netdev - Re: [PATCH bpf-next v5 00/10] BTF: BPF Type Format

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Date:   Fri, 15 Jun 2018 11:24:57 -0300
From:   Arnaldo Carvalho de Melo <acme@...nel.org>
To:     Yonghong Song <yhs@...com>
Cc:     Alexei Starovoitov <ast@...com>, Martin KaFai Lau <kafai@...com>,
        netdev@...r.kernel.org, Daniel Borkmann <daniel@...earbox.net>,
        kernel-team@...com, Wang Nan <wangnan0@...wei.com>,
        Jiri Olsa <jolsa@...nel.org>,
        Namhyung Kim <namhyung@...nel.org>,
        Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH bpf-next v5 00/10] BTF: BPF Type Format

Em Thu, Jun 14, 2018 at 09:56:43PM -0700, Yonghong Song escreveu:
> I really want to get rid of this option as well. To make pahole work
> with the default default format, I need to add bpf support to
> libdwfl in elfutils repo. I will work on that.

Right, I haven't looked into detail, but perhaps we can do like we do in
tools/perf/ where we add a feature test to check if some function is
present in a library (elfutils even) and if so, use it, otherwise, use a
copy that we carry in pahole.git.

For instance:

tools/perf/util/symbol-elf.c

#ifndef HAVE_ELF_GETPHDRNUM_SUPPORT
static int elf_getphdrnum(Elf *elf, size_t *dst)
{
        GElf_Ehdr gehdr;
        GElf_Ehdr *ehdr;

        ehdr = gelf_getehdr(elf, &gehdr);
        if (!ehdr)
                return -1;

        *dst = ehdr->e_phnum;

        return 0;
}
#endif

And we have a feature test to check if that is present, simple one, if
that builds and links, we have it, then the tools build Makefile magic
will end up defining HAVE_ELF_GETPHDRNUM_SUPPORT and our copy doesn't
get included, using what is in elfutils:

[acme@...et perf]$ cat tools/build/feature/test-libelf-getphdrnum.c 
// SPDX-License-Identifier: GPL-2.0
#include <libelf.h>

int main(void)
{
	size_t dst;

	return elf_getphdrnum(0, &dst);
}
[acme@...et perf]$ 

[acme@...et perf]$ grep elf /tmp/build/perf/FEATURE-DUMP
feature-libelf=1
feature-libelf-getphdrnum=1
feature-libelf-gelf_getnote=1
feature-libelf-getshdrstrndx=1
feature-libelf-mmap=1
[acme@...et perf]$ 

This way a new pahole version won't get to wait till places where it
gets built have these new functions and we stop using it as soon as the
library get it.

- Arnaldo