Re: [PATCHv5 bpf-next 1/1] perf tools: Rework prologue generation code

Daniel Borkmann
2022-06-20T12:43:46Z

On 6/16/22 10:22 PM, Jiri Olsa wrote:
> Some functions we use for bpf prologue generation are going to be
> deprecated. This change reworks current code not to use them.
>
> We need to replace following functions/struct:
> bpf_program__set_prep
> bpf_program__nth_fd
> struct bpf_prog_prep_result
>
> Currently we use bpf_program__set_prep to hook perf callback before
> program is loaded and provide new instructions with the prologue.
>
> We replace this function/ality by taking instructions for specific
> program, attaching prologue to them and load such new ebpf programs
> with prologue using separate bpf_prog_load calls (outside libbpf
> load machinery).
>
> Before we can take and use program instructions, we need libbpf to
> actually load it. This way we get the final shape of its instructions
> with all relocations and verifier adjustments).
>
> There's one glitch though.. perf kprobe program already assumes
> generated prologue code with proper values in argument registers,
> so loading such program directly will fail in the verifier.
>
> That's where the fallback pre-load handler fits in and prepends
> the initialization code to the program. Once such program is loaded
> we take its instructions, cut off the initialization code and prepend
> the prologue.
>
> I know.. sorry ;-)
>
> To have access to the program when loading this patch adds support to
> register 'fallback' section handler to take care of perf kprobe programs.
> The fallback means that it handles any section definition besides the
> ones that libbpf handles.
>
> The handler serves two purposes:
> - allows perf programs to have special arguments in section name
> - allows perf to use pre-load callback where we can attach init
> code (zeroing all argument registers) to each perf program
>
> Suggested-by: Andrii Nakryiko <andrii@kernel.org>
> Signed-off-by: Jiri Olsa <jolsa@kernel.org>

Hey Arnaldo, if you get a chance, please take a look.

Thanks a lot,
Daniel