Re: [PATCH v6 05/20] modpost: refactor find_fromsym() and find_tosym()

[Nick Desaulniers]

On Sun, May 21, 2023 at 9:05 AM Masahiro Yamada <masahiroy@xxxxxxxxxx> wrote:
>
> find_fromsym() and find_tosym() are similar - both of them iterate
> in the .symtab section and return the nearest symbol.
>
> The difference between them is that find_tosym() allows a negative
> distance, but the distance must be less than 20.
>
> Factor out the common part into find_nearest_sym().
>
> Signed-off-by: Masahiro Yamada <masahiroy@xxxxxxxxxx>

Thanks for the patch!
Reviewed-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>

> ---
>
> Changes in v6:
>   - Revive the check for distance less than 20
>
>  scripts/mod/modpost.c | 95 ++++++++++++++++---------------------------
>  1 file changed, 36 insertions(+), 59 deletions(-)
>
> diff --git a/scripts/mod/modpost.c b/scripts/mod/modpost.c
> index d2329ac32177..6ac0d571542c 100644
> --- a/scripts/mod/modpost.c
> +++ b/scripts/mod/modpost.c
> @@ -1104,81 +1104,58 @@ static inline int is_valid_name(struct elf_info *elf, Elf_Sym *sym)
>         return !is_mapping_symbol(name);
>  }
>
> -/**
> - * Find symbol based on relocation record info.
> - * In some cases the symbol supplied is a valid symbol so
> - * return refsym. If is_valid_name() == true, we assume this is a valid symbol.
> - * In other cases the symbol needs to be looked up in the symbol table
> - * based on section and address.
> - *  **/
> -static Elf_Sym *find_tosym(struct elf_info *elf, Elf64_Sword addr,
> -                          Elf_Sym *relsym)
> +/* Look up the nearest symbol based on the section and the address */
> +static Elf_Sym *find_nearest_sym(struct elf_info *elf, Elf_Addr addr,
> +                                unsigned int secndx, bool allow_negative,
> +                                Elf_Addr min_distance)
>  {
>         Elf_Sym *sym;
>         Elf_Sym *near = NULL;
> -       Elf64_Sword distance = 20;
> -       Elf64_Sword d;
> -       unsigned int relsym_secindex;
> -
> -       if (is_valid_name(elf, relsym))
> -               return relsym;
> -
> -       /*
> -        * Strive to find a better symbol name, but the resulting name does not
> -        * always match the symbol referenced in the original code.
> -        */
> -       relsym_secindex = get_secindex(elf, relsym);
> -       for (sym = elf->symtab_start; sym < elf->symtab_stop; sym++) {
> -               if (get_secindex(elf, sym) != relsym_secindex)
> -                       continue;
> -               if (ELF_ST_TYPE(sym->st_info) == STT_SECTION)
> -                       continue;
> -               if (!is_valid_name(elf, sym))
> -                       continue;
> -               if (sym->st_value == addr)
> -                       return sym;
> -               /* Find a symbol nearby - addr are maybe negative */
> -               d = sym->st_value - addr;
> -               if (d < 0)
> -                       d = addr - sym->st_value;
> -               if (d < distance) {
> -                       distance = d;
> -                       near = sym;
> -               }
> -       }
> -       /* We need a close match */
> -       if (distance < 20)
> -               return near;
> -       else
> -               return NULL;
> -}
> -
> -/*
> - * Find symbols before or equal addr and after addr - in the section sec.
> - * If we find two symbols with equal offset prefer one with a valid name.
> - * The ELF format may have a better way to detect what type of symbol
> - * it is, but this works for now.
> - **/
> -static Elf_Sym *find_fromsym(struct elf_info *elf, Elf_Addr addr,
> -                            unsigned int secndx)
> -{
> -       Elf_Sym *sym;
> -       Elf_Sym *near = NULL;
> -       Elf_Addr distance = ~0;
> +       Elf_Addr distance;
>
>         for (sym = elf->symtab_start; sym < elf->symtab_stop; sym++) {
>                 if (get_secindex(elf, sym) != secndx)
>                         continue;
>                 if (!is_valid_name(elf, sym))
>                         continue;
> -               if (sym->st_value <= addr && addr - sym->st_value <= distance) {
> +
> +               if (addr >= sym->st_value)
>                         distance = addr - sym->st_value;
> +               else if (allow_negative)
> +                       distance = sym->st_value - addr;
> +               else
> +                       continue;
> +
> +               if (distance <= min_distance) {
> +                       min_distance = distance;
>                         near = sym;
>                 }
> +
> +               if (min_distance == 0)
> +                       break;
>         }
>         return near;
>  }
>
> +static Elf_Sym *find_fromsym(struct elf_info *elf, Elf_Addr addr,
> +                            unsigned int secndx)
> +{
> +       return find_nearest_sym(elf, addr, secndx, false, ~0);
> +}
> +
> +static Elf_Sym *find_tosym(struct elf_info *elf, Elf_Addr addr, Elf_Sym *sym)
> +{
> +       /* If the supplied symbol has a valid name, return it */
> +       if (is_valid_name(elf, sym))
> +               return sym;
> +
> +       /*
> +        * Strive to find a better symbol name, but the resulting name does not
> +        * always match the symbol referenced in the original code.
> +        */
> +       return find_nearest_sym(elf, addr, get_secindex(elf, sym), true, 20);
> +}
> +
>  static bool is_executable_section(struct elf_info *elf, unsigned int secndx)
>  {
>         if (secndx > elf->num_sections)
> --
> 2.39.2
>


-- 
Thanks,
~Nick Desaulniers