xdp-project-bpf-examples/ktrace-CO-RE/README.org

#+Title: Example using CO-RE to access ktrace

This is an example that show how CO-RE came be utilised...
to learn CO-RE and BTF better.

* Compiling kprobes for different CPU architectures

When BPF-tracing =kprobes= then the header files need to use the correct
asm-level C-calling convention for the architecture that this is getting
compiled for, as BPF will be hook/tap directly into the kernels function
call. These are the raw CPU registers, which we later via BTF demonstrate
howto use to CO-RE to get the C-type back.

The header include file =<bpf/bpf_tracing.h>= takes care of defining macros
for accessing the members in kernels =struct pt_regs=, as the struct member
names differ between architectures. This header file is provided by [[https://github.com/libbpf/libbpf][libbpf]]
(or distros libbpf-devel package).

The architecture specific definition of =struct pt_regs= can be extracted
via generating a vmlinux.h file on the ARCH target. To ease getting the
structs this git repo contains the arch include files here:
[[file:../headers/vmlinux/arch/]]

You should not include them directly, but instead include [[file:../headers/vmlinux_local.h][vmlinux_local.h]] to
get the surrounding types defined.

** Gotcha with target bpf

This git-repo compile with clang =-target bpf=. This cause issues for
=ktrace=, because the fallback mechanism in header file
=<bpf/bpf_tracing.h>= cannot determine the ARCH from compiler defines.

To solve this our makefile system in [[file:../lib/common.mk]] detect the ARCH
(via =uname -m=) and defines the =__TARGET_ARCH_$(ARCH)= in =BPF_CFLAGS=.

Without this fix the following compile warnings occurs:
#+begin_example
 warning: implicit declaration of function 'PT_REGS_PARM1' is invalid in C99
 int BPF_KPROBE(udp_send_skb, struct sk_buff *skb)
     ^
 /usr/include/bpf/bpf_tracing.h:385:20: note: expanded from macro 'BPF_KPROBE'
#+end_example

When loading via libbpf the following error happens:
#+begin_example
 libbpf: failed to find BTF for extern 'PT_REGS_PARM1': -2
#+end_example