Option parsing: btf_features=default is not the default?

[martinetd]

This might be a feature but I'm surprised, the output with --btf_features=default and without any --btf_features seem different:

for opt in "" --btf_features=default; do
   cp .tmp_vmlinux.orig .tmp_vmlinux.btf
   LLVM_OBJCOPY=objcopy pahole -J --btf_gen_floats --lang_exclude=rust --skip_encoding_btf_inconsistent_proto --btf_gen_optimized .tmp_vmlinux.btf $opt
  echo -n "$opt: "
  sha1sum .tmp_vmlinux.btf
done

: 58fcc73788cc9700d4e5ee136334e51b6b9ad601  .tmp_vmlinux.btf
--btf_features=default: 926851f26c73a3014220764f7a1299598afeb11f  .tmp_vmlinux.btf
(ran twice to confirm it's reproducible: there's no threading enabled in the above command)

Looking at the code it's because init_btf_features() is only called when the option is actively parsed; so the default values as per btf_features defaults aren't set unless the option is called.

This also has the side effect that --reproducible_build is unset by a following --btf_features option, e.g. --reproducible_build --btf_features=default will have reproducible build unset, but --btf_features=default --reproducible_build has it set.
(If the delayed init is on purpose, this could be resolved by making the default value an enum true/false/don't-touch, but I'd rather ask first)

Thanks!

[alan-maguire]

yeah we've struggled with the terminology a bit. initially "--btf_features=default" was called "--btf_features=all" but the problem with that was it wasn't really all the BTF features; some are optionally included for split BTF like distilled_base. I get that "--btf_features=default" would appear to be sensible as a default, but the problem is that for backwards-compatibility purposes some features that are part of "default" are opt-in and only applicable to certain kernel versions. In other words the default is only a sensible default for more recent Linux kernels; it's really shorthand for "default set of BTF features that the most recent kernels support". If you can think of ways to articulate this more clearly feel free to send patches for docs etc; I agree it's confusing!