netdev - Re: [PATCH bpf-next 3/3] libbpf: Use string table index from index table if needed

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Date:   Wed, 20 Jan 2021 08:12:11 -0300
From:   Arnaldo Carvalho de Melo <acme@...nel.org>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc:     Jiri Olsa <jolsa@...nel.org>, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andriin@...com>, dwarves@...r.kernel.org,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        Yonghong Song <yhs@...com>, Hao Luo <haoluo@...gle.com>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...omium.org>,
        Joe Lawrence <joe.lawrence@...hat.com>,
        Mark Wielaard <mjw@...hat.com>
Subject: Re: [PATCH bpf-next 3/3] libbpf: Use string table index from index
 table if needed

Em Tue, Jan 19, 2021 at 05:22:08PM -0800, Andrii Nakryiko escreveu:
> On Tue, Jan 19, 2021 at 2:15 PM Jiri Olsa <jolsa@...nel.org> wrote:
> >
> > For very large ELF objects (with many sections), we could
> > get special value SHN_XINDEX (65535) for elf object's string
> > table index - e_shstrndx.
> >
> > In such case we need to call elf_getshdrstrndx to get the
> > proper string table index.
> >
> > Signed-off-by: Jiri Olsa <jolsa@...nel.org>
> > ---
> >  tools/lib/bpf/btf.c | 14 ++++++++++++--
> >  1 file changed, 12 insertions(+), 2 deletions(-)
> >
> > diff --git a/tools/lib/bpf/btf.c b/tools/lib/bpf/btf.c
> > index 3c3f2bc6c652..4fe987846bc0 100644
> > --- a/tools/lib/bpf/btf.c
> > +++ b/tools/lib/bpf/btf.c
> > @@ -863,6 +863,7 @@ static struct btf *btf_parse_elf(const char *path, struct btf *base_btf,
> >         Elf_Scn *scn = NULL;
> >         Elf *elf = NULL;
> >         GElf_Ehdr ehdr;
> > +       size_t shstrndx;
> >
> >         if (elf_version(EV_CURRENT) == EV_NONE) {
> >                 pr_warn("failed to init libelf for %s\n", path);
> > @@ -887,7 +888,16 @@ static struct btf *btf_parse_elf(const char *path, struct btf *base_btf,
> >                 pr_warn("failed to get EHDR from %s\n", path);
> >                 goto done;
> >         }
> > -       if (!elf_rawdata(elf_getscn(elf, ehdr.e_shstrndx), NULL)) {
> > +
> > +       /*
> > +        * Get string table index from extended section index
> > +        * table if needed.
> > +        */
> > +       shstrndx = ehdr.e_shstrndx;
> > +       if (shstrndx == SHN_XINDEX && elf_getshdrstrndx(elf, &shstrndx))
> > +               goto done;
> 
> just use elf_getshdrstrndx() unconditionally, it works for extended
> and non-extended numbering (see libbpf.c).

Yeah, we even have this in tools/perf/util/symbol-elf.c:

#ifndef HAVE_ELF_GETSHDRSTRNDX_SUPPORT
static int elf_getshdrstrndx(Elf *elf __maybe_unused, size_t *dst __maybe_unused)
{
        pr_err("%s: update your libelf to > 0.140, this one lacks elf_getshdrstrndx().\n", __func__);
        return -1;
}
#endif

And a feature detection for that:

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

int main(void)
{
	size_t dst;

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

Your implementation would provide a good fallback tho to avoid requiring
updating to 0.140 :-)

- Arnaldo

> > +
> > +       if (!elf_rawdata(elf_getscn(elf, shstrndx), NULL)) {
> >                 pr_warn("failed to get e_shstrndx from %s\n", path);
> >                 goto done;
> >         }
> > @@ -902,7 +912,7 @@ static struct btf *btf_parse_elf(const char *path, struct btf *base_btf,
> >                                 idx, path);
> >                         goto done;
> >                 }
> > -               name = elf_strptr(elf, ehdr.e_shstrndx, sh.sh_name);
> > +               name = elf_strptr(elf, shstrndx, sh.sh_name);
> >                 if (!name) {
> >                         pr_warn("failed to get section(%d) name from %s\n",
> >                                 idx, path);
> > --
> > 2.27.0
> >

-- 

- Arnaldo