On Fri, Dec 13, 2019 at 05:47:06PM -0800, Andrii Nakryiko wrote: > It's often important for BPF program to know kernel version or some specific > config values (e.g., CONFIG_HZ to convert jiffies to seconds) and change or > adjust program logic based on their values. As of today, any such need has to > be resolved by recompiling BPF program for specific kernel and kernel > configuration. In practice this is usually achieved by using BCC and its > embedded LLVM/Clang. With such set up #ifdef CONFIG_XXX and similar > compile-time constructs allow to deal with kernel varieties. > > With CO-RE (Compile Once – Run Everywhere) approach, this is not an option, > unfortunately. All such logic variations have to be done as a normal > C language constructs (i.e., if/else, variables, etc), not a preprocessor > directives. This patch series add support for such advanced scenarios through > C extern variables. These extern variables will be recognized by libbpf and > supplied through extra .extern internal map, similarly to global data. This > .extern map is read-only, which allows BPF verifier to track its content > precisely as constants. That gives an opportunity to have pre-compiled BPF > program, which can potentially use BPF functionality (e.g., BPF helpers) or > kernel features (types, fields, etc), that are available only on a subset of > targeted kernels, while effectively eleminating (through verifier's dead code > detection) such unsupported functionality for other kernels (typically, older > versions). Patch #3 explicitly tests a scenario of using unsupported BPF > helper, to validate the approach. > > This patch set heavily relies on BTF type information emitted by compiler for > each extern variable declaration. Based on specific types, libbpf does strict > checks of config data values correctness. See patch #1 for details. > > Outline of the patch set: > - patch #1 does a small clean up of internal map names contants; > - patch #2 adds all of the libbpf internal machinery for externs support, > including setting up BTF information for .extern data section; > - patch #3 adds support for .extern into BPF skeleton; > - patch #4 adds externs selftests, as well as enhances test_skeleton.c test to > validate mmap()-ed .extern datasection functionality. Applied. Thanks. Looking at the tests that do mkstemp()+write() just to pass a file path as .kconfig_path option into bpf_object_open_opts() it feels that file only support for externs is unnecessary limiting. I think it will simplify tests and will make the whole extern support more flexible if in addition to kconfig_path bpf_object_open_opts() would support in-memory configuration.
