[Cake] issue with Cake and bpf filter

Pete Heist pete at heistp.net
Wed Aug 22 05:51:59 EDT 2018

> On Aug 22, 2018, at 11:37 AM, Toke Høiland-Jørgensen <toke at toke.dk> wrote:
> Pete Heist <pete at heistp.net> writes:
>>> On Aug 22, 2018, at 8:17 AM, Jonathan Morton <chromatix99 at gmail.com> wrote:
>>> One difference between fq_codel and Cake is that the former - which
>>> has no shaper - will "bypass" packets when it's empty and there's no
>>> back-pressure filling it. In that case no packet classification
>>> occurs and filters will not be called. Or at least, that's how it
>>> used to be set up; I haven't looked at it recently. Cake does not
>>> rely on the same set of assumptions, so will always call the filter.
>> Aha, that sounds likely, I’ll try with htb and a rate limit. Testing
>> with fq_codel was challenging as I had to “do stuff” until my printk’s
>> were eventually called, but it’s easier now that I can use cake. I
>> suppose in my case fq_codel’s behavior would be ok in production,
>> because if there’s no queue then there’s no need to classify. Maybe in
>> some other cases (like gathering stats), it could be problematic.
> fq_codel turns off the bypass capability if you attach a tc filter to
> it, though, so if the issue you're seeing is that you filter function is
> not being called, that sounds... strange...
> How do you check if the function is being called?

With printk and "sudo tc exec bpf dbg” to tail the output. For example, in act_main of this file I just added one more line to print “act_main” if TCU_DEBUG is defined:

https://github.com/heistp/tc-users/blob/master/tc-users-bpf.c <https://github.com/heistp/tc-users/blob/master/tc-users-bpf.c>

And here’s an example of verifier weirdness (not that you need to look into this)…

If IPV6_SUPPORT_V2 is defined but TCU_DEBUG is not, the verifier error is:

"math between pkt pointer and 4294901760 is not allowed"

If IPV6_SUPPORT_V2 is defined but TCU_DEBUG *is* (which just defines the prink func and adds two printk lines, and otherwise works without IPV6_SUPPORT_V2 defined), the verifier error is:

"R3 bitwise operator &= on pointer prohibited"
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