[Bridge] [PATCH net] net: bridge: validate the NUD_PERMANENT bit when adding an extern_learn FDB entry

[Vladimir Oltean]

On Tue, Aug 10, 2021 at 01:15:32PM +0300, Nikolay Aleksandrov wrote:
> On 10/08/2021 13:09, Vladimir Oltean wrote:
> > On Tue, Aug 10, 2021 at 09:46:34AM +0300, Ido Schimmel wrote:
> >> On Mon, Aug 09, 2021 at 04:05:22PM +0000, Vladimir Oltean wrote:
> >>> On Mon, Aug 09, 2021 at 03:16:40PM +0300, Ido Schimmel wrote:
> >>>> I have at least once selftest where I forgot the 'static' keyword:
> >>>>
> >>>> bridge fdb add de:ad:be:ef:13:37 dev $swp1 master extern_learn vlan 1
> >>>>
> >>>> This patch breaks the test when run against both the kernel and hardware
> >>>> data paths. I don't mind patching these tests, but we might get more
> >>>> reports in the future.
> >>>
> >>> Is it the 'static' keyword that you forgot, or 'dynamic'? The
> >>> tools/testing/selftests/net/forwarding/bridge_vlan_aware.sh selftest
> >>> looks to me like it's testing the behavior of an FDB entry which should
> >>> roam, and which without the extern_learn flag would be ageable.
> >>
> >> static - no aging, no roaming
> >> dynamic - aging, roaming
> >> extern_learn - no aging, roaming
> >>
> >> So these combinations do not make any sense and the kernel will ignore
> >> static/dynamic when extern_learn is specified. It's needed to work
> >> around iproute2 behavior of "assume permanent"
> > 
> > Since NTF_EXT_LEARNED is part of ndm->ndm_flags and NUD_REACHABLE/NUD_NOARP
> > are part of ndm->ndm_state, it is not at all clear to me that 'extern_learn'
> > belongs to the same class of bridge neighbor attributes as 'static'/'dynamic',
> > and that it is invalid to have the full degree of freedom. If it isn't,
> > shouldn't the kernel validate that, instead of just ignoring the ndm->ndm_state?
> > If it's too late to validate, shouldn't we at least document somewhere
> > that the ndm_state is ignored in the presence of ndm_flags & NTF_EXT_LEARNED?
> > It is user API after all, easter eggs like this aren't very enjoyable.
> > 
> 
> It's too late unfortunately, ignoring other flags in that case has been the standard
> behaviour for a long time (it has never made sense to specify flags for extern_learn
> entries). I'll send a separate patch that adds a comment to document it or if you have
> another thing in mind feel free to send a patch.

No, I don't have anything else in mind, but since the topic is the same
as the "net: bridge: fix flags interpretation for extern learn fdb entries"
patch you already sent, you could as well just send a v2 for that and
add an extra phrase in a comment somewhere near a NTF_EXT_LEARNED uapi
definition, or perhaps extend this comment right here:

/* NUD_NOARP & NUD_PERMANENT are pseudostates, they never change
   and make no address resolution or NUD.
   NUD_PERMANENT also cannot be deleted by garbage collectors.
 */