[PATCHv2] tty: Set correct tty name in 'active' sysfs attribute

[PATCHv2] tty: Set correct tty name in 'active' sysfs attribute

[Hannes Reinecke]

The 'active' sysfs attribute should refer to the currently
active tty devices the console is running on, not the currently
active console.
The console structure doesn't refer to any device in sysfs,
only the tty the console is running on has.
So we need to print out the tty names in 'active', not
the console names.

Cc: Lennart Poettering <lennart@poettering.net>
Cc: Kay Sievers <kay@vrfy.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jiri Slaby <jslaby@suse.cz>
Signed-off-by: Werner Fink <werner@suse.de>
Signed-off-by: Hannes Reinecke <hare@suse.de>
---
 drivers/tty/tty_io.c | 23 +++++++++++++++++------
 1 file changed, 17 insertions(+), 6 deletions(-)

diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
index c74a00a..99af614 100644
--- a/drivers/tty/tty_io.c
+++ b/drivers/tty/tty_io.c
@@ -1271,12 +1271,13 @@ static void pty_line_name(struct tty_driver *driver, int index, char *p)
  *
  *	Locking: None
  */
-static void tty_line_name(struct tty_driver *driver, int index, char *p)
+static ssize_t tty_line_name(struct tty_driver *driver, int index, char *p)
 {
 	if (driver->flags & TTY_DRIVER_UNNUMBERED_NODE)
-		strcpy(p, driver->name);
+		return sprintf(p, "%s", driver->name);
 	else
-		sprintf(p, "%s%d", driver->name, index + driver->name_base);
+		return sprintf(p, "%s%d", driver->name,
+			       index + driver->name_base);
 }
 
 /**
@@ -3545,9 +3546,19 @@ static ssize_t show_cons_active(struct device *dev,
 		if (i >= ARRAY_SIZE(cs))
 			break;
 	}
-	while (i--)
-		count += sprintf(buf + count, "%s%d%c",
-				 cs[i]->name, cs[i]->index, i ? ' ':'\n');
+	while (i--) {
+		struct tty_driver *driver;
+		const char *name = cs[i]->name;
+		int index = cs[i]->index;
+
+		driver = cs[i]->device(cs[i], &index);
+		if (driver) {
+			count += tty_line_name(driver, index, buf + count);
+			count += sprintf(buf + count, "%c", i ? ' ':'\n');
+		} else
+			count += sprintf(buf + count, "%s%d%c",
+					 name, index, i ? ' ':'\n');
+	}
 	console_unlock();
 
 	return count;
-- 
1.7.12.4

Re: [PATCHv2] tty: Set correct tty name in 'active' sysfs attribute

[Greg Kroah-Hartman]

On Thu, Feb 06, 2014 at 03:27:43PM +0100, Hannes Reinecke wrote:
> The 'active' sysfs attribute should refer to the currently
> active tty devices the console is running on, not the currently
> active console.

That's not what Documentation/ABI/sysfs-tty says:
                 Shows the list of currently configured                         
                 console devices, like 'tty1 ttyS0'.                            
                 The last entry in the file is the active                       
                 device connected to /dev/console.                              
                 The file supports poll() to detect virtual                     
                 console switches.                         

> The console structure doesn't refer to any device in sysfs,
> only the tty the console is running on has.

That sentance doesn't make sense.

> So we need to print out the tty names in 'active', not
> the console names.

But that doesn't match the documentation.

What exactly are you trying to "fix" here?  What is the problem that the
current file has that is broken?  And as you are changing what this file
means, what will break if the information in the file changes?


thanks,

greg k-h

Re: [PATCHv2] tty: Set correct tty name in 'active' sysfs attribute

[Hannes Reinecke]

On 02/06/2014 04:29 PM, Greg Kroah-Hartman wrote:
> On Thu, Feb 06, 2014 at 03:27:43PM +0100, Hannes Reinecke wrote:
>> The 'active' sysfs attribute should refer to the currently
>> active tty devices the console is running on, not the currently
>> active console.
> 
> That's not what Documentation/ABI/sysfs-tty says:
>                  Shows the list of currently configured                         
>                  console devices, like 'tty1 ttyS0'.                            
>                  The last entry in the file is the active                       
>                  device connected to /dev/console.                              
>                  The file supports poll() to detect virtual                     
>                  console switches.                         
> 
The problem is indeed with 'console devices'. There is no such
thing; you only have tty devices where the console is running on.

>> The console structure doesn't refer to any device in sysfs,
>> only the tty the console is running on has.
> 
> That sentance doesn't make sense.
> 
>> So we need to print out the tty names in 'active', not
>> the console names.
> 
> But that doesn't match the documentation.
> 
> What exactly are you trying to "fix" here?  What is the problem that the
> current file has that is broken?  And as you are changing what this file
> means, what will break if the information in the file changes?
> 
systemd is using the 'active' sysfs attribute to figure out on which
_tty_ device to start a getty on.
As soon as the console name and the tty name are different
you have no means of figuring out which _device_ to open.
AFAICS the console 'device' (ie the current entry in 'active')
doesn't have _any_ equivalent in sysfs; it just so happens that for
most console drivers the tty driver name is identical.
But this is not a requirement, and fails for drivers which have a
different device for the console and the tty.

EG on S/390 the 3270 tty has the devices

/dev/3270/tty1

but the console driver announces the name 'tty3270'.
So as per current rules the 'active' attribute contains

tty32700

which correct as per documentation, but doesn't have _any_
equivalent in sysfs.

Martin has the grubby details here.

But of course, the documentation should be updated to match the new
behavior.

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		      zSeries & Storage
hare@suse.de			      +49 911 74053 688
SUSE LINUX Products GmbH, Maxfeldstr. 5, 90409 Nürnberg
GF: J. Hawn, J. Guild, F. Imendörffer, HRB 16746 (AG Nürnberg)

Re: [PATCHv2] tty: Set correct tty name in 'active' sysfs attribute

[Greg Kroah-Hartman]

On Thu, Feb 06, 2014 at 04:44:20PM +0100, Hannes Reinecke wrote:
> On 02/06/2014 04:29 PM, Greg Kroah-Hartman wrote:
> > On Thu, Feb 06, 2014 at 03:27:43PM +0100, Hannes Reinecke wrote:
> >> The 'active' sysfs attribute should refer to the currently
> >> active tty devices the console is running on, not the currently
> >> active console.
> > 
> > That's not what Documentation/ABI/sysfs-tty says:
> >                  Shows the list of currently configured                         
> >                  console devices, like 'tty1 ttyS0'.                            
> >                  The last entry in the file is the active                       
> >                  device connected to /dev/console.                              
> >                  The file supports poll() to detect virtual                     
> >                  console switches.                         
> > 
> The problem is indeed with 'console devices'. There is no such
> thing; you only have tty devices where the console is running on.
> 
> >> The console structure doesn't refer to any device in sysfs,
> >> only the tty the console is running on has.
> > 
> > That sentance doesn't make sense.
> > 
> >> So we need to print out the tty names in 'active', not
> >> the console names.
> > 
> > But that doesn't match the documentation.
> > 
> > What exactly are you trying to "fix" here?  What is the problem that the
> > current file has that is broken?  And as you are changing what this file
> > means, what will break if the information in the file changes?
> > 
> systemd is using the 'active' sysfs attribute to figure out on which
> _tty_ device to start a getty on.
> As soon as the console name and the tty name are different
> you have no means of figuring out which _device_ to open.
> AFAICS the console 'device' (ie the current entry in 'active')
> doesn't have _any_ equivalent in sysfs; it just so happens that for
> most console drivers the tty driver name is identical.
> But this is not a requirement, and fails for drivers which have a
> different device for the console and the tty.
> 
> EG on S/390 the 3270 tty has the devices
> 
> /dev/3270/tty1
> 
> but the console driver announces the name 'tty3270'.
> So as per current rules the 'active' attribute contains
> 
> tty32700
> 
> which correct as per documentation, but doesn't have _any_
> equivalent in sysfs.
> 
> Martin has the grubby details here.
> 
> But of course, the documentation should be updated to match the new
> behavior.

Ok, care to send an updated version, that fixes the Documentation as
well?  If Kay agrees that this is the correct solution, I'll be glad to
take it.

thanks,

greg k-h

Re: [PATCHv2] tty: Set correct tty name in 'active' sysfs attribute

[Kay Sievers]

On Thu, Feb 6, 2014 at 5:29 PM, Greg Kroah-Hartman
<gregkh@linuxfoundation.org> wrote:
> On Thu, Feb 06, 2014 at 04:44:20PM +0100, Hannes Reinecke wrote:
>> On 02/06/2014 04:29 PM, Greg Kroah-Hartman wrote:
>> > On Thu, Feb 06, 2014 at 03:27:43PM +0100, Hannes Reinecke wrote:
>> >> The 'active' sysfs attribute should refer to the currently
>> >> active tty devices the console is running on, not the currently
>> >> active console.
>> >
>> > That's not what Documentation/ABI/sysfs-tty says:
>> >                  Shows the list of currently configured
>> >                  console devices, like 'tty1 ttyS0'.
>> >                  The last entry in the file is the active
>> >                  device connected to /dev/console.
>> >                  The file supports poll() to detect virtual
>> >                  console switches.
>> >
>> The problem is indeed with 'console devices'. There is no such
>> thing; you only have tty devices where the console is running on.
>>
>> >> The console structure doesn't refer to any device in sysfs,
>> >> only the tty the console is running on has.
>> >
>> > That sentance doesn't make sense.
>> >
>> >> So we need to print out the tty names in 'active', not
>> >> the console names.
>> >
>> > But that doesn't match the documentation.
>> >
>> > What exactly are you trying to "fix" here?  What is the problem that the
>> > current file has that is broken?  And as you are changing what this file
>> > means, what will break if the information in the file changes?
>> >
>> systemd is using the 'active' sysfs attribute to figure out on which
>> _tty_ device to start a getty on.
>> As soon as the console name and the tty name are different
>> you have no means of figuring out which _device_ to open.
>> AFAICS the console 'device' (ie the current entry in 'active')
>> doesn't have _any_ equivalent in sysfs; it just so happens that for
>> most console drivers the tty driver name is identical.
>> But this is not a requirement, and fails for drivers which have a
>> different device for the console and the tty.
>>
>> EG on S/390 the 3270 tty has the devices
>>
>> /dev/3270/tty1
>>
>> but the console driver announces the name 'tty3270'.
>> So as per current rules the 'active' attribute contains
>>
>> tty32700
>>
>> which correct as per documentation, but doesn't have _any_
>> equivalent in sysfs.
>>
>> Martin has the grubby details here.
>>
>> But of course, the documentation should be updated to match the new
>> behavior.
>
> Ok, care to send an updated version, that fixes the Documentation as
> well?  If Kay agrees that this is the correct solution, I'll be glad to
> take it.

Sounds good to me. The intention clearly was to point to the device in use,
which we can find then.

I would not expect problems with this change. For common uses it is the
same name already and nothing visibly should change, and for the ones
where it isn't the same, I expect it is not too useful to find the driver
name.

Thanks,
Kay

Re: [PATCHv2] tty: Set correct tty name in 'active' sysfs attribute

[One Thousand Gnomes]

> Sounds good to me. The intention clearly was to point to the device in use,
> which we can find then.

I would concur.

> I would not expect problems with this change. For common uses it is the
> same name already and nothing visibly should change, and for the ones
> where it isn't the same, I expect it is not too useful to find the driver
> name.

Very few tools use it so if it doesn't break any distro init setups
(upstart, sysvinit, oh and that other one....) it ought to be fine.

Alan