Re: [PATCH net] net: bridge: allow users setting EXT_LEARN for user FDB entries

[Jonas Gorski]

Am So., 1. Sept. 2024 um 14:25 Uhr schrieb Nikolay Aleksandrov
<razor@xxxxxxxxxxxxx>:
>
> On 01/09/2024 14:54, Ido Schimmel wrote:
> > On Sat, Aug 31, 2024 at 11:31:50AM +0300, Nikolay Aleksandrov wrote:
> >> On 30/08/2024 17:53, Jonas Gorski wrote:
> >>> When userspace wants to take over a fdb entry by setting it as
> >>> EXTERN_LEARNED, we set both flags BR_FDB_ADDED_BY_EXT_LEARN and
> >>> BR_FDB_ADDED_BY_USER in br_fdb_external_learn_add().
> >>>
> >>> If the bridge updates the entry later because its port changed, we clear
> >>> the BR_FDB_ADDED_BY_EXT_LEARN flag, but leave the BR_FDB_ADDED_BY_USER
> >>> flag set.
> >>>
> >>> If userspace then wants to take over the entry again,
> >>> br_fdb_external_learn_add() sees that BR_FDB_ADDED_BY_USER and skips
> >>> setting the BR_FDB_ADDED_BY_EXT_LEARN flags, thus silently ignores the
> >>> update:
> >>>
> >>>    if (test_bit(BR_FDB_ADDED_BY_EXT_LEARN, &fdb->flags)) {
> >>>            /* Refresh entry */
> >>>            fdb->used = jiffies;
> >>>    } else if (!test_bit(BR_FDB_ADDED_BY_USER, &fdb->flags)) {
> >>>            /* Take over SW learned entry */
> >>>            set_bit(BR_FDB_ADDED_BY_EXT_LEARN, &fdb->flags);
> >>>            modified = true;
> >>>    }
> >>>
> >>> Fix this by relaxing the condition for setting BR_FDB_ADDED_BY_EXT_LEARN
> >>> by also allowing it if swdev_notify is true, which it will only be for
> >>> user initiated updates.
> >>>
> >>> Fixes: 710ae7287737 ("net: bridge: Mark FDB entries that were added by user as such")
> >>> Signed-off-by: Jonas Gorski <jonas.gorski@xxxxxxxx>
> >>> ---
> >>>  net/bridge/br_fdb.c | 3 ++-
> >>>  1 file changed, 2 insertions(+), 1 deletion(-)
> >>>
> >>> diff --git a/net/bridge/br_fdb.c b/net/bridge/br_fdb.c
> >>> index c77591e63841..c5d9ae13a6fb 100644
> >>> --- a/net/bridge/br_fdb.c
> >>> +++ b/net/bridge/br_fdb.c
> >>> @@ -1472,7 +1472,8 @@ int br_fdb_external_learn_add(struct net_bridge *br, struct net_bridge_port *p,
> >>>             if (test_bit(BR_FDB_ADDED_BY_EXT_LEARN, &fdb->flags)) {
> >>>                     /* Refresh entry */
> >>>                     fdb->used = jiffies;
> >>> -           } else if (!test_bit(BR_FDB_ADDED_BY_USER, &fdb->flags)) {
> >>> +           } else if (swdev_notify ||
> >>> +                      !test_bit(BR_FDB_ADDED_BY_USER, &fdb->flags)) {
> >>>                     /* Take over SW learned entry */
> >>>                     set_bit(BR_FDB_ADDED_BY_EXT_LEARN, &fdb->flags);
> >>>                     modified = true;
> >>
> >> This literally means if added_by_user || !added_by_user, so you can probably
> >> rewrite that whole block to be more straight-forward with test_and_set_bit -
> >> if it was already set then refresh, if it wasn't modified = true
> >
> > Hi Nik,
> >
> > You mean like this [1]?
> > I deleted the comment about "SW learned entry" since "extern_learn" flag
> > not being set does not necessarily mean the entry was learned by SW.
> >
> > [1]
> > diff --git a/net/bridge/br_fdb.c b/net/bridge/br_fdb.c
> > index c77591e63841..ad7a42b505ef 100644
> > --- a/net/bridge/br_fdb.c
> > +++ b/net/bridge/br_fdb.c
> > @@ -1469,12 +1469,10 @@ int br_fdb_external_learn_add(struct net_bridge *br, struct net_bridge_port *p,
> >                         modified = true;
> >                 }
> >
> > -               if (test_bit(BR_FDB_ADDED_BY_EXT_LEARN, &fdb->flags)) {
> > +               if (test_and_set_bit(BR_FDB_ADDED_BY_EXT_LEARN, &fdb->flags)) {
> >                         /* Refresh entry */
> >                         fdb->used = jiffies;
> > -               } else if (!test_bit(BR_FDB_ADDED_BY_USER, &fdb->flags)) {
> > -                       /* Take over SW learned entry */
> > -                       set_bit(BR_FDB_ADDED_BY_EXT_LEARN, &fdb->flags);
> > +               } else {
> >                         modified = true;
> >                 }
>
> Yeah, that's exactly what I meant. Since the added_by_user condition becomes
> redundant we can just drop it.

br_fdb_external_learn_add() is called from two places; once when
userspace adds a EXT_LEARN flagged fdb entry (then swdev_nofity is
set), and once when a switchdev driver reports it has learned an entry
(then swdev_notify isn't).

AFAIU the previous condition was to prevent user fdb entries from
being taken over by hardware / switchdev events, which this would now
allow to happen. OTOH, the switchdev notifications are a statement of
fact, and the kernel really has a say into whether the hardware should
keep the entry learned, so not allowing entries to be marked as
learned by hardware would also result in a disconnect between hardware
and kernel.

My change was trying to accomodate for the former one, i.e. if the
user bit is set, only the user may mark it as EXT_LEARN, but not any
(switchdev) drivers.

I have no strong feelings about what I think is right, so if this is
the wanted direction, I can send a V2 doing that.

Best Regards,
Jonas

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