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Subject: Re: [PATCH 01/13] devpts: Teach /dev/ptmx to find the associated devpts via path lookup
From: "H. Peter Anvin" <hpa@zytor.com>
Date: Sat, 09 Apr 2016 07:10:04 -0700
To: One Thousand Gnomes <gnomes@lxorguk.ukuu.org.uk>, ebiederm@xmission.com
CC: Linus Torvalds <torvalds@linux-foundation.org>,
        Peter Hurley <peter@hurleysoftware.com>, Greg KH <greg@kroah.com>,
        Jiri Slaby <jslaby@suse.com>, Aurelien Jarno <aurelien@aurel32.net>,
        Andy Lutomirski <luto@amacapital.net>,
        Florian Weimer <fw@deneb.enyo.de>, Al Viro <viro@zeniv.linux.org.uk>,
        Serge Hallyn <serge.hallyn@ubuntu.com>, Jann Horn <jann@thejh.net>,
        "security@kernel.org" <security@kernel.org>,
        "security@ubuntu.com >> security" <security@ubuntu.com>,
        security@debian.org, Willy Tarreau <w@1wt.eu>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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On April 9, 2016 6:09:09 AM PDT, One Thousand Gnomes <gnomes@lxorguk.ukuu.org.uk> wrote:
>
>> If anyone has a better idea on how userspace should connect the
>master
>> pty file descriptor the slave file descriptor, I would be willing to
>> implement that instead.
>
>If we are willing to go away from the existing mess of a tty interface
>inflicted on us by BSD and then mashed up by POSIX then a syscall of
>
>  int err = ptypair(int fd[2], int perms, int flags);
>
>[where flags is the O_ ones we usually need to cover (CLOEXEC etc) and
>maybe even some kind of "private" flag to say don't even expose it via
>devpts).
>
>would do remarkably sane things to the majoirty of use cases as it
>breaks
>the dependence on grantpt and also the historic screwup that pty pairs
>aren't allocated atomically with both file handles returned as pipe()
>does.
>
>Alan

We don't even need to do that if we'd be willing to change the user space interface... if we could rely on the POSIX interface then posix_openpt() could simply open /dev/pts/ptmx and everything would just work.

The trick here is how to make it work in the presence of some extremely bad practices in existing userspace.
-- 
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