Re: __builtin_memcpy behavior

Tristan Mayfield

Thank you to both Andrii and Toke! It's been extremely helpful to read your responses. Having conversations like these really helps me when I go into the source code and try to understand the overall intent of it. I'm going to try and summarize the conversation to confirm my understanding.

bpf_probe_read() will read any valid kernel memory (nothing new here). If the memory is already available to be read in the program (e.g. in tracepoint args), then __builtin_memcpy can be used and will potentially throw attach-time errors if reading structs incorrectly (for some reason I don't think we clarified).

CO-RE can guarantee valid memory reads because of the nature of being able to check offsets and relocations at load time rather than attach time or just returning garbage data with no errors.

To build CO-RE programs you need a vmlinux file (not to be confused with the header, vmlinux.h) which is normally found at /sys/kernel/btf/vmlinux on systems that have been compiled with pahole and CONFIG_DEBUG_INFO_BTF=y. Having the vmlinux.h file is helpful because it replaces kernel headers and makes building a bit nicer, but isn't necessary. Once compiled, CO-RE programs should be able to run on any system  that has a vmlinux file in one of the locations listed here:

For earlier kernels, it's possible to generate a vmlinux file (and this is one of the spots I'm a bit murky on) with pahole -J, but I'm not sure what you are supposed to target when running that? Just the compiled kernel binary? Something else?

BTF is just a type format that can describe C data-types. Almost like a meta-language? I've personally not looked at the source for BTF yet, but it seems to be versatile enough that it's useful for CO-RE for describing internal data structures from the kernel, but it's also useful for a variety of other things (like map declarations) and will likely be increasingly relied on in future iterations of BPF, both CO-RE and otherwise. BTF support mainly comes from the compiler (which I do believe clang 10+ works, just from my experience. I'm primarily using clang 10 right now) and libbpf supporting it, not necessarily the kernel (except for CO-RE with the vmlinux).

Again, appreciate the responses. I've been building with BPF/libbpf about a year now and still feel like I've only scratched the surface. Reading source code is great, but sometimes it just really helps to get high-level ideas as well!


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