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Date:   Wed, 13 Jan 2021 10:16:41 +0000
To:     Nick Desaulniers <ndesaulniers@google.com>
From:   Alexander Lobakin <alobakin@pm.me>
Cc:     Alexander Lobakin <alobakin@pm.me>,
        Fangrui Song <maskray@google.com>,
        clang-built-linux <clang-built-linux@googlegroups.com>,
        linux-mips@vger.kernel.org,
        Thomas Bogendoerfer <tsbogend@alpha.franken.de>,
        Ralf Baechle <ralf@linux-mips.org>,
        LKML <linux-kernel@vger.kernel.org>,
        linux-arch <linux-arch@vger.kernel.org>
Reply-To: Alexander Lobakin <alobakin@pm.me>
Subject: Re: [BUG mips llvm] MIPS: malformed R_MIPS_{HI16,LO16} with LLVM
Message-ID: <20210113101623.3020-1-alobakin@pm.me>
In-Reply-To: <CAKwvOdnvd1NaBQEJ0fPsYiGff4=tUdrcuAR0no9FUMqnOZSu6Q@mail.gmail.com>
References: <20210109171058.497636-1-alobakin@pm.me> <CAKwvOdmV2tj4Uyz1iDkqCj+snWPpnnAmxJyN+puL33EpMRPzUw@mail.gmail.com> <20210109191457.786517-1-alobakin@pm.me> <CAKwvOdnOXXaz+S1agu5kCQLm+qEkXE2Hpd2_V8yPsbUTQH7JZw@mail.gmail.com> <20210111204936.17905-1-alobakin@pm.me> <CAKwvOdnvd1NaBQEJ0fPsYiGff4=tUdrcuAR0no9FUMqnOZSu6Q@mail.gmail.com>
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From: Nick Desaulniers <ndesaulniers@google.com>
Date: Tue, 12 Jan 2021 14:14:58 -0800

> On Mon, Jan 11, 2021 at 12:50 PM Alexander Lobakin <alobakin@pm.me> wrote=
:
>>
>>> The disassembly for me produces:
>>>     399c: 3c 03 00 00   lui     $3, 0 <phy_device_free>
>>>                         0000399c:  R_MIPS_HI16  .text
>>> ...
>>>     39a8: 24 63 3a 5c   addiu   $3, $3, 14940 <phy_probe>
>>>                         000039a8:  R_MIPS_LO16  .text
>>
>> So, in your case the values of the instructions that relocs refer are:
>>
>> 0x3c030000 R_MIPS_HI16
>> 0x24633a5c R_MIPS_LO16
>>
>> Mine were:
>>
>> 0x3c010000
>> 0x24339444
>>
>> Your second one doesn't have bit 15 set, so I think this pair won't
>> break the code.
>> Try to hunt for R_MIPS_LO16 that have this bit set, i.e. they have
>> '8', '9', 'a', 'b', 'c', 'd' or 'e' as their [15:12].
>
> I don't think any of my R_MIPS_LO16 in that file have that bit set.
> See attached.

Well, seems like you got lucky here.
You can try to check for other modules if any of them have
R_MIPS_LO16 with bit 15 set.

Also, you can try to enable:
CONFIG_LD_DEAD_CODE_DATA_ELIMINATION=3Dy
CONFIG_TRIM_UNUSED_KSYMS=3Dy

to alter the build process. These options didn't change anything
in terms of relocs for me though.

> --
> Thanks,
> ~Nick Desaulniers

Thanks,
Al