[PATCH 11/12] pid: rewrite task helper functions avoiding task->pid and task->tgid

[Peter Zijlstra]

On Tue, Aug 27, 2013 at 02:35:11PM -0700, Eric W. Biederman wrote:
> Peter Zijlstra <peterz at infradead.org> writes:
> 
> > On Mon, Aug 26, 2013 at 10:37:22PM -0400, Richard Guy Briggs wrote:
> >> On Fri, Aug 23, 2013 at 08:36:21AM +0200, Peter Zijlstra wrote:
> >> > Except that's not the case, with namespaces there's a clear hierarchy
> >> > and the task_struct::pid is the one true value aka. root namespace.
> >> 
> >> Peter, I agonized over the access efficiency of dropping this one or the
> >> duplicate in task_struct::pids and this one was far easier to drop in
> >> terms of somehow always forcing
> >> task->pids[PIDTYPE_PID].pid->numbers[0].nr to point to task->pid.
> >
> > You mean there's more than 1 site that sets task_struct::pid? I thought
> > we only assign that thing once in fork.c someplace.
> 
> No there is not and that is not a concern.
> 
> Now I had thought given how the perf subsystem was constructed that only
> the god like root was even allowed to use the code.  But it turns out
> there is sysctl_perf_event_paranoid that can bet twiddled that in some
> circumstance that unprivileged users are allowed to use perf.  

Even without poking at that, a user is always allowed to use perf on his
own tasks.

> Which
> ultimately means perf will return the wrong data.

How so, perf uses pid-namespaces, have a look at
kernel/events/core.c:perf_event_[pt]id(). We store the namespace of the
task creating the event (which is also -- hopefully -- the consumer of
the data it generates). If you create an event and then switch
namespaces you've bigger issues I suppose.

> Meaning that perf is broken by design and perf has no excuse to be using
> task->pid.

It doesn't.

> Similarly every other place in the kernel that has made the
> same mistake.  I mention perf explicitly for two reasons.  perf gets the
> namespace handling horribly wrong, 

Do tell.

> perf is the only place in the kernel
> where we are accessing pids frequently enough for an extra cache line
> miss to be a concern.
> 
> When really pids in the kernel what we care about is not some stupid
> number but the stuct pid which points to that tasks, process groups, and
> sessions.  You know the object that a pid is the name for.
> 
> So yes as a long term direction task->pid and task->tgid need to be
> killed because we keep getting subsystems like perf that return the
> wrong data to userspace, or perform the wrong checks, because the
> current structure makes it seem like it is ok to do the wrong thing.

I think you should have a look at code before you start raving.. makes
you looks silly.