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

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Message-ID: <CAEf4BzZNPJZBfz7Ga9MGvGGYge4MCP1O16JVuFjdzu-bCEQFLQ@mail.gmail.com>
Date:   Tue, 19 Jan 2021 17:22:08 -0800
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Jiri Olsa <jolsa@...nel.org>
Cc:     Arnaldo Carvalho de Melo <acme@...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

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).

> +
> +       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
>