netdev - Re: [net-next:master 488/665] verifier.c:undefined reference to `_

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Message-ID: <7076fba7-4ffe-bb28-a9d7-57720768b18c@fb.com>
Date:   Sun, 12 Nov 2017 09:23:52 +0800
From:   Alexei Starovoitov <ast@...com>
To:     Fengguang Wu <fengguang.wu@...el.com>
CC:     <kbuild-all@...org>, <netdev@...r.kernel.org>,
        Daniel Borkmann <daniel@...earbox.net>
Subject: Re: [net-next:master 488/665] verifier.c:undefined reference to
 `__multi3'

On 11/12/17 9:18 AM, Fengguang Wu wrote:
> On Sun, Nov 12, 2017 at 09:14:14AM +0800, Alexei Starovoitov wrote:
>> On 11/12/17 8:23 AM, kbuild test robot wrote:
>>> tree:
>>> https://git.kernel.org/pub/scm/linux/kernel/git/davem/net-next.git
>>> master
>>> head:   7c5556decd0a629e9ee02e93653f75ba7b7da03c
>>> commit: 638f5b90d46016372a8e3e0a434f199cc5e12b8c [488/665] bpf:
>>> reduce verifier memory consumption
>>> config: mips-64r6el_defconfig (attached as .config)
>>> compiler: mips64el-linux-gnuabi64-gcc (Debian 7.2.0-11) 7.2.0
>>> reproduce:
>>>         git checkout 638f5b90d46016372a8e3e0a434f199cc5e12b8c
>>>         # save the attached .config to linux build tree
>>>         make.cross ARCH=mips
>>>
>>> All errors (new ones prefixed by >>):
>>>
>>>    kernel/bpf/verifier.o: In function `realloc_verifier_state.isra.19':
>>>>> verifier.c:(.text+0x36fc): undefined reference to `__multi3'
>>
>> that's a known issue with gcc 7 on mips that is "optimizing"
>> normal 64-bit multiply into 128-bit variant.
>> Nothing to fix on the kernel side.
>
> Good to know that! Do you think it a good idea to blacklist __multi3
> errors in mips builds?

I would do so. yes.
Though digging further this function was added to
arch/sparc/lib/multi3.S
since gcc doing the same "optimization" there.
Adding asm code doesn't look right to me. I'd rather push
gcc folks to avoid such codegen.