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

[Ido Schimmel <idosch@nvidia.com>]

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@bisdn.de>
> > ---
> >  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;
                }