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 10:51:56 -0400 (EDT)
From:   David Miller <davem@...emloft.net>
To:     jhs@...atatu.com
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

From: Jamal Hadi Salim <jhs@...atatu.com>
Date: Fri, 21 Apr 2017 06:36:19 -0400

> 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.

Which means we can never use them for anything else reliably,
there could be random crap in there.

> This allows new bits to be added over time.

No, ignoring them actually means we cannot add new bits.

> 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.

But since we didn't check and enforce, we can't use the bits for
settings however we like.

That's the entire point.

We can _never_ go back later and say "oops, add the checks now, it's
all good" because that doesn't work at all.