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

[Serge E. Hallyn]

Quoting sukadev at us.ibm.com (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 ?

An SELinux policy tagging child devpts entries with vps1_u:vps1_r:vps1_pts_t
and not allowing vps1_t access to host_pts_t entries would forbid it if
you wanted.  But failing that, the kernel doesn't break, so I don't
it's a problem.

> | > 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 ?

For userspace, but it doesn't crash the kernel.  Userspace didn't set
things up right, so it gets the wrong thing.  If I do a dup2 into fd 3
and then try to read from fd 4, I get the wrong data.  Is that the
kernel's fault?

-serge