[Devel] Re: [RFC][PATCH] Do not set /proc inode->pid for non-pid-related inodes

[Eric W. Biederman]

"Serge E. Hallyn" <serue at us.ibm.com> writes:

> Quoting Eric W. Biederman (ebiederm at xmission.com):
>> Dave Hansen <hansendc at us.ibm.com> writes:
>> > On Mon, 2007-03-19 at 20:04 -0600, Eric W. Biederman wrote:

>> >> I would also
>> >> like to see how we perform the appropriate lookups by pid namespace.
>> >
>> > What do you mean?
>> 
>> proc_pid_readdir ... next_tgid().
>
> next_tgid() is simple enough - we can always use current->pid_ns to find
> the next pidnr.

No.  We cannot use current->pid_ns.  We must get it from the mount or
something in the mount.

Using current to set the default pid_ns to mount is fine.  But if
we use current to select our files we have a moderately serious problem.

> The only hitch, as mentioned earlier, is how do we find the first task.
> Currently task 1 is statically stored as the first inode, and as Dave
> mentioned we can't do that now, because we dont' know of any one task
> which will outlive the pid_ns.

Outlive is the wrong concept.  Ideally we want something that will
live as long as there are processes in the pid_ns.

As I thought about this some more there are some problems for holding
a reference to a pid_ns for a long period of time.  Currently struct_pid
is designed so you can hang onto it forever.  struct pid_namespace isn't.
So we have some very interesting semantic questions of what happens when
the pid namespace exits.

Since we distinguish mounts by their pid namespace this looks like
something we need to sort through.


>> While I'm not categorically opposed to supporting things like that it
>> but it is something for which we need to tread very carefully because
>> it is an extension of current semantics.  I can't think of any weird
>> semantics right now but for something user visible we will have to
>> support indefinitely I don't see a reason to rush into it either.
>
> Except that unless we mandate that pid1 in any namespace can't exit, and
> put that feature off until later, we can't not address it.

What if we mandate that pid1 is the last process to exit?

Problems actually only show up in this context if other pids live
substantially longer than pid1.

>> True but we are getting close.  And it is about time we worked up
>> patches for that so our conversations can become less theoretical.
>
> Yes I really hope a patchset goes out today.

Sounds good.   I expect it will take a couple of rounds of review,
before we have all of the little things nailed down but starting that
process is a hopeful sign.

Eric
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