On Thu, Jun 16, 2022 at 05:00 PM +02, Maciej Fijalkowski wrote: > On Thu, Jun 16, 2022 at 01:02:52PM +0200, Jakub Sitnicki wrote: >> While working aarch64 JIT to allow mixing bpf2bpf calls with tailcalls, I >> noticed unexpected tailcall behavior in x86 JIT. >> >> I don't know if it is by design or a bug. The bpf_tail_call helper >> documentation says that the user should not expect the control flow to >> return to the previous program, if the tail call was successful: >> >> > If the call succeeds, the kernel immediately runs the first >> > instruction of the new program. This is not a function call, >> > and it never returns to the previous program. >> >> However, when a tailcall happens from a subprogram, that is after a bpf2bpf >> call, that is not the case. We return to the caller program because the >> stack destruction is too shallow. BPF stack of just the top-most BPF >> function gets destroyed. >> >> This in turn allows the return value of the tailcall'ed program to get >> overwritten, as the test below test demonstrates. It currently fails on >> x86: > > Disclaimer: some time has passed by since I looked into this :P > > To me the bug would be if test would have returned 1 in your case. If I > recall correctly that was the design choice, so tailcalls when mixed with > bpf2bpf will consume current stack frame. When tailcall happens from > subprogram then we would return to the caller of this subprog. We added > logic to verifier that checks if this (tc + bpf2bpf) mix wouldn't cause > stack overflow. We even limit the stack frame size to 256 in such case. > > Cilium docs explain this: > https://docs.cilium.io/en/latest/bpf/#bpf-to-bpf-calls Thanks for such a quick response. This answers my question. I should have looked in Cilium docs. I will see how to work this bit of info into the helper docs. [...]
