linux-kernel - Re: [PATCH] tty: Add EXTPROC support for LINEMODE

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Message-ID: <4C1AB305.5000602@symas.com>
Date:	Thu, 17 Jun 2010 16:43:01 -0700
From:	Howard Chu <hyc@...as.com>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
CC:	linux-kernel@...r.kernel.org, Greg KH <greg@...ah.com>
Subject: Re: [PATCH] tty: Add EXTPROC support for LINEMODE

Howard Chu wrote:
> Alan Cox wrote:
>>>>> For Alpha this value should match OSF if possible.
>>>
>>> OSF didn't define this flag, nor did it assign that particular bit to any
>>> purpose. Is that good enough?
>>
>> Fine
>>
>>>> Are you suggesting that this is completely unfixable/unworkable? Would it be
>>>> sufficient to use kernel_termios_to_user_termios() ?
>>
>> I don't see a way to fix it sanely
>>
>>>>
>>> Actually using kernel_termios_to_user_termios_1(). In all supported
>>> architectures this structure is basically aligned with but smaller than the
>>> userland struct termios.
>>
>> The relationship isn't quite so simple and it may change in the future,
>> so this seems to be a very bad idea. Besides which syscalls are *cheap*
>> so simply notifying someone to reread the terminal data they care about
>> should be fine. In that sense it seems SVR4 got it right.

> OK. I'm fine with only setting a bit in the packet header, and letting the
> application do an ioctl/tcgetattr to discover the actual state.

Just deleting that part of the patch was simple enough. The TIOCPKT_IOCTL bit 
still gets set in the packet header byte; userspace apps will just have to do 
an ioctl to retrieve the state when the bit is set.

I've also added locking to the pty_signal() function. The check for tty->link
seems a bit paranoid, but a few other functions do it as well.

/* Send a signal to the slave */
static int pty_signal(struct tty_struct *tty, int sig)
{
          unsigned long flags;
          struct pid *pgrp;

          if (tty->link) {
                  spin_lock_irqsave(&tty->link->ctrl_lock, flags);
                  pgrp = get_pid(tty->link->pgrp);
                  spin_unlock_irqrestore(&tty->link->ctrl_lock, flags);

                  kill_pgrp(pgrp, sig, 1);
                  put_pid(pgrp);
          }
          return 0;
}

That covers all the feedback so far. I'll be reposting the entire patch again 
shortly, unless you have any additional thoughts.

-- 
   -- Howard Chu
   CTO, Symas Corp.           http://www.symas.com
   Director, Highland Sun     http://highlandsun.com/hyc/
   Chief Architect, OpenLDAP  http://www.openldap.org/project/
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