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

From: Arnaldo Carvalho de Melo <[email-masked]>
Date: Wed, 20 Jan 2021 08:12:11 -0300

Em Tue, Jan 19, 2021 at 05:22:08PM -0800, Andrii Nakryiko escreveu:
> On Tue, Jan 19, 2021 at 2:15 PM Jiri Olsa <[email-masked]> 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 <[email-masked]>
> > ---
> >  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@five 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@five 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