linux-kernel - Re: [PATCH 01/16] devpts: Attempting to get it right

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Date:	Tue, 19 Apr 2016 14:00:48 -0500
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Andy Lutomirski <luto@...capital.net>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	security@...ian.org, "security\@kernel.org" <security@...nel.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	"security\@ubuntu.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: [PATCH 01/16] devpts: Attempting to get it right

"H. Peter Anvin" <hpa@...or.com> writes:

> It's really too bad we can't just use follow_link :-/

Well follow_link is actually impossible to use as it doesn't exist
anymore.  The routine now is get_link. ;-)

That said just to be certain of where everything stands I took a look to
verify that we can't.

I got about half way there.
I tweaked init_special_inode, and d_flags_for_inode to set inode->i_link
to "pts/ptmx", and to set DCACHE_SYMLINK_TYPE (aka made it so
d_is_symlink returned true) on all instances of /dev/ptmx except
/dev/pts/ptmx.

Things sort of worked and things also acted very very weird.

It is tempting because we would not need special vfs helpers, and would
not require much code, but since it did not work cleanly I give that
case a pass.

I think old udev on distro's like centos5 and centos6 are actually
a limit, as udev does something like calling stat on the device node
after creation to ensure everything was created properly.

Eric