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

[Jann Horn <jann@thejh.net>]

[-- Attachment #1: Type: text/plain, Size: 2429 bytes --]

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?

[-- Attachment #2: Digital signature --]
[-- Type: application/pgp-signature, Size: 819 bytes --]