linux-kernel - Re: [PATCH 3/5] LoongArch: Support relocation against _GLOBAL_OFFSET_TABLE

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Message-ID: <caf8ec4b-f74d-e6ef-17f2-9e899d41b3a9@loongson.cn>
Date:   Thu, 28 Jul 2022 17:14:55 +0800
From:   Jinyang He <hejinyang@...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>
Subject: Re: [PATCH 3/5] LoongArch: Support relocation against
 _GLOBAL_OFFSET_TABLE_

On 07/28/2022 02:41 PM, Xi Ruoyao wrote:

> On Thu, 2022-07-28 at 09:02 +0800, Jinyang He wrote:
>
>> Hi, Ruoyao,
>>
>> It is possible to create a symbol when linking, which means
>> maybe we can add _GLOBAL_OFFSET_TABLE_ in 'module.lds.h'.
>> What do you think about it?
> Hi Jinyang,
>
> I think you are right, this patch can be replaced with the following,
> which is much simpler.
>
> -- >8 --
>
> With the stack-based relocations, the assembler emits three relocations
> to push the PC-relative offset of a GOT entry:
>
>      R_LARCH_SOP_PUSH_PCREL _GLOBAL_OFFSET_TABLE_
>      R_LARCH_SOP_PUSH_GPREL foo
>      R_LARCH_SOP_ADD
>
> In normal relocatable ELF object files, "_GLOBAL_OFFSET_TABLE_" is
> undefined in the symtab and the BFD linker generates it on final link.
> Define it for kernel modules so we will be able to really use
> R_LARCH_SOP_PUSH_GPREL relocation.
>
> Note that we need to use "HIDDEN" to make it a local symbol because each
> kernel module has its own GOT and the reference to it should not be
> resolved to the GOT in the main kernel image.
>
> Suggested-by: Jinyang He <hejinyang@...ngson.cn>
> Signed-off-by: Xi Ruoyao <xry111@...111.site>
> ---
>   arch/loongarch/include/asm/module.lds.h | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/loongarch/include/asm/module.lds.h b/arch/loongarch/include/asm/module.lds.h
> index 57bbd0cedd26..42b7cca0b947 100644
> --- a/arch/loongarch/include/asm/module.lds.h
> +++ b/arch/loongarch/include/asm/module.lds.h
> @@ -4,5 +4,5 @@ SECTIONS {
>   	. = ALIGN(4);
>   	.plt : { BYTE(0) }
>   	.plt.idx : { BYTE(0) }
> -	.got : { BYTE(0) }
> +	.got : { HIDDEN(_GLOBAL_OFFSET_TABLE_ = .); BYTE(0) }
Not sure but maybe re-align is needed here.
Since '{BYTE(0)}' actually use 1byte. _GLOABL_OFFSET_TABLE_ may have
difference with really GOT table entry. Have no machine and without 
test... :-(

Thanks,
Jinyang
>   }