Re: [PATCH v2] module: Ignore _GLOBAL_OFFSET_TABLE_ when warning for undefined symbols

[Marco Elver]

On Thu, 14 Jan 2021 at 22:54, Fangrui Song <maskray@xxxxxxxxxx> wrote:
> clang-12 -fno-pic (since
> https://github.com/llvm/llvm-project/commit/a084c0388e2a59b9556f2de0083333232da3f1d6)
> can emit `call __stack_chk_fail@PLT` instead of `call __stack_chk_fail`
> on x86.  The two forms should have identical behaviors on x86-64 but the
> former causes GNU as<2.37 to produce an unreferenced undefined symbol
> _GLOBAL_OFFSET_TABLE_.
>
> (On x86-32, there is an R_386_PC32 vs R_386_PLT32 difference but the
> linker behavior is identical as far as Linux kernel is concerned.)
>
> Simply ignore _GLOBAL_OFFSET_TABLE_ for now, like what
> scripts/mod/modpost.c:ignore_undef_symbol does. This also fixes the
> problem for gcc/clang -fpie and -fpic, which may emit `call foo@PLT` for
> external function calls on x86.
>
> Note: ld -z defs and dynamic loaders do not error for unreferenced
> undefined symbols so the module loader is reading too much.  If we ever
> need to ignore more symbols, the code should be refactored to ignore
> unreferenced symbols.
>
> Reported-by: Marco Elver <elver@xxxxxxxxxx>
> Link: https://github.com/ClangBuiltLinux/linux/issues/1250
> Signed-off-by: Fangrui Song <maskray@xxxxxxxxxx>

Tested-by: Marco Elver <elver@xxxxxxxxxx>

Thank you for the patch!

> ---
>  kernel/module.c | 20 ++++++++++++++++++--
>  1 file changed, 18 insertions(+), 2 deletions(-)
> ---
> Changes in v2:
> * Fix Marco's email address
> * Add a function ignore_undef_symbol similar to scripts/mod/modpost.c:ignore_undef_symbol
>
> diff --git a/kernel/module.c b/kernel/module.c
> index 4bf30e4b3eaa..278f5129bde2 100644
> --- a/kernel/module.c
> +++ b/kernel/module.c
> @@ -2348,6 +2348,20 @@ static int verify_exported_symbols(struct module *mod)
>         return 0;
>  }
>
> +static int ignore_undef_symbol(Elf_Half emachine, const char *name)

Why not 'bool' return-type?

> +{
> +       /* On x86, PIC code and Clang non-PIC code may have call foo@PLT. GNU as

Not sure if checkpatch.pl warns about this, but this multi-line
comment does not follow the normal kernel-style (see elsewhere in
file):

/*
 * ...
 */

> +        * before 2.37 produces an unreferenced _GLOBAL_OFFSET_TABLE_ on x86-64.
> +        * i386 has a similar problem but may not deserve a fix.
> +        *
> +        * If we ever have to ignore many symbols, consider refactoring the code to
> +        * only warn if referenced by a relocation.
> +        */
> +       if (emachine == EM_386 || emachine == EM_X86_64)
> +               return !strcmp(name, "_GLOBAL_OFFSET_TABLE_");
> +       return 0;
> +}
> +
>  /* Change all symbols so that st_value encodes the pointer directly. */
>  static int simplify_symbols(struct module *mod, const struct load_info *info)
>  {
> @@ -2395,8 +2409,10 @@ static int simplify_symbols(struct module *mod, const struct load_info *info)
>                                 break;
>                         }
>
> -                       /* Ok if weak.  */
> -                       if (!ksym && ELF_ST_BIND(sym[i].st_info) == STB_WEAK)
> +                       /* Ok if weak or ignored.  */
> +                       if (!ksym &&
> +                           (ELF_ST_BIND(sym[i].st_info) == STB_WEAK ||
> +                            ignore_undef_symbol(info->hdr->e_machine, name)))
>                                 break;
>
>                         ret = PTR_ERR(ksym) ?: -ENOENT;
> --
> 2.30.0.296.g2bfb1c46d8-goog
>