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

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Date:   Fri, 22 Jan 2021 14:55:51 -0800
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Jiri Olsa <jolsa@...hat.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>,
        Mark Wielaard <mjw@...hat.com>
Subject: Re: [PATCH 2/3] bpf_encoder: Translate SHN_XINDEX in symbol's
 st_shndx values

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