netdev - Re: [PATCH v2] sched: report if filter is too large to dump

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Date:   Wed, 21 Feb 2018 09:38:52 +0100
From:   Roman Kapl <code@...pl.cz>
To:     Phil Sutter <phil@....cc>, netdev@...r.kernel.org,
        "David S . Miller" <davem@...emloft.net>,
        Jiri Pirko <jiri@...nulli.us>,
        Cong Wang <xiyou.wangcong@...il.com>,
        Jiri Benc <jbenc@...hat.com>
Subject: Re: [PATCH v2] sched: report if filter is too large to dump

On 02/21/2018 08:45 AM, Phil Sutter wrote:
> Hi Roman,
>
> On Mon, Feb 19, 2018 at 09:32:51PM +0100, Roman Kapl wrote:
>> So far, if the filter was too large to fit in the allocated skb, the
>> kernel did not return any error and stopped dumping. Modify the dumper
>> so that it returns -EMSGSIZE when a filter fails to dump and it is the
>> first filter in the skb. If we are not first, we will get a next chance
>> with more room.
>>
>> I understand this is pretty near to being an API change, but the
>> original design (silent truncation) can be considered a bug.
>>
>> Note: The error case can happen pretty easily if you create a filter
>> with 32 actions and have 4kb pages. Also recent versions of iproute try
>> to be clever with their buffer allocation size, which in turn leads to
> I'm curious, what does it lead to? :)
>
> Thanks, Phil

tc will dump all filters up to the big one, then it will stop silently. 
So it will seem as if you have less filters.

The new behavior is the same, but tc will at leas print the EMSGSIZE 
error. It does not handle it in any other way.