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

[Thomas Monjalon <thomas@monjalon.net>]

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.

>    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.

> 3. To fix this bug, one should check that the device one attempts to
>    hotplug does not already exists as a probed rte_device.
>    An existing rte_devargs is not sufficient, because a blacklisted
>    device would have an rte_devargs without having a probed rte_device,
>    and the current behavior is to supersede the current blacklist and
>    forcibly insert the new device, as if it was newly whitelisted.
>    This check can only happen at rte_dev level.
> 
> 4. Your confusion about rte_devargs_remove is understandable, the API is
>    muddy. The reason for these quirks is because I wanted a user
>    to be able to remove any devargs, even without having a direct
>    reference to it: you only had to define the bus and the device id
>    (name), and it would find it and remove it. It might be preferrable
>    to force the user to find the rte_device, and from it, use the actual
>    rte_devargs reference to remove it, but then, it would be impossible
>    to remove devargs for non-existing devices (spoiler: that's the
>    blacklisted ones).
> 
> 5. It bears repeating: blacklisted mode is horrible and should be removed.
>    It is all-around abominable, forces unsightly designs to exist and be
>    used, makes managers ask questions about "why do you add this quirky
>    `-w 00:00.0` parameter to your command line and what is your timeline
>    for not needing it?", makes at least one team integrating OVS ask
>    themselves "why not --no-pci? but then why can't I hotplug PCI ports?",
>    and I would not be surprised if it killed puppies as a hobby.
> 
>    So far, I was able to collect "but it simplifies testing bot
>    configuration" as a plus, which I do not agree with.
> 
>    And anyone trying to package DPDK on their platform, expecting users
>    not to know or care about it, would be better off developping a
>    proper autoconf tool, instead of baking it in the entrails of the
>    EAL, which are ugly enough as it is. /rant

Nice rant :)