Den ons 16 jan. 2019 kl 16:41 skrev Daniel Borkmann <dan...@iogearbox.net>: > [...] > > > For the fixed prologue: how does, say x86, deal with BPF stack usage > > in the tail call case? If the caller doesn't use the bpf stack, but > > the callee does. From a quick glance in the code, the x86 prologue > > still uses aux->stack_depth. If the callee has a different stack usage > > that the caller, and then the callee does a function call, wouldn't > > this mess up the frame? (Yeah, obviously missing something! :-)) > > Basically in this case verifier sets stack size to MAX_BPF_STACK when it > finds a tail call in the prog, meaning the callee will be reusing <= stack > size than the caller and then upon exit unwinds it via leave+ret. >
Ugh, so for "dynamic" tail calls this would mean "more expensive functions calls with maximum stack usage per call"? I.e. each tail call consumes MAX_BPF_STACK plus regular pro-/epilogue plus depth tracking. I'd still prefer optimizing to regular functions calls, than tail calls -- or is that naive? What is most common in larger bpf deployments, say, Katran or Cilium? Cheers! Björn > Cheers, > Daniel
