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

From: Andrii Nakryiko <andrii.nakryiko@gmail.com>
To: Jiri Olsa <jolsa@redhat.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 14:55:51 -0800	[thread overview]
Message-ID: <CAEf4BzaRrMp1+2dgv_1WrkBt+=KF1BJnN_KGwZKx5gDg7t++Yg@mail.gmail.com> (raw)
In-Reply-To: <20210122204654.GB70760@krava>

On Fri, Jan 22, 2021 at 12:47 PM Jiri Olsa <jolsa@redhat.com> wrote:
>
> 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

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
section index in all cases, we need to check again *after* we got
symbol, and if it's not extended, then set index manually.

>
> >
> > 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

I think skipping symbols is nicer. If ELF is totally broken, then all
symbols are going to be ignored anyway. If it's some one-off issue for
a specific symbol, we'll just ignore it (unfortunately, silently).

>
> >
> >
> > 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
>