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

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Date:	Fri, 11 Dec 2015 15:11:56 -0600
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Al Viro <viro@...IV.linux.org.uk>
Cc:	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>,
	Andy Lutomirski <luto@...capital.net>,
	Florian Weimer <fw@...eb.enyo.de>,
	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@...r.kernel.org
Subject: Re: [PATCH] devpts: Sensible /dev/ptmx & force newinstance

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.

If it would help I am happy to add a helper to change the filp path that
lives next to d_dentry_open so that this is not insane to maintain.  I
am not ready to consider yet another attempt by kernel people to solve
userspace problems that is a half thought out mess that means we will
still have bugs and problems 20 years hence.

We need to actually solve the problem and anything less than that is
just stupid.

Eric
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