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

[Arnaldo Carvalho de Melo <acme@kernel.org>]

Em Fri, Jan 22, 2021 at 09:37:48PM +0100, Jiri Olsa escreveu:
> 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

I think it is because this patch ends up calling 

extern GElf_Sym *gelf_getsymshndx (Elf_Data *__symdata, Elf_Data *__shndxdata,
                                   int __ndx, GElf_Sym *__sym,
                                   Elf32_Word *__xshndx);

Which expects a pointer to a Elf32_Word, right Jiri?

Jiri, are you going to submit a new version of this patch? I processed
the first one, collecting Andrii's Acked-by, if you're busy I can try to
address the other concerns from Andrii, please let me know.

- Arnaldo
 
> > 
> > 
> > 
> > >  {
> > >         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
> 

-- 

- Arnaldo