libbpf VS linux

The whole BPF verifier and development process is so botched, it's ridiculous. It's like maintainers decided to make this as hard as possible out of pettiness and "they have to use C APIs instead" or something.

- Loading an eBPF module without the CAP_BPF (and in some cases without the CAP_NET_ADMIN which you need for XDP) capabilities will generate a "unknown/invalid memory access" error which is super useless as an error message.

- In my personal opinion a bytecode format for both little endian (bpfel) and big endian (bpfeb) machines is kinda unnecessary. I mean, it's a virtual bytecode format for a reason, right!?

- Compiling eBPF via clang to the bpf bytecode format without debug symbols will make every following error message down the line utterly useless. Took me a while to figure out what "unknown scalar" really means. If you forget that "-g" flag you're totally fucked.

- Anything pointer related that eBPF verifier itself doesn't support will lead to "unknown scalar" errors which are actually out of bounds errors most of the time (e.g. have to use if pointer < size(packet) around it), which only happen in the verification process and can only be shown using the bpftool. If you miss them, good luck getting a better error message out of the kernel while loading the module.

- The bpftool maintainer is kind of unfriendly, he's telling you to read a book about the bytecode format if your code doesn't compile and you're asking about examples on how to use pointers inside a BPF codebase because it seems to enforce specific rules in terms of what kind of method (__always_static) are allowed to modify or allocate memory. There's a lot of limitations that are documented _nowhere_ on the internet, and seemingly all developers are supposed to know them by reading the bpftool codebase itself!? Who's the audience for using the bpftool then? Developers of the bpftool itself?

- The BCC tools (bpf compiler collection) are still using examples that can't compile on an up-to-date kernel. [1] If you don't have the old headers, you'll find a lot of issues that show you the specific git hash where the "bpf-helpers.h" file was still inside the kernel codebase.

- The libbpf repo contain also examples that won't compile. Especially the xdp related ones [2]

- There's also an ongoing migration of all projects (?) to xdp-tools, which seems to be redundant in terms of bpf related topics, but also has only a couple examples that somehow work [3]

- Literally the only userspace eBPF generation framework that worked outside a super outdated enterprise linux environment is the cilium ebpf project [4], but only because they're using the old "bpf-helpers.h" file that are meanwhile removed from the kernel itself. [5] They're also incomplete for things like the new "__u128" and "__bpf_helper_methods" syntax which are sometimes missing.

- The only working examples that can also be used for reference on "what's available" in terms of eBPF and kernel userspace APIs is a forked repo of the bootlin project [6] which literally taught me how to use eBPF in practice.

- All other (official?) examples show you how to make a bpf_printk call, but _none_ of them show you how to even interact with bpf maps (whose syntax changed like 5 times over the course of the last years, and 4 of them don't run through the verifier, obviously). They're also somewhat documented in the wiki of the libbpf project, without further explanation on why or what [7]. Without that bootlin repo I still would have no idea other than how to make a print inside a "kretprobe". Anything more advanced is totally undocumented.

- OpenSnitch even has a workflow that copies their own codebase inside the kernel codebase, just to make it compile - because all other ways are too redundant or too broken. Not kidding you. [8]

Note that none of any BPF related projects uses any kind of reliable version scheme, and none of those project uses anything "modern" like conan (or whatever) as a package manager. Because that would have been too easy to use, and too easy on documenting on what breaks when. /s

Overall I have to say, BPF was the worst development experience I ever had. Writing a kernel module is _easier_ than writing a BPF module, because then you have at least reliable tooling. In the BPF world, anything will and can break at any unpredictable moment. If you compare that to the experience of other development environments like say, JVM or even the JS world, where debuggers that interact with JIT compilers are the norm, well ... then you've successfully been transferred back to the PTSD moments of the 90s.

Honestly I don't know how people can use BPF and say "yeah this has been a great experience and I love it" and not realize how broken the tooling is on every damn level.

I totally recommend reading the book [9] and watching the YouTube videos of Liz Rice [10]. They're awesome, and they show you how to tackle some of the problems I mentioned. I think that without her work, BPF would have had zero chance of success.

What's missing in the BPF world is definitely better tooling, better error messages (e.g. "did you forget to do this?" or even "unexpected statement" would be sooooo much better than the current state), and an easier way to debug an eBPF program. Documentation on what's available and what is not is also necessary, because it's impossible to find out right now. If I am not allowed to use pointers or whatever, then say so in the beginning.

[1] https://github.com/iovisor/bcc

[2] https://github.com/libbpf/libbpf

[3] https://github.com/xdp-project/xdp-tools

[4] https://github.com/cilium/ebpf/

[5] https://github.com/cilium/ebpf/tree/master/examples/headers

[6] https://elixir.bootlin.com/linux/latest/source/tools/testing...

[7] https://github.com/libbpf/libbpf/wiki/Libbpf-1.0-migration-g...

[8] https://github.com/evilsocket/opensnitch/blob/master/ebpf_pr...

[9] https://isovalent.com/learning-ebpf/

[10] (e.g.) https://www.youtube.com/watch?v=L3_AOFSNKK8

Wasm-bpf is a WebAssembly eBPF library, toolchain and runtime powered by CO-RE(Compile Once – Run Everywhere) libbpf and WAMR. It can help you build almost every eBPF programs or use cases to Wasm.

Starting in v0.2, libbpf supports multiple entry-point BPF programs within the same executable and linkable format (ELF) section. Therefore, you can attach multiple BPF programs to the same event (such as tracepoints or kprobes) without worrying about ELF section name clashes. For details, see Add libbpf full support for BPF-to-BPF calls. Now, you can naturally define multiple handlers for an event like this:

These are a bit easier to see what's going on:

https://github.com/torvalds/linux/commit/d5cf50dafc9dd5faa1e...

https://github.com/torvalds/linux/blob/d5cf50dafc9dd5faa1e61...

Unfortunately Github doesn't have a way to render symbols for whitespace, but you can tell by selecting the spaces that the previous version had leading tabs. Linus changed it so that the tokens `default` and the number e.g. `12` are also separated by a tab. This is tricky, because the token "default" is seven characters, it will always give this added tab a width of 1 char which makes it always layout the same as if it were a space no matter if you use tab widths of 1, 2, 4, or 8.

There was also a Gentoo effort to run atop FreeBSD[0]. The challenge of course is that afaik none of the BSD kernel ABIs are considered stable. The stable interface is the BSD libc. That said, with binfmt_misc, I don't see a reason you couldn't just run (at least some) FreeBSD binaries on Linux with a thin syscall translation layer (rather something like qemu-system) and then your layer hooked via binfmt_misc. I'm not aware of anyone who has done this for FreeBSD, but prior efforts existed as alternate binfmts for SysVr4/5 ELF binaries[2]. Either way would take some elbow grease, but you *might* even be able just reuse binfmt_elf and just have a new interpreter for FreeBSD elf.

[0] https://wiki.gentoo.org/wiki/Gentoo_FreeBSD

[1] https://docs.kernel.org/admin-guide/binfmt-misc.html

[2] https://github.com/torvalds/linux/blob/master/fs/binfmt_elf....

> The original less-than check was deemed incorrect

It was only deemed incorrect because of an information leak. Not because it's a valid use-case for user space to copy smaller portions of *hwrpb into user space. https://github.com/torvalds/linux/commit/21c5977a836e399fc71...

Correct. And the combined work needs to carry the MIT license text and copyright attributions for the MIT software authors. With binary distribution it must also be overt, not hidden in some source code drop, but directly accompanying the binary.

Many people who talk about relicensing never credit the MIT developers or distribute the MIT license text. "Because it's GPL now."

I don't think that you believe that, but many developers do.

Some don't see the need for source code scans for Open Source compliance, because the license.txt says GPL, so it's GPL. Prime example is the Linux kernel. There is code under different licenses in there, but people don't even read https://github.com/torvalds/linux/blob/master/COPYING till the end ("In addition, other licenses may also apply.") and conclude it's simply GPL 2 and nothing else.

Also be aware that sublicensing is not the same as relicensing.

> Does he have something against it?

He notoriously hates GNU Emacs, yes.

https://marc.info/?m=122955159617722

https://github.com/torvalds/linux/blob/master/Documentation/...

So If we would only count code and not comments, it is only 9489 LoC Rust. Which would be about 0.03% and if we take all lines and not only LoC it would be around 0.05%

[0] https://github.com/XAMPPRocky/tokei

[1] https://github.com/torvalds/linux/commit/b401b621758e46812da...

AIUI fsync is built on futex_waitv which has been upstreamed. So this has to be more than that.

https://github.com/torvalds/linux/commit/a0eb2da92b715d0c97b...

git clone --no-checkout --depth 1 https://github.com/torvalds/linux.git $dir