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

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

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



>  {
>         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?

> +       }
> +
> +       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...

And please do fix elf_symtab__for_each_symbol().

> +
>  static int collect_symbols(struct btf_elf *btfe, bool collect_percpu_vars)
>  {
>         struct funcs_layout fl = { };
> +       Elf32_Word sym_sec_idx;
>         uint32_t core_id;
>         GElf_Sym sym;
>

[...]