Subject: Re: Are BPF tail calls only supposed to work with pinned maps?

From: Toke Høiland-Jørgensen <[email-masked]>
Date: Thu, 26 Sep 2019 15:12:30 +0200

Daniel Borkmann <[email-masked]> writes:

> Hi Toke,
>
> On Thu, Sep 26, 2019 at 01:23:38PM +0200, Toke Høiland-Jørgensen wrote:
> [...]
>> While working on a prototype of the XDP chain call feature, I ran into
>> some strange behaviour with tail calls: If I create a userspace program
>> that loads two XDP programs, one of which tail calls the other, the tail
>> call map would appear to be empty even though the userspace program
>> populates it as part of the program loading.
>> 
>> I eventually tracked this down to this commit:
>> c9da161c6517 ("bpf: fix clearing on persistent program array maps")
>
> Correct.
>
>> Which clears PROG_ARRAY maps whenever the last uref to it disappears
>> (which it does when my loader exits after attaching the XDP program).
>> 
>> This effectively means that tail calls only work if the PROG_ARRAY map
>> is pinned (or the process creating it keeps running). And as far as I
>> can tell, the inner_map reference in bpf_map_fd_get_ptr() doesn't bump
>> the uref either, so presumably if one were to create a map-in-map
>> construct with tail call pointer in the inner map(s), each inner map
>> would also need to be pinned (haven't tested this case)?
>
> There is no map in map support for tail calls today.

Not directly, but can't a program do:

tail_call_map = bpf_map_lookup(outer_map, key);
bpf_tail_call(tail_call_map, idx);

>> Is this really how things are supposed to work? From an XDP use case PoV
>> this seems somewhat surprising...
>> 
>> Or am I missing something obvious here?
>
> The way it was done like this back then was in order to break up cyclic
> dependencies as otherwise the programs and maps involved would never get
> freed as they reference themselves and live on in the kernel forever
> consuming potentially large amount of resources, so orchestration tools
> like Cilium typically just pin the maps in bpf fs (like most other maps
> it uses and accesses from agent side) in order to up/downgrade the agent
> while keeping BPF datapath intact.

Right. I can see how the cyclic reference thing gets thorny otherwise.
However, the behaviour was somewhat surprising to me; is it documented
anywhere?

I think I'll probably end up creating a new map type for chaining
programs anyway, so this is not a huge show-stopper for me; but it had
me scratching my head for a while there... ;)

-Toke