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

Arnaldo Carvalho de Melo
2022-08-01T17:44:55Z

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@kernel.org>
> > Signed-off-by: Jiri Olsa <jolsa@kernel.org>
>
> Hey Arnaldo, if you get a chance, please take a look.

Sry, applied.

- Arnaldo