Reproducible BTF tarballs

[brycekahle]

I believe that if you removed a BTF .tar.xz file and re-ran the script, you would get a different output. The actual BTF should be the same, but because tar is preserving mtime, users, and possible other data, the .tar.xz file will always be different.

I can see two potential avenues to explore:

1. Extract a datetime from the package repo to use as the mtime for the BTF file itself
2. Force the same datetime for all files. This StackOverflow answer talks about a way to do that.

WDYT?

[brycekahle]

For context, I think this is valuable so we can re-run the script on the archive when things change, such as pahole arguments, and only update what actually changes.

[rafaeldtinoco]

Sorry has been some busy last weeks. I wonder if the BTF ids coming from pahole won't be random. From libbpf, the BTF ids are created as the types are added to the "BTF" object.

I believe pahole has this process parallelized which would cause the IDs to, maybe, be different from one run to another.

[brycekahle]

We aren't using -j, so it isn't an issue: acmel/dwarves#42