linux-kernel - Re: [PATCH v2] module: Ignore _GLOBAL_OFFSET_TABLE

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Message-ID: <20210115080920.GA9637@linux-8ccs>
Date:   Fri, 15 Jan 2021 09:09:20 +0100
From:   Jessica Yu <jeyu@...nel.org>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Fangrui Song <maskray@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Sam Ravnborg <sam@...nborg.org>, Marco Elver <elver@...gle.com>
Subject: Re: [PATCH v2] module: Ignore _GLOBAL_OFFSET_TABLE_ when warning for
 undefined symbols

+++ Nick Desaulniers [14/01/21 14:01 -0800]:
>On Thu, Jan 14, 2021 at 1:54 PM 'Fangrui Song' via Clang Built Linux
><clang-built-linux@...glegroups.com> wrote:
>>
>> 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 <elver@...gle.com>
>> Link: https://github.com/ClangBuiltLinux/linux/issues/1250
>> Signed-off-by: Fangrui Song <maskray@...gle.com>
>
>Thanks for the patch.
>
>Reviewed-by: Nick Desaulniers <ndesaulniers@...gle.com>
>
>Jessica, would you mind adding when applying:
>
>Cc: <stable@...r.kernel.org>
>
>as I suspect we might want this fixed in stable tree's branches, too.
>It might of interest to add:
>
>Link: https://sourceware.org/bugzilla/show_bug.cgi?id=27178
>
>too.

Sure, will do!

Thanks,

Jessica