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

[H. Peter Anvin]

On December 11, 2015 3:00:49 PM PST, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
>On Fri, Dec 11, 2015 at 2:58 PM, Jann Horn <jann@xxxxxxxxx> 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@xxxxxxxxxxxx> wrote:
>>> > Andy Lutomirski <luto@xxxxxxxxxxxxxx> writes:
>>> >
>>> >> On Fri, Dec 11, 2015 at 2:07 PM, H. Peter Anvin <hpa@xxxxxxxxx>
>wrote:
>>> >>> On 12/11/15 13:48, Andy Lutomirski wrote:
>>> >>>> On Fri, Dec 11, 2015 at 1:11 PM, Eric W. Biederman
>>> >>>> <ebiederm@xxxxxxxxxxxx> wrote:
>>> >>>>> Al Viro <viro@xxxxxxxxxxxxxxxxxx> 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.
-- 
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