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

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Date:   Thu, 20 Apr 2017 13:38:14 -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 11:50 AM, David Miller wrote:
> From: Jamal Hadi Salim <jhs@...atatu.com>
> Date: Thu, 20 Apr 2017 09:27:00 -0400
>
>> The issue Jiri is bringing up is unrelated. He is talking about
>> a bitmap and conflating it with a data structure. They are not
>> the same issue.
>
> Bitmaps can have the same exact problem as padding if we didn't code
> it correctly.
>
> The issue is _purely_, "did we check unused 'fields' and enforce them
> to be a certain value"
>
> If not, we lose, and can't use those "fields" in the future.
>
> This rule applies whether you are speaking about padding or a bitmask.
>

There are no examples of such issues with bitmasks encapsulated in TLVs
that exist.
I grep iproute2 code and there are tons of example of bitmask flags
being sent in TLVs. They all start with:

u64/32/16 mybitflags = 0;

if i want foo then
        mybitflags |= BRIDGE_FLAGS_SELF;
if i want bar then
        mybitflags |= xxxx

addattr16/32/64(&req.n, sizeof(req), ATTR_XXX, mybitflags);

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.

cheers,
jamal