Enable BTF headers for Linux kernel

From: Francis Laniel <[email protected]>

Hi.

First of all, I hope you are fine and the same for your relatives.

Extended Berkeley Packet Filter (eBPF) is a virtual machine inside the kernel which permits executing safe code [1].
This code can, for example, be used to hook kernel events.

BPF Compile Once - Run Everywhere (BPF CO-RE) is thought as an evolution of conventional BPF program [2].
BPF CO-RE relies on BPF Type Format (BTF) to be executed on different kernels. This is done thanks to libbpf, the BPF loader, which resolves symbols contained into the BPF binary against symbols offered by the kernel.

Kernel shares its BTF symbols through /sys/kernel/btf/vmlinux which is enabled by CONFIG_DEBUG_INFO_BTF.
Nonetheless, CONFIG_DEBUG_INFO_BTF relies on pahole to convert DWARF symbols to BTF [3].
Thus, this series enable BTF headers for Linux kernel by:
1. First, adding pahole as host package by compiling it from source taken from its git repository.
2. Second, adding pahole as kernel dependencies if user selected BR2_PACKAGE_HOST_PAHOLE.

I understand Buildroot and BPF can seem to be far from each other at first glance.
But Buildroot, while mainly used to generate embedded Linux systems, is also used to generate "qemu ready" image [4].
More particularly, minikube, a tool used to deploy Kubernetes locally, relies on buildroot to generate its system image [5, 6].

I use minikube daily to test a tool we develop for Kubernetes and I wrote a patch to enable CONFIG_DEBUG_INFO_BTF [7, 8].
Thus, I thought it could be a good idea to share this patch upstream as this feature would be welcomed by the community [9].

Regarding the series itself, I tested it by compiling a system for qemu_x86_defconfig with BR2_LINUX_KERNEL_NEEDS_HOST_PAHOLE.
Then, I booted the system using qemu to check if /sys/kernel/btf/vmlinux exists, which is the case:
Welcome to Buildroot
buildroot login: root
# [ -f /sys/kernel/btf/vmlinux ] && echo yes
yes

If you have any remarks regarding this series, feel free to share as I would be happy to handle them to increase this contribution quality.

Changes since v1:
* Added pahole COPYING hash.
* Jumped pahole version from v1.22 to v1.23.
* Added host-elfutils as host-pahole dependencies (it does not build without).
* Built dynamically libppf for pahole instead of statically.
* Added BR2_LINUX_KERNEL_NEEDS_HOST_PAHOLE which acts like BR2_LINUX_KERNEL_NEEDS_HOST_LIBELF. If CONFIG_DEBUG_INFO_BTF is set and this option not, kernel .config will not be written and an error will be printed.

Francis Laniel (2):
package/pahole: new host package
linux: add BR2_LINUX_KERNEL_NEEDS_HOST_PAHOLE

 DEVELOPERS                    |  3 +++
 linux/Config.in               | 12 ++++++++++++
 linux/linux.mk                | 12 ++++++++++++
 package/Config.in.host        |  1 +
 package/pahole/Config.in.host |  8 ++++++++
 package/pahole/pahole.hash    |  3 +++
 package/pahole/pahole.mk      | 18 ++++++++++++++++++
 7 files changed, 57 insertions(+)
 create mode 100644 package/pahole/Config.in.host
 create mode 100644 package/pahole/pahole.hash
 create mode 100644 package/pahole/pahole.mk

Best regards and thank you in advance for your reviews.

---
[1] https://man7.org/linux/man-pages/man2/bpf.2.html
[2] https://nakryiko.com/posts/bpf-portability-and-co-re/
[3] https://elixir.bootlin.com/linux/v5.15.10/source/lib/Kconfig.debug#L315
[4] https://petermalmgren.com/qemu-buildroot/
[5] https://github.com/kubernetes/minikube
[6] https://github.com/kubernetes/minikube/blob/7dc8836303721c4486c007945beb5272cf28601c/deploy/iso/minikube-iso/configs/minikube_defconfig
[7] https://github.com/kinvolk/inspektor-gadget
[8] https://github.com/kinvolk/minikube/commit/44327414939f8acc08e095e789bc41f7dc73099e
[9] https://github.com/kubernetes/minikube/pull/12707#issuecomment-986990849
--
2.30.2

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