The Linux Concept Journey — BTF (BPF Type Format)

BTF (BPF Type Format) is a compact metadata format that describes the data types and function signatures used by BPF programs. Created as a space efficient alternative to DWARF format (https://medium.com/@boutnaru/the-linux-concept-journey-dwarf-debugging-with-attributed-record-format-5e3b98103bb0). BTF uses a deduplication algorithm to maintain a small footprint while remaining expressive enough to represent complex C programs. The ELF File Format defines how metadata is stored for loaders like libbpf, while the Kernel API ensures the kernel can validate this information for use in map visualization\JITed code annotation\enhanced verifier logs (https://docs.ebpf.io/concepts/btf/).

Overall, BTF is a compact binary format that simplifies eBPF development by embedding software data type details directly into the Linux kernel. Unlike the bulky DWARF debug data (often 200MB+), BTF deduplicates information into a lightweight section (approx. 5MB). Hence, making it feasible to load into memory at runtime. During the kernel build, if “CONFIG_DEBUG_INFO_BTF” (https://www.kernelconfig.io/CONFIG_DEBUG_INFO_BTF) is enabled, the “link-vmlinux.sh” (https://elixir.bootlin.com/linux/v6.19.3/source/scripts/link-vmlinux.sh) script invokes the “pahole” tool (https://linux.die.net/man/1/pahole). This tool parses existing DWARF data to generate BTF information, which is then appended to the vmlinux binary in the “.BTF” section (https://bootlin.com/blog/the-dwarves-beneath-the-kernel-forging-btf-for-ebpf/). The same is also perfromed when compiling a BPF program- as shown in the screenshot below (https://www.ebpf.top/post/btf-bpf-type-format/).

Lastly, BTF provides a structural map of its internal data types and memory layouts. It is a basic building block of CO-RE (Compile-Once, Run-Everywhere) — more on that in futreu writeups. Thus, BTF allows eBPF programs to remain portable across different kernel versions. It works alongside the compiler\Clang and the “libbpf” loader to automatically relocate field offsets that shift between releases. Beyond portability, BTF transforms the observability experience by allowing tools like “bpftool” (https://bpftool.dev/) to “pretty print” raw hex data into human readable structures and enabling developers to use “vmlinux.h” to access all kernel types (https://ebpfchirp.substack.com/p/what-is-btf-bpf-type-format).

See you in my next writeup ;-) You can follow me on twitter — @boutnaru (https://twitter.com/boutnaru). Also, you can read my other writeups on medium — https://medium.com/@boutnaru. You can find my free eBooks at https://TheLearningJourneyEbooks.com.

https://www.ebpf.top/post/btf-bpf-type-format/