netdev - Re: [PATCH net-next v4 1/2] net sched actions: dump more than TCA_ACT_MAX

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Date:   Fri, 21 Apr 2017 06:36:19 -0400
From:   Jamal Hadi Salim <jhs@...atatu.com>
To:     David Miller <davem@...emloft.net>
Cc:     eric.dumazet@...il.com, jiri@...nulli.us, netdev@...r.kernel.org,
        xiyou.wangcong@...il.com
Subject: Re: [PATCH net-next v4 1/2] net sched actions: dump more than
 TCA_ACT_MAX_PRIO actions per batch

On 17-04-20 01:58 PM, David Miller wrote:
> From: Jamal Hadi Salim <jhs@...atatu.com>
> Date: Thu, 20 Apr 2017 13:38:14 -0400
>

>> There are no examples of such issues with bitmasks encapsulated in
>> TLVs

>> It does not make much sense to have a TLV for each of these
>> bits when i can fit a bunch of them in u16/32/64.
>
> I have not ruled out bitmasks.  I'm only saying that the kernel must
> properly reject bits it doesn't recognize when they are set.
>

It is the other way round from what i see: It ignores them.
This allows new bits to be added over time.
Note: It is a bug - which must be fixed - if user space sets
something the kernel doesnt want it to set. Even then, the only good
use case i can think of for something like this is the kernel
is exposing something to user space for read-only and user space
is being silly and setting read-only bits on requests to the kernel.
But even that is not a catastrophic issue; kernel should just ignore it.

> Each bit must have a strict semantic, even unused ones, otherwise
> unused ones may never safely be used in the future.
>

I think we are pretty good at this.
It would be interesting to have a fuzzer which sets random bits on a
TLV bitmask and see what bugs show up.

cheers,
jamal