[Devel] Re: [PATCH 5/7]: Determine pts_ns from a pty's inode.

[Serge E. Hallyn]

Quoting Serge E. Hallyn (serue at us.ibm.com):
> Quoting sukadev at us.ibm.com (sukadev at us.ibm.com):
> > 
> > From: Sukadev Bhattiprolu <sukadev at us.ibm.com>
> > Subject: [PATCH 5/7]: Determine pts_ns from a pty's inode.
> > 
> > The devpts interfaces currently operate on a specific pts namespace
> > which they get from the 'current' task.
> > 
> > With implementation of containers and cloning of PTS namespaces, we want
> > to be able to access PTYs in a child-pts-ns from a parent-pts-ns. For
> > instance we could bind-mount and pivot-root the child container on
> > '/vserver/vserver1' and then access the "pts/0" of 'vserver1' using 
> > 
> > 	$ echo foo > /vserver/vserver1/dev/pts/0
> > 	
> > The task doing the above 'echo' could be in parent-pts-ns. So we find
> > the 'pts-ns' of the above file from the inode representing the above
> > file rather than from the 'current' task.
> > 
> > Note that we need to find and hold a reference to the pts_ns to prevent
> > the pts_ns from being freed while it is being accessed from 'outside'.
> > 
> > This patch implements, 'pts_ns_from_inode()' which returns the pts_ns
> > using 'inode->i_sb->s_fs_info'.
> > 
> > Since, the 'inode' information is not visible inside devpts code itself,
> > this patch modifies the tty driver code to determine the pts_ns and passes
> > it into devpts.
> > 
> > TODO:
> > 	What is the expected behavior when '/dev/tty' or '/dev/ptmx' are
> > 	accessed from parent-pts-ns. i.e:
> > 
> > 		$ echo "foobar" > /vserver/vserver1/dev/tty)
> > 		
> > 	This patch currently ignores the '/vserver/vserver1' part (that
> 
> The way this is phrased it almost sounds like you're considering using
> the pathnames to figure out the ptsns to use :).
> 
> It's not clear to me what is the sane thing to do.
> 
> what you're doing here - have /dev/ptmx and /dev/tty always use
> current->'s ptsns - isn't ideal.
> 
> It would be nicer to not have a 'devpts ns', and instead have a
> full device namespace.  However, then it still isn't clear how to tie
> /vs/vs1/dev/ptmx to vs1's device namespace, since there is no device
> fs to which to tie the devns.
> 
> We could tie the devns to a device inode on mknod, using the devns of
> the creating task.  Then when starting up vs1, you just have to always
> let vs1 create /dev/ptmx and /dev/tty.  I can't think of anything
> better offhand.
> 
> Other ideas?

I suppose you could just create /dev/pts/ptmx and /dev/pts/tty.
Recommend that in containers /dev/ptmx and /dev/tty be symlinks
into /dev/pts.  Applications don't need to change.  If
ptmx_open() sees that inode->i_sb is a devptsfs, it gets the
namespace from the sb.  If not, then it was a device in /dev
and it gets the nmespace from current.

-serge
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