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

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Message-ID: <CALCETrWVbWz-cOaOZrcU1MLPyviknssEWY5OoKLVqkoDi-9kjA@mail.gmail.com>
Date:	Fri, 11 Dec 2015 13:48:35 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Al Viro <viro@...iv.linux.org.uk>, Greg KH <greg@...ah.com>,
	Jiri Slaby <jslaby@...e.com>, "H. Peter Anvin" <hpa@...or.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 1:11 PM, Eric W. Biederman
<ebiederm@...ssion.com> wrote:
> Al Viro <viro@...IV.linux.org.uk> writes:
>
>> On Fri, Dec 11, 2015 at 01:40:40PM -0600, Eric W. Biederman wrote:
>>
>>> +    inode = path.dentry->d_inode;
>>> +    filp->f_path = path;
>>> +    filp->f_inode = inode;
>>> +    filp->f_mapping = inode->i_mapping;
>>> +    path_put(&old);
>>
>> Don't.  You are creating a fairly subtle constraint on what the code in
>> fs/open.c and fs/namei.c can do, for no good reason.  You can bloody
>> well maintain the information you need without that.
>
> There is a good reason.  We can not write a race free version of ptsname
> without it.

As long as this is for new userspace code, would it make sense to just
add a new ioctl to ask "does this ptmx fd match this /dev/pts fd?"

--Andy
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