Re: [dpdk-dev] [PATCH] eal: fix floating device argument pointer

["Gaëtan Rivet" <gaetan.rivet@6wind.com>]

On Wed, Oct 24, 2018 at 04:43:45PM +0200, Thomas Monjalon wrote:
> 24/10/2018 00:39, Gaëtan Rivet:
> > Hi,
> > 
> > On Mon, Oct 22, 2018 at 09:25:22AM +0200, Thomas Monjalon wrote:
> > > 22/10/2018 07:49, Qi Zhang:
> > > > After we insert a devargs into devargs_list, following bus->scan may
> > > > destroy it due to another rte_devargs_insert. Its better not to use
> > > > a devargs pointer after it has been inserted.
> > > 
> > 
> > A bus scan calls rte_devargs_insert? Mapping devargs to device is the
> > responsibility of the bus scan, if it calls potentially destructive
> > functions, it must rebuild the map.
> > 
> > > I think the problem is in:
> > > 
> > > rte_devargs_insert(struct rte_devargs *da)
> > > {
> > >     int ret;
> > > 
> > >     ret = rte_devargs_remove(da);                                                                    
> > >     if (ret < 0)
> > >         return ret;
> > >     TAILQ_INSERT_TAIL(&devargs_list, da, next);
> > >     return 0;
> > > }
> > > 
> > > We insert a structure which is freed!
> > 
> > Not usually, I hope!
> > 
> > > 
> > > See http://git.dpdk.org/dpdk/commit/?id=55744d83d525
> > > 
> > > Gaetan, what can be the fix?
> > 
> > 1. rte_devargs_insert is misdefined.
> >    It is designed as a function that can never fail.
> >    The function should return void instead.
> > 
> > 2. rte_devargs_remove(da), will not remove da itself.
> >    It will remove whichever rte_devargs matches da within the internal
> >    list. If da does not match any in the list, it does nothing.
> >    As da is a newly-callocated structure, it is actually safe to
> >    continue using it after having called rte_devargs_remove(), because
> >    it cannot possibly have been inserted in the meantime (so would not
> >    have been freed, even if another devargs matched it).
> 
> If the devargs pointer passed in parameter is the same as the one
> in the list, it will be freed.
> 

This would only happen if one did:

    rte_devargs_insert(dev->devargs);

> >    The actual issue is that the matching rte_devargs within the list
> >    would be referenced by a device after a successful scan, meaning that
> >    this reference is not safe if someone attemps to insert the same
> >    device after the bus->scan(). If my understanding is correct, the above
> >    fix is not necessary, but probing should be guarded against
> >    re-entrancy.
> 
> We may want to probe again with different parameters.
> 

Sure, but in this case the fix is to check whether the device is already
probed, and if so remove it before probing it again with the new
devargs.

> 
> Nice rant :)

:)

-- 
Gaëtan Rivet
6WIND