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

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Date:   Mon, 1 Aug 2022 14:44:47 -0300
From:   Arnaldo Carvalho de Melo <acme@...nel.org>
To:     Daniel Borkmann <daniel@...earbox.net>
Cc:     Jiri Olsa <jolsa@...nel.org>, Alexei Starovoitov <ast@...nel.org>,
        Andrii Nakryiko <andrii@...nel.org>,
        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

Em Mon, Jun 20, 2022 at 02:43:22PM +0200, Daniel Borkmann escreveu:
> 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.

Sry, applied.

- Arnaldo