linux-kernel - Re: [PATCH] module: Ignore _GLOBAL_OFFSET_TABLE

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Date:   Thu, 14 Jan 2021 08:57:10 -0800
From:   Fāng-ruì Sòng <maskray@...gle.com>
To:     Jessica Yu <jeyu@...nel.org>, Sam Ravnborg <sam@...nborg.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Marco Elver <melver@...gle.com>
Subject: Re: [PATCH] module: Ignore _GLOBAL_OFFSET_TABLE_ when warning for
 undefined symbols

On Thu, Jan 14, 2021 at 6:06 AM Jessica Yu <jeyu@...nel.org> wrote:
>
> +++ Fangrui Song [13/01/21 21:48 -0800]:
> >clang-12 -fno-pic (since
> >https://github.com/llvm/llvm-project/commit/a084c0388e2a59b9556f2de0083333232da3f1d6)
> >can emit `call __stack_chk_fail@...` 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@...` 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 <melver@...gle.com>
> >Link: https://github.com/ClangBuiltLinux/linux/issues/1250
> >Signed-off-by: Fangrui Song <maskray@...gle.com>
> >---
> > kernel/module.c | 10 ++++++++--
> > 1 file changed, 8 insertions(+), 2 deletions(-)
> >
> >diff --git a/kernel/module.c b/kernel/module.c
> >index 4bf30e4b3eaa..2e2deea99289 100644
> >--- a/kernel/module.c
> >+++ b/kernel/module.c
> >@@ -2395,8 +2395,14 @@ 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. Also allow _GLOBAL_OFFSET_TABLE_:
> >+                       * GNU as before 2.37 produces an unreferenced _GLOBAL_OFFSET_TABLE_
> >+                       * for call foo@PLT on x86-64.  If the code ever needs to ignore
> >+                       * more symbols, refactor the code to only warn if referenced by
> >+                       * a relocation.
> >+                       */
> >+                      if (!ksym && (ELF_ST_BIND(sym[i].st_info) == STB_WEAK ||
> >+                                    !strcmp(name, "_GLOBAL_OFFSET_TABLE_")))
> >                               break;
>
> Hi Fangrui,
>
> Thanks for the patch. I am puzzled why we don't already mirror modpost
> here, that particular line of code in modpost to ignore _GLOBAL_OFFSET_TABLE_
> has been there long before my time. Let's properly mirror modpost
> then, and create a similar helper function ignore_undef_symbol() (and
> stick the _GLOBAL_OFFSET_TABLE_ check in there) to account for future
> cases like this.
>
> Thanks,
>
> Jessica

Hi Jessica,

I guess __this_module in scripts/mod/modpost.c:ignore_undef_symbol is
not a problem.
For PPC64 _restgpr0_* and _savegpr0_*, I am not sure ignoring the
undefined functions in kernel/module.c is right.
(I know they can be produced by gcc -Os in some cases
(https://reviews.llvm.org/D79977), but I want to learn whether that is
a real issue before adding them.)

If we ever need to ignore more symbols, the code should be refactored
to not warn for unreferenced undefined symbols as my description says.