Can someone tell me what's the advantage of eBPF over a user mode driver? The article makes it look it eBPF is have your cake and eat it too solution which is too good to be true? Can you run graphics drivers in eBPF for example?
AFAIK, an ebpf function can only access memory it got handed as an argument or as result from a very limited number of kernel functions. Your function will not load if you don't have boundary checks. Fighting the ebpf validator is a bit like fighting Rust's borrow checker; annoying, at times it's too conservative and rejects perfectly correct code, but it will protect you from panics. Loops will only be accepted if the validator can prove they'll end in time; this means it can be a pain to make the validator to accept a loop. Also, ebpf is a processor-independent byte code, so vectorizing code is not possible (unless the byte code interpreter itself does it).
Given all its restrictions, I doubt something complex like a graphics driver would be possible. But then, I know nothing about graphics driver programming.
> Fighting the ebpf validator is a bit like fighting Rust's borrow checker
I think this undersells how annoying it is. There's a bit of an impedance mismatch. Typically you write code in C and compile it with clang to eBPF bytecode, which is then checked by the kernel's eBPF verifier. But in some cases clang is smart enough to optimize away bounds checks, but the eBPF verifier isn't smart enough to realize the bound checks aren't needed. This requires manual hacking to trick clang into not optimizing things in a way that will confuse the verifier, and sometimes you just can't get the C code to work and need to write things in eBPF bytecode by hand using inline assembly. All of these problems are massively compounded if you need to support several different kernel versions. At least with the Rust borrow checker there is a clearly defined set of rules you can follow.
This is the wiki. I haven't kept up, but this isn't a kernel module.
"eBPF is a technology that can run programs in a privileged context such as the operating system kernel. It is the successor to the Berkeley Packet Filter (BPF, with the "e" originally meaning "extended") filtering mechanism in Linux and is also used in non-networking parts of the Linux kernel as well."
No, you can't run arbitrary general-purpose programs in eBPF, and you cannot run graphics drivers in it. You generally can't run programs with unprovably bounded loops in eBPF, and your program can interact with the kernel only through a small series of explicitly enumerated "helpers" (for any given type of eBPF program, you probably have about 20 of these in total).
