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

[sukadev at us.ibm.com]

Serge E. Hallyn [serue at us.ibm.com] wrote:
| Quoting sukadev at us.ibm.com (sukadev at us.ibm.com):
| > Serge E. Hallyn [serue at us.ibm.com] wrote:
| > | > | 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.
| > | > 
| > | > But we would still depend on user-space remounting /dev/pts after
| > | > the clone right ? Until they do that we would access the parent
| > | > container's /dev/pts/ptmx ?
| > | 
| > | Yes.  Which is the right thing to do imo.
| > 
| > Hmm, that sounds reasonable, although slightly inconsistent with pid-ns,
| > where pid starts at 1 regardless of whether /proc is remounted.
| 
| Very different cases.  The pid is the task's pid in the new pidns.
| The task ALSO has a different pid in the parent pidns.
| 
| The pts only has an identity in one ptsns.
| 
| > But even so, if user fails to establish the symlink, clones the pts ns
| > and tries to create a pty, we would end up with different pts nses again ?
| 
| Yes.  So what?

We would end up allocating a pts index from child-pts-ns (i.e index 0)
and attempt to open /dev/pts/0 which could be an existing pty in the
parent pts ns ?
| 
| > i.e
| > 	/dev/ptmx is still a char dev in root fs
| > 	clone(pts_ns)
| > 		( In child, (before remount /dev/pts))
| > 		open("/dev/ptmx")
| > 		open("/dev/pts/0")
| > 
| > Since ptmx is not in devpts, we use current_pts_ns() or child-pts-ns
| > Since /dev/pts is not remounted in child, we get the parent pts-ns from
| > 
| > If we can somehow detect the incorrect configuration and fail either
| > open, we should be ok :-)
| 
| I completely disagree with this sentiment.  The kernel doesn't need
| to detect an "incorrect configuration" if it isn't dangerous.  One
| man's "incorrect configuration" is another man's useful trick.

Myabe configuration is the wrong word, but unless I am missing something
above, spanning two pts-nses is an error condition ?