linux-kernel - Re: [PATCH] devpts: Sensible /dev/ptmx & force newinstance

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Date:	Fri, 11 Dec 2015 14:24:05 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Al Viro <viro@...iv.linux.org.uk>, Greg KH <greg@...ah.com>,
	Jiri Slaby <jslaby@...e.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Aurelien Jarno <aurelien@...el32.net>,
	Florian Weimer <fw@...eb.enyo.de>,
	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@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] devpts: Sensible /dev/ptmx & force newinstance

On Fri, Dec 11, 2015 at 2:18 PM, H. Peter Anvin <hpa@...or.com> wrote:
> On 12/11/15 14:12, Andy Lutomirski wrote:
>>>
>>> For the newinstance case st_dev should match between the master and the
>>> slave.  Unfortunately this is not the case for a legacy ptmx, as a
>>> stat() on the master descriptor still returns the st_dev, st_rdev, and
>>> st_ino for the ptmx device node.
>>
>> Sure, but I'm not talking about stat.  I'm saying that we could add a
>> new ioctl that works on any ptmx fd (/dev/ptmx or /dev/pts/ptmx) that
>> answers the question "does this ptmx logically belong to the given
>> devpts filesystem".
>>
>> Since it's not stat, we can make it do whatever we want, including
>> following a link to the devpts instance that isn't f_path or f_inode.
>>
>
> Sure.  My thinking, though, was whether or not we can do something that
> works on legacy kernels, and/or is less intrusive than new ioctls.
>
> What is the actual operation we need?

To do the whole shebang at once:

ioctl(ptmx_fd, TIOCWHATEVER, fd_to_devpts_mount);

returns the slave number if fd_to_devpts_mount points to the right
place or an error if not.

ptsname(fd) logically does:

fd_to_devpts_mount = open("/dev/pts", O_RDONLY | O_DIRECTORY);
int n = ioctl(fd, TIOCWHATEVER, fd_to_devpts_mount);
close(fd_to_devpts_mount);
if (n < 0)
  return some error;
return "/dev/pts/" + n;

I think that all kinds of variants are possible.

--Andy

>
>         -hpa
>



-- 
Andy Lutomirski
AMA Capital Management, LLC
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