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

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Date:   Mon, 20 Jun 2022 14:43:22 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Jiri Olsa <jolsa@...nel.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Andrii Nakryiko <andrii@...nel.org>
Cc:     linux-perf-users@...r.kernel.org, netdev@...r.kernel.org,
        bpf@...r.kernel.org, Ingo Molnar <mingo@...nel.org>,
        Namhyung Kim <namhyung@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        Ian Rogers <irogers@...gle.com>
Subject: Re: [PATCHv5 bpf-next 1/1] perf tools: Rework prologue generation
 code

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@...nel.org>
> Signed-off-by: Jiri Olsa <jolsa@...nel.org>

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

Thanks a lot,
Daniel