linux-kernel - Re: [PATCHv3] tty: Set correct tty name in 'active' sysfs attribute

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Date:	Tue, 25 Feb 2014 10:38:04 +0100
From:	David Herrmann <dh.herrmann@...il.com>
To:	Hannes Reinecke <hare@...e.de>
Cc:	linux-kernel <linux-kernel@...r.kernel.org>,
	Ray Strode <halfline@...il.com>,
	Lennart Poettering <lennart@...ttering.net>,
	Kay Sievers <kay@...y.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Jiri Slaby <jslaby@...e.cz>, Werner Fink <werner@...e.de>
Subject: Re: [PATCHv3] tty: Set correct tty name in 'active' sysfs attribute

Hi

On Tue, Feb 25, 2014 at 8:51 AM, Hannes Reinecke <hare@...e.de> wrote:
> On 02/24/2014 03:58 PM, David Herrmann wrote:
>> Hi
>>
>> On Mon, Feb 24, 2014 at 3:50 PM, Hannes Reinecke <hare@...e.de> wrote:
>>> 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.
>>> But we need to take care for the virtual console to always display
>>> 'tty0' so as not to break existing programs.
>>>
>>> Cc: Lennart Poettering <lennart@...ttering.net>
>>> Cc: Kay Sievers <kay@...y.org>
>>> Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
>>> Cc: Jiri Slaby <jslaby@...e.cz>
>>> Signed-off-by: Werner Fink <werner@...e.de>
>>> Signed-off-by: Hannes Reinecke <hare@...e.de>
>>> ---
>>>  drivers/tty/tty_io.c | 24 ++++++++++++++++++------
>>>  1 file changed, 18 insertions(+), 6 deletions(-)
>>>
>>> diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
>>> index c74a00a..96eb462 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 +3546,20 @@ 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);
>>> +               /* Some programs rely on 'tty0' for console */
>>> +               if (driver && (index > 0 || driver->major != TTY_MAJOR)) {
>>
>> As Ray mentioned in the previous discussion, this has to be
>> cs[i]->index instead of "index" as ->device() changes the parameter.
>> See drivers/tty/vt/vt.c vt_console_device().
>>
> Positive?
> I thought this was precisely the problem, ->device() changing the
> index '0' into something non-zero.
> The reports we had were that the line 'tty0' changed into 'tty1'.
> Hence ->device() converted cs[i]->index (which is '0') into index
> (which is '1').
> Hence the check would be correct, wouldn't it?

If "cs[i]" points to tty0, then cs[i]->index is 0. If you call
->device(), it will store 1 (or !=0) in "index". Thus, "(driver &&
(index > 0))" will be true and you will write tty1 into the file
instead of tty0. So you don't want to check whether the new value is
non-zero, but whether the *previous* value was 0, turning this into:

if (driver && (cs[i]->index > 0 || driver->major != TTY_MAJOR))

So loosely speaking, we use the new code only for devices which either
are not a VT or have an idx > 0. Otherwise, we use our fallback.

Thanks
David
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