Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754601AbbLKWwX (ORCPT ); Fri, 11 Dec 2015 17:52:23 -0500 Received: from mail-ob0-f171.google.com ([209.85.214.171]:35611 "EHLO mail-ob0-f171.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752822AbbLKWwV (ORCPT ); Fri, 11 Dec 2015 17:52:21 -0500 MIME-Version: 1.0 In-Reply-To: <87y4d0sjff.fsf@x220.int.ebiederm.org> References: <1CB621EF-1647-463B-A144-D611DB150E15@zytor.com> <20151208223135.GA8352@kroah.com> <87oae0h2bo.fsf@x220.int.ebiederm.org> <56677DE3.5040705@zytor.com> <20151209012311.GA11794@kroah.com> <84B136DF-55E4-476A-9CB2-062B15677EE5@zytor.com> <20151209013859.GA12442@kroah.com> <20151209083225.GA30452@1wt.eu> <87wpskyds7.fsf_-_@x220.int.ebiederm.org> <20151211210400.GL20997@ZenIV.linux.org.uk> <87h9jovgf7.fsf@x220.int.ebiederm.org> <566B4931.5080806@zytor.com> <87y4d0sjff.fsf@x220.int.ebiederm.org> From: Andy Lutomirski Date: Fri, 11 Dec 2015 14:52:01 -0800 Message-ID: Subject: Re: [PATCH] devpts: Sensible /dev/ptmx & force newinstance To: "Eric W. Biederman" Cc: "H. Peter Anvin" , Al Viro , Greg KH , Jiri Slaby , Linus Torvalds , Aurelien Jarno , Florian Weimer , Serge Hallyn , Jann Horn , "security@kernel.org" , "security@ubuntu.com >> security" , security@debian.org, Willy Tarreau , "linux-kernel@vger.kernel.org" Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3379 Lines: 79 On Fri, Dec 11, 2015 at 2:35 PM, Eric W. Biederman wrote: > Andy Lutomirski writes: > >> On Fri, Dec 11, 2015 at 2:07 PM, H. Peter Anvin wrote: >>> On 12/11/15 13:48, Andy Lutomirski wrote: >>>> On Fri, Dec 11, 2015 at 1:11 PM, Eric W. Biederman >>>> wrote: >>>>> Al Viro 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?" >>>> >>> >>> For the newinstance case st_dev should match between the master and the >>> slave. Unfortunately this is not the case for a legacy ptmx, as a >>> stat() on the master descriptor still returns the st_dev, st_rdev, and >>> st_ino for the ptmx device node. >> >> Sure, but I'm not talking about stat. I'm saying that we could add a >> new ioctl that works on any ptmx fd (/dev/ptmx or /dev/pts/ptmx) that >> answers the question "does this ptmx logically belong to the given >> devpts filesystem". >> >> Since it's not stat, we can make it do whatever we want, including >> following a link to the devpts instance that isn't f_path or f_inode. > > The useful ioctl to add in my opinion would be one that actually opens > the slave, at which point ptsname could become ttyname, and that closes > races in grantpt. Unfortunately, ptsname is POSIX, so we can't get rid of it. It's a bad idea, but it's in the standard. > > I even posted an implementation earlier in the discussion and no one was > interested. > > Honestly the more weird special cases we add to devpts the less likely > userspace will be to get things right. We have been trying since 1998 > and devpts is still a poor enough design we have not been able to get > rid of /usr/lib/pt_chown. Adding another case where we have to sand on > one foot and touch our nose does not seem to likely to achieve > widespread adoption. How many version of libc are there now? Old libc can stay buggy, I think. Given that this mess is partially a libc mis-design, I don't see why it needs to be fixed entirely in the kernel. For new systems, it would be really nice if we can make a /dev/ptmx symlink be 100% functional. In any event, this is semi-moot. If we actually want to retire the newinstance=0 thing from the kernel, we apparently need a magic /dev/ptmx node. That doesn't mean that new userspace needs to *use* that magic node. So why not implement the magic node without fiddling with f_path? --Andy -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/