linux-kernel - Re: [PATCH 01/13] devpts: Teach /dev/ptmx to find the associated devpts via path lookup

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Message-ID: <83FE8CD2-C0A2-4ADB-AEBD-8DD89AD4F88A@zytor.com>
Date:	Sat, 09 Apr 2016 07:10:04 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>,
	ebiederm@...ssion.com
CC:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Peter Hurley <peter@...leysoftware.com>,
	Greg KH <greg@...ah.com>, Jiri Slaby <jslaby@...e.com>,
	Aurelien Jarno <aurelien@...el32.net>,
	Andy Lutomirski <luto@...capital.net>,
	Florian Weimer <fw@...eb.enyo.de>,
	Al Viro <viro@...iv.linux.org.uk>,
	Serge Hallyn <serge.hallyn@...ntu.com>,
	Jann Horn <jann@...jh.net>,
	"security@...nel.org" <security@...nel.org>,
	"security@...ntu.com >> security" <security@...ntu.com>,
	security@...ian.org, Willy Tarreau <w@....eu>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 01/13] devpts: Teach /dev/ptmx to find the associated devpts via path lookup

On April 9, 2016 6:09:09 AM PDT, One Thousand Gnomes <gnomes@...rguk.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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