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Comment by monocasa | Hacker News Reader

The end goal with bpf is to allow arbitrary untrusted programs to load bpf programs. If you were just loading kernel modules you wouldn't be able to maintain kernel integrity and let arbitrary programs load code.

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I can't find the LKML thread, but there is some fundamental problem with unloading modules safely. BPF programs might not have such limitations.

Oh, neat! Yeah, I didn't think of this but that totally makes sense.

BPF programs declare their resources to the kernel (maps, etc.), but modules are reliant on the __exit function cleaning everything up properly. The correctness of __exit is unverified, difficult, and practically one of the least tested pathways which makes it traditionally fraught with bugs.

A BPF->kernel module compiler would ensure all necessary cleanup happens in __exit automatically

A BPF to kernel module compiler wouldn't let the kernel verify the program in a real way. There's still work to be done, bit the end goal of BPF is pretty obviously to allow non root users to load programs.

Doing the verification offline is a non starter. Appending the verification information and reverifying it at load time is more work than a BPF runtime as it is as you have to reproject ISA semantics in a more complex way.

Hmm I think bpf today is used in fully trusted environments. Kernel level verification is unnecessary except in untrusted containerized environments or when running untrusted applications, both use cases being relatively rare/specialized.

I think the main benefit of bpf is that it prevents you from shooting yourself in the foot. Running totally untrusted code in the kernel just seems like a recipe for disaster and for that reason it makes sense bpf is still limited to root.

That's where it is today, but the end goal is removing that restriction. It probably would have happened quicker if Spectre/Meltdown hadn't come out of nowhere. Like it used to be that KVM required CAP_SYS_ADMIN as well, but now that's been opened up to whoever has permissions to the device file. Start requiring "own the box anyway" privileges while the feature bakes, but open it up as it becomes more mature and attackers have a go at it.

It's sort of like how originally you could only jump forward, then the opened it up to any DAG, then they allowed probably bounded loops.

And there's been OSes that don't require root for their in kernel virtual machines, XOK and AEGIS being the prominent examples.

Sure but I don’t see any compelling use cases to motivate opening it up. Do you have an example of one?

Even if the kernel opens it up without a compelling use case, it seems likely that distribution policy will keep it default locked to root.

Which is my point here. I don’t see a compelling reason to have a JIT in the kernel when AOT BPF seems to cover 90% of all existing use cases. In fact I may even write a bpf to kernel module compiler myself.

* Syscall tracing, sandboxing, and monitoring

* KVM device MMIO emulation

* OS personality emulation (like WSL but doesn't require root)

* New synchronization primitives to user space (like XOK's wake predicates)

* A lot of others..

Modern BPF is the exploring the same cornerstones as exokernels and really opens up a whole bunch of concepts that haven't been seen in mainstream kernels, particularly if non privileged users are allowed to invoke it.

Thanks for the examples but those all still seem like things vast majority of Linux users can do today, since vast majority of Linux users have root access. Both desktop and server.

Mobile users like android don’t have root but I don’t see why an untrusted mobile app would need bpf.

Only benefit of allowing non-root that I can see is enabling untrusted containers in cloud environments to do the same. All large cloud providers use KVM/zen (not containers) for untrusted users in which case they already have root.

Can you give an example of a scenario where the user doesn’t have root yet still would want to do those things?

Loading BPF requires root, so does loading kernel modules. I.e. if you have permissions to load BPF, you can already load arbitrary code.

At the moment.

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