Re: Weird failure with bpftool when building 6.11-rc4 with clang+rust+lto

[Matthew Maurer <matthew.r.maurer@gmail.com>]

Sorry that this isn't a solution, but I can tell you some background:

Linux currently relies on the `--lang_exclude` flag to `pahole` to
filter Rust debugging information out of the output BTF. This is done
because various downstream tools (for example, bpftool) do not handle
Rust types correctly. The `--lang_exclude` flag works by checking the
language flag on the compilation unit the type is referenced in. Once
LTO is enabled however, things can migrate from one compilation unit
to another, leading to C code having Rust code and types referenced
inside them. The resulting type will be considered C-language type by
`pahole`, but is actually a Rust type. Even if you fixed bpftool,
without additional patches/hacks, once the kernel boots it would
likely fail to parse its own BTF debugging information, and disable
BPF loading.

The most confusing part to me here is that I only encountered this
issue with x-lang LTO enabled, which is not available in the kernel
you're building from. If this is happening without x-lang LTO enabled,
it likely means that there's another way for debug symbols to leak
across CUs during LTO. That's where I'd start looking - use `pahole`
to dump the contents of `vmlinux.o` and see if you can find a
C-language CU referencing a Rust type. Then, try to figure out how
that's possible. With x-lang LTO it was obvious, inlining caused a
bunch of issues.

The last possibility I can think of is that somehow in your build
configuration `pahole` is not being invoked with the `--lang_exclude`
flag when building `vmlinux`. I don't know why that would be, but it
might be worth double checking.

On Tue, Aug 20, 2024 at 5:13 AM Neal Gompa <ngompa@fedoraproject.org> wrote:
>
> Hey all,
>
> While working on enabling Rust in the Fedora kernel[1], we've managed
> to get the setup almost completely working, but we have a build
> failure with the clang+lto build variant[2][3].
>
> Based on the build failure log[4][5], it looks like there's some
> random mixing of Rust inside of C code or something of the sort (which
> obviously would be invalid).
>
> Can someone help with this?
>
> Thanks in advance and best regards,
>
> [1]: https://gitlab.com/cki-project/kernel-ark/-/merge_requests/3295
> [2]: https://s3.amazonaws.com/arr-cki-prod-trusted-artifacts/index.html?prefix=trusted-artifacts/1419488480/build_x86_64/7618803903/artifacts/
> [3]: https://s3.amazonaws.com/arr-cki-prod-trusted-artifacts/index.html?prefix=trusted-artifacts/1419488480/build_aarch64/7618803917/artifacts/
> [4]: https://s3.amazonaws.com/arr-cki-prod-trusted-artifacts/trusted-artifacts/1419488480/build_x86_64/7618803903/artifacts/build-failure.log
> [5]: https://s3.amazonaws.com/arr-cki-prod-trusted-artifacts/trusted-artifacts/1419488480/build_aarch64/7618803917/artifacts/build-failure.log
>
>
> --
> Neal Gompa (FAS: ngompa)
>