Messages in this thread | | | From | Andy Lutomirski <> | Date | Fri, 11 Dec 2015 15:16:48 -0800 | Subject | Re: [PATCH] devpts: Sensible /dev/ptmx & force newinstance |
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On Fri, Dec 11, 2015 at 3:07 PM, H. Peter Anvin <hpa@zytor.com> wrote: > On December 11, 2015 3:00:49 PM PST, Andy Lutomirski <luto@amacapital.net> wrote: >>On Fri, Dec 11, 2015 at 2:58 PM, Jann Horn <jann@thejh.net> wrote: >>> On Fri, Dec 11, 2015 at 02:52:01PM -0800, Andy Lutomirski wrote: >>>> On Fri, Dec 11, 2015 at 2:35 PM, Eric W. Biederman >>>> <ebiederm@xmission.com> wrote: >>>> > Andy Lutomirski <luto@amacapital.net> writes: >>>> > >>>> >> On Fri, Dec 11, 2015 at 2:07 PM, H. Peter Anvin <hpa@zytor.com> >>wrote: >>>> >>> On 12/11/15 13:48, Andy Lutomirski wrote: >>>> >>>> On Fri, Dec 11, 2015 at 1:11 PM, Eric W. Biederman >>>> >>>> <ebiederm@xmission.com> wrote: >>>> >>>>> Al Viro <viro@ZenIV.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?" >>>> >>>> >>>> >>> >>>> >>> 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. >>> >>> But then ptsname could become "open the slave, call ttyname() on it, >>close >>> the slave". Unless opening the slave would have side effects? >> >>Hmm, fair enough. So maybe that does make sense after all. >> >>Anyway, I still think there are two pieces here: >> >>1. Fix /dev/ptmx so that we can banish newinstance=0. >> >>2. Fix libc. If that needs kernel help, then so be it. >> >>ISTM we could still implement the "open the slave" operation for (2) >>as an ioctl that does the appropriate magic the fd is /dev/ptmx as >>opposed to /dev/pts/ptmx. >> >> >>--Andy > > I want to be clear: > > If /dev/ptmx -> pts/ptmx and devpts is mounted with the proper options, I believe ask the remaining parts of userspace should be fine, and pt_chown can be removed even with glibc. > > The magic ptmx we are talking about is all about dealing with a mismanaged /dev. > -- > Sent from my Android device with K-9 Mail. Please excuse brevity and formatting.
I think you're right, modulo the one stupidity that a configuration like that is prone to breakage with container apps running on the same system.
Hmm. Could userspace be changed to set newinstance=1 on its /dev/pts mount to work around that?
--Andy
-- Andy Lutomirski AMA Capital Management, LLC
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