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

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Message-ID: <B0092104-8559-42D5-BE05-9A2EA3BC9A59@zytor.com>
Date:	Tue, 19 Apr 2016 12:16:13 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	ebiederm@...ssion.com,
	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	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 April 19, 2016 11:44:40 AM PDT, ebiederm@...ssion.com wrote:
>Linus Torvalds <torvalds@...ux-foundation.org> writes:
>
>> What this does is get rid of the horrible notion of having that
>>
>>     struct inode *ptmx_inode
>>
>> be the interface between the pty code and devpts. By de-emphasizing
>the 
>> ptmx inode, a lot of things actually get cleaner, and we will have a
>much 
>> saner way forward.
>
>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.
>
>The only way we have today to query which instance of devpts the pty is
>on is through fstat and look st_dev to see if the file is on the
>correct
>filesystem.  This works when /dev/pts/ptmx is used and fails when
>/dev/ptmx is used.
>
>Does anyone else care?
>
>If no one cares I will stop worrying about it and just get on with
>fixing the rest of this mess which there definitely seems to be the
>will
>to do.
>
>Eric

We could add another ioctl for that purpose of we need to.  Perhaps an ioctl which returns a file descriptor to the slave device?

However, since we are now defining ptmx to explicitly look up pts/ by name it seems like /dev/ptmx -> /dev/pts/# is true by definition.  If what you worry about is namespace reshuffling then ptsname() is the wrong interface in the first place since it returns a pathname.

Fwiw, in klibc ptsname() is basically just an sprintf().
-- 
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