On Tue, Jun 7, 2022 at 5:12 AM Michal Koutný <mkoutny@xxxxxxxx> wrote: > > On Mon, Jun 06, 2022 at 12:41:06PM -0700, Yosry Ahmed <yosryahmed@xxxxxxxxxx> wrote: > > I don't know if there is a standard way to handle this, but I think > > you should know the configs of your kernel when you are loading a bpf > > program? > > Isn't this one of purposes of BTF? (I don't know, I'm genuinely asking.) > > > If the CONFIG_CGROUPS=1 but CONFIG_MEMCG=0 I think everything will > > work normally except that task_memcg() will always return NULL so no > > stats will be collected, which makes sense. > > I was not able to track down what is the include chain to > tools/testing/selftests/bpf/progs/cgroup_vmscan.c, i.e. how is the enum > value memory_cgrp_id defined. memory_cgrp_id is defined in "vmlinux.h" (generated from BTF) which is included through "bpf_iter.h". If the kernel is not compiled with CONFIG_MEMCG then this enum value will not be defined and the bpf prog should not compile. > > (A custom kernel module build requires target kernel's header files, I > could understand that compiling a BPF program requires them likewise and > that's how this could work. > Although, it goes against my undestanding of the CO-RE principle.) > > > There will be some overhead to running bpf programs that will always > > do nothing, but I would argue that it's the userspace's fault here for > > loading bpf programs on a non-compatible kernel. > > Yeah, running an empty program is non-issue in my eyes, I was rather > considering whether the program uses proper offsets. > > Michal >
