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

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Date:	Sat, 09 Apr 2016 09:45:46 -0500
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>,
	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\@kernel.org" <security@...nel.org>,
	"security\@ubuntu.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

"H. Peter Anvin" <hpa@...or.com> writes:

> 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.

At a quick skim it does look like userspace uses posix_openpt for the
most part.  Certainly portable apps that can run on FreeBSD do.
And just grepping through binaries all of the ones I have checked so far
are calling posix_openpt.

Peter if you or someone could start updating the userspace version of
posix_openpt to use /dev/pts/ptmx when available over /dev/ptmx in
parallel to the kernel work to always allow mount of devpts to give
distinct instances that would be great.

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

Yeah.  I am going to look and see if I can move this controversial bit
to a separate patch so we can discuss it more conviniently.

Eric