Quoting C Anthony Risinger (anthony@xtfx.me):

On Mon, May 23, 2011 at 4:05 PM, Eric W. Biederman
[off-list ref] wrote:

quoted

This tree adds the files /proc/<pid>/ns/net, /proc/<pid>/ns/ipc,
/proc/<pid>/ns/uts that can be opened to refer to the namespaces of a
process at the time those files are opened, and can be bind mounted to
keep the specified namespace alive without a process.

This tree adds the setns system call that can be used to change the
specified namespace of a process to the namespace specified by a system
call.

i just have a quick question regarding these, apologies if wrong place
to respond -- i trimmed to lists only.

if i understand correctly, mount namespaces (for example), allow one
to build such constructs as "private /tmp" and similar that even
`root` cannot access ... and there are many reasons `root` does not
deserve to completely know/interact with user processes (FUSE makes a
good example ... just because i [user] have SSH access to a machine,
why should `root`?)

would these /proc additions break such guarantees?  IOW, would it now
become possible for `root` to inject stuff into my private namespaces,
and/or has these guarantees never existed and i am mistaken?  is there
any kind of ACL mechanism that endows the origin process (or similar)
with the ability to dictate who can hold and/or interact with these
references?

If for instance you have a file open in your private /tmp, then root
in another mounts ns can open the file through /proc/$$/fd/N anyway.
If it's a directory, he can now traverse the whole fs.

-serge