On Fri, 17 Sept 2021 at 22:57, Alexei Starovoitov <alexei.starovoitov@xxxxxxxxx> wrote: > > From: Alexei Starovoitov <ast@xxxxxxxxxx> > > Hi All, > > This is very early RFC that introduces CO-RE support in the kernel. > There are several reasons to add such support: > 1. It's a step toward signed BPF programs. > 2. It allows golang like languages that struggle to adopt libbpf > to take advantage of CO-RE powers. > 3. Currently the field accessed by 'ldx [R1 + 10]' insn is recognized > by the verifier purely based on +10 offset. If R1 points to a union > the verifier picks one of the fields at this offset. > With CO-RE the kernel can disambiguate the field access. > > This set wires all relevant pieces and passes two selftests with CO-RE > in the kernel. > > The main goal of RFC is to get feedback on patch 3. > It's passing CO-RE relocation into the kernel via bpf_core_apply_relo() > helper that is called by the loader program. > It works, but there is no clean way to add error reporting here. > So I'm thinking that the better approach would be to pass an array > of 'struct bpf_core_relo_desc' into PROG_LOAD command similar to > how func_info and line_info are passed. > Such approach would allow for the use case 3 above (which > current approach in patch 3 doesn't support). +1 to having good error reporting, it's hard to debug CO-RE failures as they are. PROG_LOAD seems nice, provided that relocation happens before verification. Some questions: * How can this handle kernels that don't have built-in BTF? Not a problem for myself, but some people have to deal with BTF-less distro kernels by using pahole to generate external BTF from debug symbols. Can we accommodate that? * Does in-kernel CO-RE need to account for packed structs w/ bitfields in them? If relocation happens after verification this could be a problem: [1]. * Tangent: libbpf CO-RE has this res->validate flag, which turns off some checks for bitfields. I've never fully understood why that is, maybe Andrii can explain it? Lorenz 1: https://lore.kernel.org/bpf/CACAyw9_R4_ib0KvcuQC4nSOy5+Hn8-Xq-G8geDdLsNztX=0Fsw@xxxxxxxxxxxxxx/ -- Lorenz Bauer | Systems Engineer 6th Floor, County Hall/The Riverside Building, SE1 7PB, UK www.cloudflare.com
