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

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Message-ID: <76f43f37-1d5b-c80d-480d-52708652acf2@loongson.cn>
Date: Tue, 26 Nov 2024 21:24:59 +0800
From: Tiezhu Yang <yangtiezhu@...ngson.cn>
To: Josh Poimboeuf <jpoimboe@...nel.org>
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 11/26/2024 07:00 PM, Tiezhu Yang wrote:
> On 11/26/2024 03:19 PM, Josh Poimboeuf wrote:
>> 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.
>
> OK.
>
>>
>> Maybe there should be an arch-specific function arch_adjusted_addend().

Add adjust_offset() in
tools/objtool/include/objtool/elf.h,
like this:

static inline unsigned long adjust_offset(struct elf *elf, struct reloc 
*reloc,
                                           unsigned long offset)
{
         if (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));

         return offset;
}

then call it in check.c, like this:

offset = reloc->sym->offset + reloc_addend(reloc);
offset = adjust_offset(file->elf, reloc, offset);

What do you think?

Thanks,
Tiezhu