used. I don't see why this couldn't be done in-band with any IPC.


>> Another use case: In my job, I'm struggling with different serial
>> consoles (some using ipmi SoL, some using telnet to a service
>> processor, some connected with a real RS-232 link). If I could send
>> BREAK over a pty, I could extend ipmiconsole to translate it to the SOL
>> message, telnet to translate it to the telnet escape, amtterm to send a
>> corresponding message... Then I could send a BREAK to any of my systems
>> simply by pressing 'C-A b' in screen(1) without having to think how is
>> this particular machine connected and what the correct sequence is for
>> that protocol.

I need to think more on this.

Regards,
Peter Hurley

PS - it's interesting that you mention a service processor, because the
hacked support for remote supervisor adapter is and has been the #1
barrier to splitting the 8250 driver. I literally have spent days trying
to come up with an acceptable solution.

Re: [PATCH 1/1] pty: BREAK for pseudoterminals

[Peter Hurley]

On 02/16/2015 08:04 AM, Peter Hurley wrote:
> On 02/05/2015 02:11 PM, Nan Li wrote:
>> This will greatly enhance the usefulness of QEMU virtual serial ports, because the Linux kernel interprets a break on the serial console as a SysRq, but there is currently no way to pass this signal over a pseudo-terminal. This patch will work for transmitting BREAK from master to slave through pseudo-terminal.
> 
> pty is an IPC mechanism, not a virtualization driver.
> 
> Those programs that know they're consuming from a slave pty may not
> have bothered to set termios for BREAKs, and may not be prepared
> for NULLs in the input stream.

I'm not even seeing how this works as advertised.

In QEMU, the *master* end is the source/sink for the guest VM, while the
*slave* pty is sink/source for the host. Like this:

Host <--> slave pty <===> master pty <--> qemu <==> emulated serial port

So this patch enables the guest VM to transmit BREAK to the host.
Why is that useful?

Regards,
Peter Hurley

>> Signed-off-by: Nan Li <nli@suse.com>
>> ---
>>  drivers/tty/pty.c | 19 +++++++++++++++++++
>>  1 file changed, 19 insertions(+)
>>
>> diff --git a/drivers/tty/pty.c b/drivers/tty/pty.c
>> index e72ee62..ac8893a 100644
>> --- a/drivers/tty/pty.c
>> +++ b/drivers/tty/pty.c
>> @@ -31,6 +31,7 @@
>>  static struct tty_driver *ptm_driver;
>>  static struct tty_driver *pts_driver;
>>  static DEFINE_MUTEX(devpts_mutex);
>> +#define BREAK_STRING '\0'
>>  #endif
>>  
>>  static void pty_close(struct tty_struct *tty, struct file *filp)
>> @@ -674,6 +675,23 @@ static void pty_unix98_shutdown(struct tty_struct *tty)
>>  	devpts_kill_index(tty->driver_data, tty->index);
>>  }
>>  
>> +static int pty_unix98_break_ctl(struct tty_struct *tty, int break_state)
>> +{
>> +	int c;
>> +	struct tty_struct *to = tty->link;
>> +
>> +	if (break_state) {
>> +		/* Stuff the break data into the input queue of the other end */
>> +		c = tty_insert_flip_char(to->port, BREAK_STRING, TTY_BREAK);
>> +		/* And shovel */
>> +		if (c)
>> +			tty_flip_buffer_push(to->port);
>> +		else
>> +			return -ENOMEM;
>> +	}
>> +	return 0;
>> +}
>> +
>>  static const struct tty_operations ptm_unix98_ops = {
>>  	.lookup = ptm_unix98_lookup,
>>  	.install = pty_unix98_install,
>> @@ -686,6 +704,7 @@ static const struct tty_operations ptm_unix98_ops = {
>>  	.chars_in_buffer = pty_chars_in_buffer,
>>  	.unthrottle = pty_unthrottle,
>>  	.ioctl = pty_unix98_ioctl,
>> +	.break_ctl = pty_unix98_break_ctl,
>>  	.resize = pty_resize,
>>  	.shutdown = pty_unix98_shutdown,
>>  	.cleanup = pty_cleanup
>>
> 

Re: [PATCH 1/1] pty: BREAK for pseudoterminals

[Petr Tesarik]

On Tue, 17 Feb 2015 14:03:58 -0500
Peter Hurley <peter@hurleysoftware.com> wrote:

> On 02/16/2015 08:04 AM, Peter Hurley wrote:
> > On 02/05/2015 02:11 PM, Nan Li wrote:
> >> This will greatly enhance the usefulness of QEMU virtual serial ports, because the Linux kernel interprets a break on the serial console as a SysRq, but there is currently no way to pass this signal over a pseudo-terminal. This patch will work for transmitting BREAK from master to slave through pseudo-terminal.
> > 
> > pty is an IPC mechanism, not a virtualization driver.
> > 
> > Those programs that know they're consuming from a slave pty may not
> > have bothered to set termios for BREAKs, and may not be prepared
> > for NULLs in the input stream.
> 
> I'm not even seeing how this works as advertised.
> 
> In QEMU, the *master* end is the source/sink for the guest VM, while the
> *slave* pty is sink/source for the host. Like this:

Yes, you're totally right. This patch implements break handling on the
"wrong" side of the pty pair. I think you have already pointed this
out in one of the previous mails, and we're waiting for a patch that
implements it on the slave side.

Regards,
Petr Tesarik

> Host <--> slave pty <===> master pty <--> qemu <==> emulated serial
> port
> 
> So this patch enables the guest VM to transmit BREAK to the host.
> Why is that useful?
> 
> Regards,
> Peter Hurley
> 
> >> Signed-off-by: Nan Li <nli@suse.com>
> >> ---
> >>  drivers/tty/pty.c | 19 +++++++++++++++++++
> >>  1 file changed, 19 insertions(+)
> >>
> >> diff --git a/drivers/tty/pty.c b/drivers/tty/pty.c
> >> index e72ee62..ac8893a 100644
> >> --- a/drivers/tty/pty.c
> >> +++ b/drivers/tty/pty.c
> >> @@ -31,6 +31,7 @@
> >>  static struct tty_driver *ptm_driver;
> >>  static struct tty_driver *pts_driver;
> >>  static DEFINE_MUTEX(devpts_mutex);
> >> +#define BREAK_STRING '\0'
> >>  #endif
> >>  
> >>  static void pty_close(struct tty_struct *tty, struct file *filp)
> >> @@ -674,6 +675,23 @@ static void pty_unix98_shutdown(struct
> >> tty_struct *tty) devpts_kill_index(tty->driver_data, tty->index);
> >>  }
> >>  
> >> +static int pty_unix98_break_ctl(struct tty_struct *tty, int
> >> break_state) +{
> >> +	int c;
> >> +	struct tty_struct *to = tty->link;
> >> +
> >> +	if (break_state) {
> >> +		/* Stuff the break data into the input queue of
> >> the other end */
> >> +		c = tty_insert_flip_char(to->port, BREAK_STRING,
> >> TTY_BREAK);
> >> +		/* And shovel */
> >> +		if (c)
> >> +			tty_flip_buffer_push(to->port);
> >> +		else
> >> +			return -ENOMEM;
> >> +	}
> >> +	return 0;
> >> +}
> >> +
> >>  static const struct tty_operations ptm_unix98_ops = {
> >>  	.lookup = ptm_unix98_lookup,
> >>  	.install = pty_unix98_install,
> >> @@ -686,6 +704,7 @@ static const struct tty_operations
> >> ptm_unix98_ops = { .chars_in_buffer = pty_chars_in_buffer,
> >>  	.unthrottle = pty_unthrottle,
> >>  	.ioctl = pty_unix98_ioctl,
> >> +	.break_ctl = pty_unix98_break_ctl,
> >>  	.resize = pty_resize,
> >>  	.shutdown = pty_unix98_shutdown,
> >>  	.cleanup = pty_cleanup
> >>
> > 
> 

Re: [PATCH 1/1] pty: BREAK for pseudoterminals

[One Thousand Gnomes]

On Fri,  6 Feb 2015 03:11:29 +0800
Nan Li <nli@suse.com> wrote:

> This will greatly enhance the usefulness of QEMU virtual serial ports, because the Linux kernel interprets a break on the serial console as a SysRq, but there is currently no way to pass this signal over a pseudo-terminal. This patch will work for transmitting BREAK from master to slave through pseudo-terminal.

Break is a signalling event not a 0 character. pty doesn't support modem
or other serial signalling events. Whether some kind of virtio tty<->host
interface should exist and do that I don't know, but pty does not.

Alan