Comment by vardump | Hacker News Reader

Comment by vardump | Hacker News Reader

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Except for example when the user needs to either do something only doable in kernel space or to diagnose and analyze kernel behavior.

I would have loved an option like this for example when working with a power management issue last time (PCI-e surprise removal and hot-plug can be annoying!)... Sure, I can use remote kernel debuggers etc., but those can be sometimes a hassle to set up, not always the most convenient option.

Like when an issue occurs on the other side of the world, and kernel dumps and tracing leads nowhere. Having an extra option for easy injection of code in kernel would be a nice thing to have in the toolbox.

In other words, one-off scenarios. It wouldn't be something you'd release for wider use.

Knee-jerk reactions have been somewhat off-putting lately. All of us should keep in mind other people might have a completely different mindset, problems and needs than our own. We should celebrate new ideas and novel points of view and not to brainlessly use existing dogma or cheap talking points to immediately slam them.

(I've developed (among other things) commercial kernel drivers for Windows. And "for fun" for Linux just to keep my skills updated.)

That's just a side effect of the way Linux puts everything into a giant trusted binary. The GP is correct that we should separate out services (probably to separate sockets until Spectre-like attacks are fixed), even if it's also true that when dealing with Linux-as-it-is-today the solution to everything being in the kernel is to put more stuff in the kernel.

Any sandboxing adds a performance overhead. There is no free lunch in Operating Systems.

A tiny performance overhead is worthwhile in name of security.

For example, I don't remember the last time I cared about disabling bounds checking, even in C++ code (VC++ allows to keep it on).

Tell that the various libc maintainers. They are blocking the C11 Annex K (safe bounds checks) for over a decade now.

And talking about performance With good compilers my secure memcpy_s is actually faster than the glibc or BSD libc memcpy, with compile-time constexpr.

I would blame ISO C, by making Annex K optional, and not caring to improve C's safety in general.

I am quite happy with rust these days. But it still lacks great ‘no allocation’ libraries.

Then you are illusional. None of their safety guarantees are real. Memory, type, concurrency, none. But talking to them fall on deaf ears. Its called hype driven development and very popular amongst HN folks. There exist plenty of real safe languages though.

Then Oracle, Microsoft, Amazon and Google are illusional as well, given their investment into production code Rust.

Any references to support that claim?

The whole point of BPF is that you can't run this code in a separate context _and_ have the code work. The original use case was packet filtering in interrupts because waiting for user space took too long.

If I remember correctly Intel has implemented a user space TCP/IP stack exactly for that.

Which doesn't work if you want to still share an adapter across multiple applications.

I don't see any reason why it can't br made to work between multiple applications while being still faster than traditional kernel based stacks.

The whole point of DPDK is to avoid the context switches by dedicating the adapter to a single application, and letting the application take over management. Once you have multiplexing, you're right back to where you started. There has been a secure multiplexing scheme based on packet buffers... the Berkeley Packet Filter (BPF), ie. what this paper is talking about as prior art.

No, you can simply have memory mapped ring buffers between processes and all of the stuff not required for the specific application cut away. You don't need to have any more context switches that way.

No need to have a traditional socket API while still being able to do access it from multiple applications.

I have no interest to create such beast, but I'd be truly shocked if it couldn't at least beat a generic kernel based stack.

It'll lose some performance compared to the single application approach, but there might still be a niche for this kind of way.

Sure, you'll need to copy memory, but the data should be almost always in L3 cache anyways.

Once you're doing copies (in your example two copies!) it's game over.

My intention was however to only have one copy (other than initial DMA to (hopefully) directly L3 cache).

Yeah, it hurts if it's 400 Gbps ethernet. L3 bandwidth is like 50-90 GB/s. X86 just doesn't have enough bandwidth even to the caches! Better have pretty high CPU frequency, as I think L3 gets faster in proportion (but not completely sure). 200 Gbps should be somewhat fine.

Also pretty bad if the data travels over a QPI link... better have both processes in same NUMA region. And that ethernet PCI-e adapter... :-)

Regardless, I do think it'd still work way faster than anything a reasonably general kernel stack could do. Might be a reasonable compromise when process & permission isolation is required.

Bpf can handle 60 million packets per second, adding a user/kernel context switch will kill that by a factor of 1000x. So while your code may not care about it, there are definitely low latency applications where milliseconds equate to dollars.

Some high integrity OSes do that full in user space, while running Linux on the side as it isn't certified for such scenarios.

> The GP is correct that we should separate out services...

That's why I said "It wouldn't be something you'd release for wider use.".

I wanted to point out there are use cases where a full-fledged kernel module would be an overkill, and some safety would be desirable.

Not safety and security from others, but from myself, so that small dumb mistakes are less likely to cause kernel crashes. For research, debugging, curiosity, and so on. Not necessarily for production use!

Although, now that I think about it, it would be nice to be able to sandbox trusted code as a part of a kernel module as well. Writing kernel code is hard work, because it needs to be truly correct.

There's often code in kernel drivers that's not performance critical, but must be done in kernel mode and must be crash-free and secure to execute. So why not take advantage of sandboxing for additional safety?

That is what Windows 10 introduced with its secure kernel hypervisor.

https://docs.microsoft.com/en-us/windows-hardware/design/dev...

What would this sandboxing achieve? If you mean kernel internally using multiple protection rings, tricky to do in Linux, because the API in that either has to be tight or it's still not too secure. E.g. if you just take RCU or softirqd into near userspace, as done in RT patchset, but then introduce context switching overhead, life is going to be relatively ok, but you've covered only relatively small parts of the drivers. And RT breaks drivers without even introducing more overhead. It would take a major overhaul.

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