On Sat, Jan 23, 2021 at 10:51 AM Jiri Olsa <jolsa@xxxxxxxxxx> wrote:
>
> On Fri, Jan 22, 2021 at 02:55:51PM -0800, Andrii Nakryiko wrote:
> > On Fri, Jan 22, 2021 at 12:47 PM Jiri Olsa <jolsa@xxxxxxxxxx> 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
>
> the latest upstream code seems to set it always,
> but I agree we should be careful
oh, then maybe it's not necessary. I honestly don't even know where
the authoritative source code of libelf is, so I just found some
random source code with Google.
>
> Mark, any insight in here? thanks
>
> > section index in all cases, we need to check again *after* we got
> > symbol, and if it's not extended, then set index manually.
>
> hum, then we should use '!=', right?
>
> if (sym->st_shndx != SHN_XINDEX)
> *sym_sec_idx = sym->st_shndx;
yeah, sorry, that was a typo
>
> SNIP
>
> > > > 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).
>
> agreed, I'll use this
sounds good
>
> thanks,
> jirka
>
