netdev - Re: [PATCH net-next v5 1/4] net/sched: user-space can't set unknown tcfa

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Message-ID: <7b874aed-c06f-18e7-16e3-9046c2de43cc@mojatatu.com>
Date:   Wed, 1 Aug 2018 10:34:23 -0400
From:   Jamal Hadi Salim <jhs@...atatu.com>
To:     Paolo Abeni <pabeni@...hat.com>, netdev@...r.kernel.org
Cc:     Cong Wang <xiyou.wangcong@...il.com>,
        Jiri Pirko <jiri@...nulli.us>,
        Daniel Borkmann <daniel@...earbox.net>,
        Marcelo Ricardo Leitner <marcelo.leitner@...il.com>,
        Eyal Birger <eyal.birger@...il.com>,
        "David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCH net-next v5 1/4] net/sched: user-space can't set unknown
 tcfa_action values

On 31/07/18 10:40 AM, Paolo Abeni wrote:

> If we choose to reject unknown opcodes, such user-space configuration
> will fail.
> 

I think that is a good thing. The kernel should not be accepting things
it doesnt understand. This is a good opportunity to enforce that.

> What would happen before this patch is that configurations using such
> TC_ACT_XXXX value would be successful. This is why I proposed to keep
> the fixup.
>

Note: Such behavior can only occur if tc(user space) allows you
to pass illegitimate values which today can only happen when you have a 
new user space but older kernel (with "old" starting with your current
changes).
iow, fixing a policy in a kernel which has no support for TC_ACT_XXXX
to translate intent to be TC_ACT_OK/PIPE is problematic (as i was
showing earlier).

> I initially thought the kernel behavior in the above scenario would
> match exactly TC_ACT_UNSPEC processing, but as you noted with the
> example in your previous email, TC_ACT_UNSPEC processing is actually a
> bit different.
> 

I worry: I dont think we can get a good default for most use
cases. No point in maintaining faulty  expectations
(because  IMO: the user will - eventually - fix their scripts if they
dont see expected behavior).

cheers,
jamal