Subject: Re: [PATCH net 00/13] cls_u32 cleanups and fixes.

From: Al Viro <[email-masked]>
Date: Sun, 9 Sep 2018 15:15:38 +0100

On Sun, Sep 09, 2018 at 08:58:50AM -0400, Jamal Hadi Salim wrote:
> 
> Since you have the momentum here: i noticed something
> unusual while i was trying to craft a test that would
> vet some of your changes. This has nothing to do with
> your changes, same happens on my stock debian laptop
> with kernel:
> 4.17.0-0.bpo.3-amd64 #1 SMP Debian 4.17.17-1~bpo9+1 (2018-08-27)
> 
> Looking at git - possibly introduced around the time u32
> lockless was being introduced and maybe even earlier
> than that.

It's always been that way, actually - before that point the old
knode simply got reused, which excluded any chance of changing
n->sel.

> Unfortunately i dont have time to dig
> further.
> 
> To reproduce what i am referring to, here's a setup:
> 
> $tc filter add dev eth0 parent ffff: protocol ip prio 102 u32 \
> classid 1:2 match ip src 192.168.8.0/8
> $tc filter replace dev eth0 parent ffff: protocol ip prio 102 \
> handle 800:0:800 u32 classid 1:2 match ip src 1.1.0.0/24
> 
> u32_change() code path should have allowed changing of the
> keynode.

Umm...  Interesting - TCA_U32_SEL is not the only thing that
gets ignored there; TCA_U32_MARK gets the same treatment.
And then there's a lovely question what to do with n->pf -
it's an array of n->sel.nkeys counters, and apparently we
want (at least in common cases) to avoid resetting those.

*If* we declare that ->nkeys mismatch means failure, it's
all relatively easy to implement.  Alternatively, we could
declare that selector change means resetting the stats.
Preferences?