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

[Jiri Olsa <jolsa@redhat.com>]

On Fri, Jan 22, 2021 at 12:16:42PM -0800, Andrii Nakryiko wrote:
> On Fri, Jan 22, 2021 at 8:46 AM Jiri Olsa <jolsa@kernel.org> wrote:
> >
> > For very large ELF objects (with many sections), we could
> > get special value SHN_XINDEX (65535) for symbol's st_shndx.
> >
> > This patch is adding code to detect the optional extended
> > section index table and use it to resolve symbol's section
> > index.
> >
> > Adding elf_symtab__for_each_symbol_index macro that returns
> > symbol's section index and usign it in collect functions.
> >
> > Signed-off-by: Jiri Olsa <jolsa@kernel.org>
> > ---
> >  btf_encoder.c | 59 +++++++++++++++++++++++++++++++++++++--------------
> >  elf_symtab.c  | 39 +++++++++++++++++++++++++++++++++-
> >  elf_symtab.h  |  2 ++
> >  3 files changed, 83 insertions(+), 17 deletions(-)
> >
> > diff --git a/btf_encoder.c b/btf_encoder.c
> > index 5557c9efd365..56ee55965093 100644
> > --- a/btf_encoder.c
> > +++ b/btf_encoder.c
> > @@ -63,13 +63,13 @@ static void delete_functions(void)
> >  #define max(x, y) ((x) < (y) ? (y) : (x))
> >  #endif
> >
> > -static int collect_function(struct btf_elf *btfe, GElf_Sym *sym)
> > +static int collect_function(struct btf_elf *btfe, GElf_Sym *sym,
> > +                           Elf32_Word sym_sec_idx)
> 
> nit: we use size_t or int for this, no need for libelf types here, imo

ok

> 
> 
> 
> >  {
> >         struct elf_function *new;
> >         static GElf_Shdr sh;
> > -       static int last_idx;
> > +       static Elf32_Word last_idx;
> >         const char *name;
> > -       int idx;
> >
> >         if (elf_sym__type(sym) != STT_FUNC)
> >                 return 0;
> > @@ -90,12 +90,10 @@ static int collect_function(struct btf_elf *btfe, GElf_Sym *sym)
> >                 functions = new;
> >         }
> >
> > -       idx = elf_sym__section(sym);
> > -
> > -       if (idx != last_idx) {
> > -               if (!elf_section_by_idx(btfe->elf, &sh, idx))
> > +       if (sym_sec_idx != last_idx) {
> > +               if (!elf_section_by_idx(btfe->elf, &sh, sym_sec_idx))
> >                         return 0;
> > -               last_idx = idx;
> > +               last_idx = sym_sec_idx;
> >         }
> >
> >         functions[functions_cnt].name = name;
> > @@ -542,14 +540,15 @@ static bool percpu_var_exists(uint64_t addr, uint32_t *sz, const char **name)
> >         return true;
> >  }
> >
> > -static int collect_percpu_var(struct btf_elf *btfe, GElf_Sym *sym)
> > +static int collect_percpu_var(struct btf_elf *btfe, GElf_Sym *sym,
> > +                             Elf32_Word sym_sec_idx)
> 
> nit: same, size_t or just int would be fine
> 
> >  {
> >         const char *sym_name;
> >         uint64_t addr;
> >         uint32_t size;
> >
> >         /* compare a symbol's shndx to determine if it's a percpu variable */
> > -       if (elf_sym__section(sym) != btfe->percpu_shndx)
> > +       if (sym_sec_idx != btfe->percpu_shndx)
> >                 return 0;
> >         if (elf_sym__type(sym) != STT_OBJECT)
> >                 return 0;
> > @@ -585,12 +584,13 @@ static int collect_percpu_var(struct btf_elf *btfe, GElf_Sym *sym)
> >         return 0;
> >  }
> >
> > -static void collect_symbol(GElf_Sym *sym, struct funcs_layout *fl)
> > +static void collect_symbol(GElf_Sym *sym, struct funcs_layout *fl,
> > +                          Elf32_Word sym_sec_idx)
> >  {
> >         if (!fl->mcount_start &&
> >             !strcmp("__start_mcount_loc", elf_sym__name(sym, btfe->symtab))) {
> >                 fl->mcount_start = sym->st_value;
> > -               fl->mcount_sec_idx = sym->st_shndx;
> > +               fl->mcount_sec_idx = sym_sec_idx;
> >         }
> >
> >         if (!fl->mcount_stop &&
> > @@ -598,9 +598,36 @@ 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)
> 
> This is a generic function, why don't you want to move it into elf_symtab.h?
> 
> > +{
> > +       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))
> > +                       return false;
> 
> You also ignored my feedback about not fetching symbol twice. Why?

ugh, I did not read your last 2 comments.. let me check that, sry

> 
> > +       }
> > +
> > +       return true;
> > +}
> > +
> > +#define elf_symtab__for_each_symbol_index(symtab, id, sym, sym_sec_idx)                \
> > +       for (id = 0;                                                            \
> > +            id < symtab->nr_syms &&                                            \
> > +            elf_sym__get(symtab->syms, symtab->syms_sec_idx_table,             \
> > +                         id, &sym, &sym_sec_idx);                              \
> > +            id++)
> 
> This should be in elf_symtab.h next to elf_symtab__for_each_symbol.
> 
> And thinking a bit more, the variant with just ignoring symbols that
> we failed to get is probably a safer alternative. I.e., currently
> there is no way to communicate that we terminated iteration with
> error, so it's probably better to skip failed symbols and still get
> the rest, no? I was hoping to discuss stuff like this on the previous
> version of the patch...

I was thinking of that, but then I thought it's better to fail,
than have possibly wrong data in BTF, because the ELF data is
possibly damaged? no idea.. so I took the safer way

jirka