netdev - Re: [PATCH net-next v8 2/3] net sched actions: dump more than TCA_ACT_MAX

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Date:   Tue, 25 Apr 2017 16:29:40 -0400
From:   Jamal Hadi Salim <jhs@...atatu.com>
To:     Jiri Pirko <jiri@...nulli.us>
Cc:     davem@...emloft.net, xiyou.wangcong@...il.com,
        eric.dumazet@...il.com, netdev@...r.kernel.org
Subject: Re: [PATCH net-next v8 2/3] net sched actions: dump more than
 TCA_ACT_MAX_PRIO actions per batch

On 17-04-25 12:04 PM, Jiri Pirko wrote:
> Tue, Apr 25, 2017 at 03:01:22PM CEST, jhs@...atatu.com wrote:
>> On 17-04-25 08:13 AM, Jiri Pirko wrote:
>>> Tue, Apr 25, 2017 at 01:54:06PM CEST, jhs@...atatu.com wrote:

>>>>
>>>> +static inline bool tca_flags_valid(u32 act_flags)
>>>> +{
>>>> +	u32 invalid_flags_mask  = ~VALID_TCA_FLAGS;
>>>> +
>>>> +	if (act_flags & invalid_flags_mask)
>>>> +		return false;
>>>
>>> I don't see how this resolves anything. VALID_TCA_FLAGS is set in stone
>>> not going to change anytime in future, right?
>>
>> Every time a new bit gets added VALID_TCA_FLAGS changes.
>
> You mean flag that user can set? If that is the case, you are breaking
> UAPI for newer app running on older kernel.
>

Ok, let me try to explain with more clarity. The rules Iam
trying to follow are:
if i see any bit set that i dont understand I will reject.

So lets in first kernel I have support for bit 0.
My validation check is to make sure only bit 0 is set.
The valid_flags currently then only constitutes bit 0.
i.e
If you set bit 2 or 3, the function above will reject and i return
the error to the user.

That is expected behavior correct?

3 months down the road:
I add two flags - bit 1 and 2.
So now my valid_flags changes to bits 1, 2 and 0.

The function above will now return true for bits 0-2 but
will reject if you set bit 3.

That is expected behavior, correct?

On u32/16/8:
I am choosing u32 so it allows me to add more upto 32 bit flags.
Not all 32 are needed today but it is better insurance.
If I used u8 then the 24 of those 32 bits i dont use will be used
as pads in the TLV. So it doesnt make sense for me to use a u8/16.

Does that make more sense?

cheers,
jamal