linux-kernel - Re: Does anyone care about a race free ptsname?

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Date:	Tue, 19 Apr 2016 20:48:41 -0500
From:	"Serge E. Hallyn" <serge@...lyn.com>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Andy Lutomirski <luto@...capital.net>, security@...ian.org,
	"security@...nel.org" <security@...nel.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	"security@...ntu.com >> security" <security@...ntu.com>,
	Peter Hurley <peter@...leysoftware.com>,
	Serge Hallyn <serge.hallyn@...ntu.com>,
	Willy Tarreau <w@....eu>,
	Aurelien Jarno <aurelien@...el32.net>,
	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>,
	Jann Horn <jann@...jh.net>, Greg KH <greg@...ah.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Jiri Slaby <jslaby@...e.com>, Florian Weimer <fw@...eb.enyo.de>
Subject: Re: Does anyone care about a race free ptsname?

On Tue, Apr 19, 2016 at 04:39:07PM -0700, H. Peter Anvin wrote:
> On 04/19/2016 04:23 PM, Linus Torvalds wrote:
> > On Tue, Apr 19, 2016 at 11:44 AM, Eric W. Biederman
> > <ebiederm@...ssion.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

That's what i was thinking, since as I said many users just want

        blah = ptsname(fd);
        slavefd = open(blah, ...);

so they really just want an open fd.

> 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