linux-kernel - Re: [PATCH ghak90 V9 02/13] audit: add container id

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Date:   Fri, 21 Aug 2020 15:36:49 -0400
From:   Paul Moore <paul@...l-moore.com>
To:     Richard Guy Briggs <rgb@...hat.com>
Cc:     containers@...ts.linux-foundation.org, linux-api@...r.kernel.org,
        Linux-Audit Mailing List <linux-audit@...hat.com>,
        linux-fsdevel@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
        netdev@...r.kernel.org, netfilter-devel@...r.kernel.org,
        sgrubb@...hat.com, Ondrej Mosnacek <omosnace@...hat.com>,
        dhowells@...hat.com, simo@...hat.com,
        Eric Paris <eparis@...isplace.org>,
        Serge Hallyn <serge@...lyn.com>, ebiederm@...ssion.com,
        nhorman@...driver.com, Dan Walsh <dwalsh@...hat.com>,
        mpatel@...hat.com
Subject: Re: [PATCH ghak90 V9 02/13] audit: add container id

On Wed, Jul 29, 2020 at 4:06 PM Richard Guy Briggs <rgb@...hat.com> wrote:
> On 2020-07-05 11:09, Paul Moore wrote:
> > On Sat, Jun 27, 2020 at 9:22 AM Richard Guy Briggs <rgb@...hat.com> wrote:

...

> > > @@ -212,6 +219,33 @@ void __init audit_task_init(void)
> > >                                              0, SLAB_PANIC, NULL);
> > >  }
> > >
> > > +/* rcu_read_lock must be held by caller unless new */
> > > +static struct audit_contobj *_audit_contobj_hold(struct audit_contobj *cont)
> > > +{
> > > +       if (cont)
> > > +               refcount_inc(&cont->refcount);
> > > +       return cont;
> > > +}
> > > +
> > > +static struct audit_contobj *_audit_contobj_get(struct task_struct *tsk)
> > > +{
> > > +       if (!tsk->audit)
> > > +               return NULL;
> > > +       return _audit_contobj_hold(tsk->audit->cont);
> > > +}
> > > +
> > > +/* rcu_read_lock must be held by caller */
> > > +static void _audit_contobj_put(struct audit_contobj *cont)
> > > +{
> > > +       if (!cont)
> > > +               return;
> > > +       if (refcount_dec_and_test(&cont->refcount)) {
> > > +               put_task_struct(cont->owner);
> > > +               list_del_rcu(&cont->list);
> >
> > You should check your locking; I'm used to seeing exclusive locks
> > (e.g. the spinlock) around list adds/removes, it just reads/traversals
> > that can be done with just the RCU lock held.
>
> Ok, I've redone the locking yet again.  I knew this on one level but
> that didn't translate consistently to code...
>
> > > +               kfree_rcu(cont, rcu);
> > > +       }
> > > +}
> >
> > Another nitpick, but it might be nice to have similar arguments to the
> > _get() and _put() functions, e.g. struct audit_contobj, but that is
> > some serious bikeshedding (basically rename _hold() to _get() and
> > rename _hold to audit_task_contid_hold() or similar).
>
> I have some idea what you are trying to say, but I think you misspoke.
> Did you mean rename _hold to _get, rename _get to
> audit_task_contobj_hold()?

It reads okay to me, but I know what I'm intending here :)  I agree it
could be a bit confusing.  Let me try to put my suggestion into some
quick pseudo-code function prototypes to make things a bit more
concrete.

The _audit_contobj_hold() function would become:
   struct audit_contobj *_audit_contobj_hold(struct task_struct *tsk);

The _audit_contobj_get() function would become:
   struct audit_contobj *_audit_contobj_get(struct audit_contobj *cont);

The _audit_contobj_put() function would become:
   void _audit_contobj_put(struct audit_contobj *cont);

Basically swap the _get() and _hold() function names so that the
arguments are the same for both the _get() and _set() functions.  Does
this make more sense?

> > >  /**
> > >   * audit_alloc - allocate an audit info block for a task
> > >   * @tsk: task
> > > @@ -232,6 +266,9 @@ int audit_alloc(struct task_struct *tsk)
> > >         }
> > >         info->loginuid = audit_get_loginuid(current);
> > >         info->sessionid = audit_get_sessionid(current);
> > > +       rcu_read_lock();
> > > +       info->cont = _audit_contobj_get(current);
> > > +       rcu_read_unlock();
> >
> > The RCU locks aren't strictly necessary here, are they?  In fact I
> > suppose we could probably just replace the _get() call with a
> > refcount_set(1) just as we do in audit_set_contid(), yes?
>
> I don't understand what you are getting at here.  It needs a *contobj,
> along with bumping up the refcount of the existing contobj.

Sorry, you can disregard.  My mental definition for audit_alloc() is
permanently messed up; I usually double check myself before commenting
on related code, but I must have forgotten here.

-- 
paul moore
www.paul-moore.com