netdev - Re: [PATCH net-next v6 2/3] net sched actions: dump more than TCA_ACT_MAX

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Date:   Fri, 21 Apr 2017 11:40:00 -0400
From:   Jamal Hadi Salim <jhs@...atatu.com>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     Jiri Pirko <jiri@...nulli.us>, davem@...emloft.net,
        xiyou.wangcong@...il.com, netdev@...r.kernel.org
Subject: Re: [PATCH net-next v6 2/3] net sched actions: dump more than
 TCA_ACT_MAX_PRIO actions per batch

On 17-04-21 11:20 AM, Eric Dumazet wrote:
> On Fri, 2017-04-21 at 11:12 -0400, Jamal Hadi Salim wrote:
>> On 17-04-21 09:12 AM, Jiri Pirko wrote:
>>> Fri, Apr 21, 2017 at 12:55:31PM CEST, jhs@...atatu.com wrote:
>>>> From: Jamal Hadi Salim <jhs@...atatu.com>
>>

>> Jiri, there is a balance between extensibility and performance.
>> It is senseless to use a TLV just so i can set a 0/1(true/false).
>
> You assume that the (user space) did sensible things.
>

If it didnt it is a bug. Seriously.
Look: If this was the case a lot of things would break in the
kernel. We have bit flags everywhere. We add new ones frequently
to these bitmaps.

> Sometimes they do not, and sets some bits to 1 while they should not.
>

That is a bug.  You cant blame the compiler.

> If old kernel just ignored theses bits, application just ran fine and
> was _qualified_.
>
> Now customers might use these _working_ applications.
>
> Then, Jamal comes and change the kernel to give a meaning to these bits.
>

As happens frequently, not just Jamal - Eric also;->

> Now the customer is running the new kernel and the old application
> breaks horribly.
>
> Who is at fault ? Jamal of course, not the application authors that
> might be out of business, and could not have any test that could have
> spot the (future) issue.
>
> Please Jamal, can we stop this for good ?

Eric: Your are speaking in generalities and you starting premise is
wrong. Anyone setting random bits in a netlink bitmask that has not
been defined is creating bug. We have many examples of how netlink
bitmasks are being used and constantly extended. Please take a look.

If i was the first person starting this today, then yes you will be
making a lot of sense.
For the pads - the arguement that malloc-ing the datastructure may put
random values in the pads was a reasonable arguement. But this is not.

cheers,
jamal