Re: [PATCH v4 bpf-next 0/4] Add libbpf-provided extern variables support

[Alexei Starovoitov <alexei.starovoitov@gmail.com>]

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.