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Date:   Fri, 15 Jan 2021 09:14:42 +0100
From:   Jessica Yu <jeyu@kernel.org>
To:     Marco Elver <elver@google.com>
Cc:     Fangrui Song <maskray@google.com>,
        LKML <linux-kernel@vger.kernel.org>,
        clang-built-linux <clang-built-linux@googlegroups.com>,
        Sam Ravnborg <sam@ravnborg.org>
Subject: Re: [PATCH v2] module: Ignore _GLOBAL_OFFSET_TABLE_ when warning for
 undefined symbols
Message-ID: <20210115081442.GB9637@linux-8ccs>
References: <20210114211840.GA5617@linux-8ccs>
 <20210114215416.993167-1-maskray@google.com>
 <CANpmjNOrD76O2_Zpwo5RJ2d12gczuQpG9bJkXYLY_sOVeEVZGQ@mail.gmail.com>
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+++ Marco Elver [15/01/21 08:03 +0100]:
>On Thu, 14 Jan 2021 at 22:54, Fangrui Song <maskray@google.com> 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@google.com>
>> Link: https://github.com/ClangBuiltLinux/linux/issues/1250
>> Signed-off-by: Fangrui Song <maskray@google.com>
>
>Tested-by: Marco Elver <elver@google.com>
>
>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):
>
>/*
> * ...
> */

+1 to Marco's comments. Otherwise, patch looks good to me.

Thanks Fangrui!

Jessica