linux-kernel - Re: [PATCH v4 04/10] objtool: Handle PC relative relocation type

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Message-ID: <20241126071935.2vmsna54zsa5cdho@jpoimboe>
Date: Mon, 25 Nov 2024 23:19:35 -0800
From: Josh Poimboeuf <jpoimboe@...nel.org>
To: Tiezhu Yang <yangtiezhu@...ngson.cn>
Cc: Huacai Chen <chenhuacai@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>, loongarch@...ts.linux.dev,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 04/10] objtool: Handle PC relative relocation type

On Fri, Nov 22, 2024 at 12:49:59PM +0800, Tiezhu Yang wrote:
> When compling with Clang on LoongArch, there exists 32 bit PC relative
> relocation type, it needs to get the offset with "S + A - PC" according
> to the spec of "ELF for the LoongArch Architecture".
> 
> This is preparation for later patch on LoongArch, there is no effect for
> the other archs with this patch.
> 
> Link: https://github.com/loongson/la-abi-specs/blob/release/laelf.adoc
> Signed-off-by: Tiezhu Yang <yangtiezhu@...ngson.cn>
> ---
>  tools/objtool/check.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/tools/objtool/check.c b/tools/objtool/check.c
> index 19d1263e64e4..8733ca620cca 100644
> --- a/tools/objtool/check.c
> +++ b/tools/objtool/check.c
> @@ -2126,6 +2126,11 @@ static int add_jump_table(struct objtool_file *file, struct instruction *insn,
>  		if (reloc->sym->type == STT_SECTION) {
>  			/* Addend field in the relocation entry associated with the symbol */
>  			offset = reloc_addend(reloc);
> +			/* Handle the special cases compiled with Clang on LoongArch */
> +			if (file->elf->ehdr.e_machine == EM_LOONGARCH &&
> +			    reloc_type(reloc) == R_LARCH_32_PCREL)
> +				offset = reloc->sym->offset + reloc_addend(reloc) -
> +					 (reloc_offset(reloc) - reloc_offset(table));

Please, no more "special cases".  It makes the common code unreadable.
We should avoid checking 'elf->ehdr.e_machine' in check.c.

Maybe there should be an arch-specific function arch_adjusted_addend().

-- 
Josh