Subject: Re: Does anyone care about a race free ptsname?
To: Linus Torvalds <torvalds@linux-foundation.org>,
        "Eric W. Biederman" <ebiederm@xmission.com>
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Cc: Andy Lutomirski <luto@amacapital.net>, security@debian.org,
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        One Thousand Gnomes <gnomes@lxorguk.ukuu.org.uk>,
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        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Jiri Slaby <jslaby@suse.com>, Florian Weimer <fw@deneb.enyo.de>
From: "H. Peter Anvin" <hpa@zytor.com>
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Date: Tue, 19 Apr 2016 16:39:07 -0700
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On 04/19/2016 04:23 PM, Linus Torvalds wrote:
> On Tue, Apr 19, 2016 at 11:44 AM, Eric W. Biederman
> <ebiederm@xmission.com> wrote:
>>
>> I will take a look in a minute.   Before I do that I want to mention
>> why I care about /dev/pts/ptmx.
>>
>> There is a posix function that is widely used called ptsname.  It's
>> function is to take a master file descriptor and returns the path to the
>> slave.
>>
>> All we have in the kernel to support ptsname is an ioctl TIOCGPTN that
>> returns the pty number in the appropriate instance of devpts.
> 
> Don't bother with that completely mis-designed interface.It's crap.
> 
> So we'll keep it working for legacy models, but the whole "return an
> integer index" is just pure shit. It's not worth worrying about.
> 
> We can (and probably should) just introduce a new ioctl or even a
> system call that just does the sane thing and returns the pathname
> from the kernel.
> 

What do you think of the idea of TIOCPTSOPEN (or whatever) to get a file
descriptor for the slave device given the master device?  Then we can
use realpath() or a readlink on /proc/self/fd/# to get the pathname if
needed.

(Incidentally, we added getcwd() as a system call.  Should we add
[f]realpath() as a system call too?)

	-hpa