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

From: Andrii Nakryiko <andrii.nakryiko@gmail.com>
To: Jiri Olsa <jolsa@kernel.org>
Cc: Arnaldo Carvalho de Melo <acme@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: Tue, 19 Jan 2021 17:22:08 -0800	[thread overview]
Message-ID: <CAEf4BzZNPJZBfz7Ga9MGvGGYge4MCP1O16JVuFjdzu-bCEQFLQ@mail.gmail.com> (raw)
In-Reply-To: <20210119221220.1745061-4-jolsa@kernel.org>

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).

> +
> +       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
>