Re: [PATCH 2/3] bpf_encoder: Translate SHN_XINDEX in symbol's st_shndx values

From: Jiri Olsa <jolsa@redhat.com>
To: Andrii Nakryiko <andrii.nakryiko@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>,
	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 2/3] bpf_encoder: Translate SHN_XINDEX in symbol's st_shndx values
Date: Fri, 22 Jan 2021 21:46:54 +0100	[thread overview]
Message-ID: <20210122204654.GB70760@krava> (raw)
In-Reply-To: <CAEf4BzZquSn0Th7bpVuM0M4XbTPU5-9jDPPd5RJBS5AH2zqaMA@mail.gmail.com>

On Thu, Jan 21, 2021 at 03:32:40PM -0800, Andrii Nakryiko wrote:

SNIP

> > @@ -598,9 +599,36 @@ static void collect_symbol(GElf_Sym *sym, struct funcs_layout *fl)
> >                 fl->mcount_stop = sym->st_value;
> >  }
> >
> > +static bool elf_sym__get(Elf_Data *syms, Elf_Data *syms_sec_idx_table,
> > +                        int id, GElf_Sym *sym, Elf32_Word *sym_sec_idx)
> > +{
> > +       if (!gelf_getsym(syms, id, sym))
> > +               return false;
> > +
> > +       *sym_sec_idx = sym->st_shndx;
> > +
> > +       if (sym->st_shndx == SHN_XINDEX) {
> > +               if (!syms_sec_idx_table)
> > +                       return false;
> > +               if (!gelf_getsymshndx(syms, syms_sec_idx_table,
> > +                                     id, sym, sym_sec_idx))
> 
> 
> gelf_getsymshndx() is supposed to work even for cases that don't use
> extended numbering, so this should work, right?
> 
> if (!gelf_getsymshndx(syms, syms_sec_idx_table, id, sym, sym_sec_idx))
>     return false;
> 

it seems you're right, gelf_getsymshndx seem to work for
both cases, I'll check

> if (sym->st_shndx == SHN_XINDEX)
>   *sym_sec_idx = sym->st_shndx;

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

> 
> return true;
> 
> ?
> 
> > +                       return false;
> > +       }
> > +
> > +       return true;
> > +}
> > +
> > +#define elf_symtab__for_each_symbol_index(symtab, id, sym, sym_sec_idx)                \
> > +       for (id = 0, elf_sym__get(symtab->syms, symtab->syms_sec_idx_table,     \
> > +                                 id, &sym, &sym_sec_idx);                      \
> > +            id < symtab->nr_syms;                                              \
> > +            id++, elf_sym__get(symtab->syms, symtab->syms_sec_idx_table,       \
> > +                               id, &sym, &sym_sec_idx))
> 
> what do we want to do if elf_sym__get() returns error (false)? We can
> either stop or ignore that symbol, right? But currently you are
> returning invalid symbol data.
> 
> so either
> 
> for (id = 0; id < symtab->nr_syms && elf_sym__get(symtab->syms,
> symtab->syms_sec_idx_table, d, &sym, &sym_sec_idx); id++)
> 
> or
> 
> for (id = 0; id < symtab->nr_syms; id++)
>   if (elf_sym__get(symtab->syms, symtab->syms_sec_idx_table, d, &sym,
> &sym_sec_idx))

if we go ahead with skipping symbols, this one seems good

> 
> 
> But the current variant looks broken. Oh, and
> elf_symtab__for_each_symbol() is similarly broken, can you please fix
> that as well?
> 
> And this new macro should probably be in elf_symtab.h, along the
> elf_symtab__for_each_symbol.

thanks,
jirka