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

From: Toke Høiland-Jørgensen <[email-masked]>
Date: Thu, 26 Sep 2019 13:23:38 +0200

Hi Daniel

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")

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)?

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?

-Toke