From: Arnaldo Carvalho de Melo <acme@kernel.org>
To: Andrii Nakryiko <andrii.nakryiko@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>, Alexei Starovoitov <ast@kernel.org>,
Daniel Borkmann <daniel@iogearbox.net>,
Andrii Nakryiko <andriin@fb.com>,
dwarves@vger.kernel.org, Networking <netdev@vger.kernel.org>,
bpf <bpf@vger.kernel.org>, Yonghong Song <yhs@fb.com>,
Hao Luo <haoluo@google.com>, Martin KaFai Lau <kafai@fb.com>,
Song Liu <songliubraving@fb.com>,
John Fastabend <john.fastabend@gmail.com>,
KP Singh <kpsingh@chromium.org>,
Joe Lawrence <joe.lawrence@redhat.com>,
Mark Wielaard <mjw@redhat.com>
Subject: Re: [PATCH bpf-next 3/3] libbpf: Use string table index from index table if needed
Date: Wed, 20 Jan 2021 08:12:11 -0300 [thread overview]
Message-ID: <20210120111211.GP12699@kernel.org> (raw)
In-Reply-To: <CAEf4BzZNPJZBfz7Ga9MGvGGYge4MCP1O16JVuFjdzu-bCEQFLQ@mail.gmail.com>
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@kernel.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@kernel.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@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
next prev parent reply other threads:[~2021-01-20 11:30 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-01-19 22:12 [PATCH 0/3] dwarves,libbpf: Add support to use optional extended section index table Jiri Olsa
2021-01-19 22:12 ` [PATCH 1/3] elf_symtab: Add support for SHN_XINDEX index to elf_section_by_name Jiri Olsa
2021-01-20 1:23 ` Andrii Nakryiko
2021-01-20 12:20 ` Jiri Olsa
2021-01-19 22:12 ` [PATCH 2/3] bpf_encoder: Translate SHN_XINDEX in symbol's st_shndx values Jiri Olsa
2021-01-20 2:03 ` Andrii Nakryiko
2021-01-20 12:25 ` Jiri Olsa
2021-01-19 22:12 ` [PATCH bpf-next 3/3] libbpf: Use string table index from index table if needed Jiri Olsa
2021-01-20 1:22 ` Andrii Nakryiko
2021-01-20 11:12 ` Arnaldo Carvalho de Melo [this message]
2021-01-20 12:26 ` Jiri Olsa
2021-01-19 23:17 ` [PATCH 0/3] dwarves,libbpf: Add support to use optional extended section index table Joe Lawrence
2021-01-21 20:22 [PATCHv2 " Jiri Olsa
2021-01-21 20:22 ` [PATCH bpf-next 3/3] libbpf: Use string table index from index table if needed Jiri Olsa
2021-01-21 23:46 ` Andrii Nakryiko
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