netdev - Re: [PATCH 2/3] bpf_encoder: Translate SHN_XINDEX in symbol's st

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Date:   Sat, 23 Jan 2021 21:08:15 +0100
From:   Mark Wielaard <mark@...mp.org>
To:     Jiri Olsa <jolsa@...hat.com>,
        Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc:     Jiri Olsa <jolsa@...nel.org>,
        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>
Subject: Re: [PATCH 2/3] bpf_encoder: Translate SHN_XINDEX in symbol's
 st_shndx values

Hi Jiri,

On Sat, 2021-01-23 at 19:51 +0100, Jiri Olsa wrote:
> On Fri, Jan 22, 2021 at 02:55:51PM -0800, Andrii Nakryiko wrote:
> > 
> > > I don't understand this..  gelf_getsymshndx will return both
> > > symbol and proper index, no? also sym_sec_idx is already
> > > assigned from previou call
> > 
> > Reading (some) implementation of gelf_getsymshndx() that I found
> > online, it won't set sym_sec_idx, if the symbol *doesn't* use
> > extended
> > numbering. But it will still return symbol data. So to return the
> 
> the latest upstream code seems to set it always,
> but I agree we should be careful
> 
> Mark, any insight in here? thanks

GElf_Sym *
gelf_getsymshndx (Elf_Data *symdata, Elf_Data *shndxdata, int ndx,
                  GElf_Sym *dst, Elf32_Word *dstshndx)

Will always set *dst, but only set *dstshndx if both it and shndxdata
are not NULL and no error occurred (the function returns NULL and set
libelf_error in case of error).

So as long as shndxdata != NULL you can rely on *dstshndx being set.
Otherwise you get the section index from dst->st_shndx.

Cheers,

Mark