Re: [PATCH net v2] bonding: fix feature flag setting at init time

[Jarod Wilson <jarod@redhat.com>]

On Wed, Dec 2, 2020 at 12:55 PM Jay Vosburgh <jay.vosburgh@canonical.com> wrote:
>
> Jarod Wilson <jarod@redhat.com> wrote:
>
> >Don't try to adjust XFRM support flags if the bond device isn't yet
> >registered. Bad things can currently happen when netdev_change_features()
> >is called without having wanted_features fully filled in yet. Basically,
> >this code was racing against register_netdevice() filling in
> >wanted_features, and when it got there first, the empty wanted_features
> >led to features also getting emptied out, which was definitely not the
> >intended behavior, so prevent that from happening.
>
>         Is this an actual race?  Reading Ivan's prior message, it sounds
> like it's an ordering problem (in that bond_newlink calls
> register_netdevice after bond_changelink).

Sorry, yeah, this is not actually a race condition, just an ordering
issue, bond_check_params() gets called at init time, which leads to
bond_option_mode_set() being called, and does so prior to
bond_create() running, which is where we actually call
register_netdevice().

>         The change to bond_option_mode_set tests against reg_state, so
> presumably it wants to skip the first(?) time through, before the
> register_netdevice call; is that right?

Correct. Later on, when the bonding driver is already loaded, and
parameter changes are made, bond_option_mode_set() gets called and if
the mode changes to or from active-backup, we do need/want this code
to run to update wanted and features flags properly.


-- 
Jarod Wilson
jarod@redhat.com