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Subject: Re: Does anyone care about a race free ptsname?
From: "H. Peter Anvin" <hpa@zytor.com>
Date: Tue, 19 Apr 2016 12:16:13 -0700
To: ebiederm@xmission.com, Linus Torvalds <torvalds@linux-foundation.org>
CC: Andy Lutomirski <luto@amacapital.net>, security@debian.org,
        "security@kernel.org" <security@kernel.org>,
        Al Viro <viro@zeniv.linux.org.uk>,
        "security@ubuntu.com >> security" <security@ubuntu.com>,
        Peter Hurley <peter@hurleysoftware.com>,
        Serge Hallyn <serge.hallyn@ubuntu.com>, Willy Tarreau <w@1wt.eu>,
        Aurelien Jarno <aurelien@aurel32.net>,
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        Jann Horn <jann@thejh.net>, Greg KH <greg@kroah.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Jiri Slaby <jslaby@suse.com>, Florian Weimer <fw@deneb.enyo.de>
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On April 19, 2016 11:44:40 AM PDT, ebiederm@xmission.com wrote:
>Linus Torvalds <torvalds@linux-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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