linux-kernel - Re: [PATCH v4 0/4] LoongArch: Support new relocation types

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Date:   Mon, 1 Aug 2022 10:16:09 +0800
From:   Youling Tang <tangyouling@...ngson.cn>
To:     Xi Ruoyao <xry111@...111.site>, loongarch@...ts.linux.dev
Cc:     linux-kernel@...r.kernel.org, WANG Xuerui <kernel@...0n.name>,
        Huacai Chen <chenhuacai@...nel.org>,
        Jinyang He <hejinyang@...ngson.cn>,
        Lulu Cheng <chenglulu@...ngson.cn>
Subject: Re: [PATCH v4 0/4] LoongArch: Support new relocation types

Hi, Ruoyao

On 07/30/2022 10:52 AM, Xi Ruoyao wrote:
> On Sat, 2022-07-30 at 10:24 +0800, Xi Ruoyao wrote:
>> On Sat, 2022-07-30 at 01:55 +0800, Xi Ruoyao wrote:
>>> On Fri, 2022-07-29 at 20:19 +0800, Youling Tang wrote:
>>>
>>>> On 07/29/2022 07:45 PM, Xi Ruoyao wrote:
>>>>> Hmm... The problem is the "addresses" of per-cpu symbols are
>>>>> faked: they
>>>>> are actually offsets from $r21.  So we can't just load such an
>>>>> offset
>>>>> with PCALA addressing.
>>>>>
>>>>> It looks like we'll need to introduce an attribute for GCC to
>>>>> make
>>>>> an
>>>>> variable "must be addressed via GOT", and add the attribute into
>>>>> PER_CPU_ATTRIBUTES.
>>>
>>>> Yes, we need a GCC attribute to specify the per-cpu variable.
>>>
>>> GCC patch adding "addr_global" attribute for LoongArch:
>>> https://gcc.gnu.org/pipermail/gcc-patches/2022-July/599064.html
>>>
>>> An experiment to use it:
>>> https://github.com/xry111/linux/commit/c1d5d70
>>
>> Correction: https://github.com/xry111/linux/commit/c1d5d708
>>
>> It seems 7-bit SHA is not enough for kernel repo.
>
> If addr_global is rejected or not implemented (for example, building the
> kernel with GCC 12), *I expect* the following hack to work (I've not
> tested it because I'm AFK now).  Using visibility in kernel seems
> strange, but I think it may make some sense because the modules are some
> sort of similar to an ELF shared object being dlopen()'ed, and our way
> to inject per-CPU symbols is analog to ELF interposition.
>
> arch/loongarch/include/asm/percpu.h:
>
>    #if !__has_attribute(__addr_global__) && defined(MODULE)
>    /* Magically remove "static" for per-CPU variables.  */
>    # define ARCH_NEEDS_WEAK_PER_CPU
>    /* Force GOT-relocation for per-CPU variables.  */
>    # define PER_CPU_ATTRIBUTES __attribute__((__visibility__("default")))
>    #endif
>
> arch/loongarch/Makefile:
>
>    # Hack for per-CPU variables, see PER_CPU_ATTRIBUTES in
>    # include/asm/percpu.h
>    if (call gcc-does-not-support-addr-global)
>      KBUILD_CFLAGS_MODULE += -fPIC -fvisibility=hidden
>    endif
>
Using the old toolchain (GCC 12) can successfully load the nf_tables.ko
module after applying the above patch.

Thanks,
Youling