On Wed, Jan 04, 2023 at 10:59:15AM -0800, Andrii Nakryiko wrote: > > > >> to actual BPF helpers by then where we go and say, that kfunc has proven itself in production > > >> and from an API PoV that it is ready to be a proper BPF helper, and until this point > > > > > > "Proper BPF helper" model is broken. > > > static void *(*bpf_map_lookup_elem)(void *map, const void *key) = (void *) 1; > > > > > > is a hack that works only when compiler optimizes the code. > > > See gcc's attr(kernel_helper) workaround. > > > This 'proper helper' hack is the reason we cannot compile bpf programs with -O0. > > > And because it's uapi we cannot even fix this > > > With kfuncs we will be able to compile with -O0 and debug bpf programs with better tools. > > > These tools don't exist yet, but we have a way forward whereas with helpers > > > we are stuck with -O2. > > > > But specifically about how the BPF helper model is broken, that's at > least an exaggeration. BPF helper call is defined at BPF ISA level, it > has to be a `call <some constant>;`, and as long as compiler generates > such code, it's all good. From C standpoint UAPI is just a function > call: > > bpf_map_lookup_elem(&map, ...); > > As long as this compiles and generates proper `call 1;` assembly > instruction, we are good. If/when both Clang and GCC support an > alternative way to define helper and not as a static func pointer, -O0 > builds (at least in the aspect of calling BPF helpers, I suspect other > stuff will break still) will just work. And what's better, > bpf_helper_defs.h would be able to pick the best option based on > compiler's support with end users not having to care or notice the > difference. Right and that's what gcc did with attribute((kernel_helper(1)), but we didn't like it because gcc and clang would diverge. Now you're arguing it's just a bpf_helper_defs.h change and we should have allowed it? Also consider that 'call <some constant>' or more precise 'call absolute_address' as an instruction exist in only one CPU architecture. It's BPF ISA. It's a mistake that I made 8 years ago and inability to fix it bothers me. Now we have 100 times more developers than we had 8 years ago. I expect 100 time more UAPI and ABI mistakes. Minimizing unfixable mistakes is what I'm after.
