linux-kernel - Re: [PATCH v2 52/62] objtool/klp: Introduce klp diff subcommand for diffing object files

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Message-ID: <tencent_984AF5DAF2F307EA419A747031F0920C7C09@qq.com>
Date: Wed, 14 May 2025 18:32:32 +0800
From: laokz <laokz@...mail.com>
To: Josh Poimboeuf <jpoimboe@...nel.org>
Cc: x86@...nel.org, linux-kernel@...r.kernel.org,
 Petr Mladek <pmladek@...e.com>, Miroslav Benes <mbenes@...e.cz>,
 Joe Lawrence <joe.lawrence@...hat.com>, live-patching@...r.kernel.org,
 Song Liu <song@...nel.org>, Jiri Kosina <jikos@...nel.org>,
 Marcos Paulo de Souza <mpdesouza@...e.com>, Weinan Liu <wnliu@...gle.com>,
 Fazla Mehrab <a.mehrab@...edance.com>,
 Chen Zhongjin <chenzhongjin@...wei.com>, Puranjay Mohan <puranjay@...nel.org>
Subject: Re: [PATCH v2 52/62] objtool/klp: Introduce klp diff subcommand for
 diffing object files

On 5/14/2025 4:45 AM, Josh Poimboeuf wrote:
> On Tue, May 13, 2025 at 10:49:59PM +0800, laokz wrote:
>> On 5/10/2025 4:17 AM, Josh Poimboeuf wrote:
>>> +
>>> +#define sym_for_each_reloc(elf, sym, reloc)				\
>>> +	for (reloc = find_reloc_by_dest_range(elf, sym->sec,		\
>>> +					      sym->offset, sym->len);	\
>>> +	     reloc && reloc_offset(reloc) <  sym->offset + sym->len;	\
>>> +	     reloc = rsec_next_reloc(sym->sec->rsec, reloc))
>>
>> This macro intents to walk through ALL relocations for the 'sym'. It seems
>> we have the assumption that, there is at most one single relocation for the
>> same offset and find_reloc_by_dest_range only needs to do 'less than' offset
>> comparison:
>>
>> 	elf_hash_for_each_possible(reloc, reloc, hash,
>> 				   sec_offset_hash(rsec, o)) {
>> 		if (reloc->sec != rsec)
>> 			continue;
>> 		if (reloc_offset(reloc) >= offset &&
>> 		    reloc_offset(reloc) < offset + len) {
>> less than ==>		if (!r || reloc_offset(reloc) < reloc_offset(r))
>> 					r = reloc;
>>
>> Because if there were multiple relocations for the same offset, the returned
>> one would be the last one in section entry order(hash list has reverse order
>> against section order), then broken the intention.
> 
> Right.  Is that a problem?  I don't believe I've ever seen two
> relocations for the same offset.
> 

Thanks for the clarification. I asked this because I noticed the 
patchset have done some code refactoring, so guess if we could make it 
more general to other architectures which not support objtool yet. Such 
as RISC-V, it is not unusual having multiple relocs for same offset, 
like vmlinux.o might have:

000c 0000010a00000017 R_RISCV_PCREL_HI20 0000000000000000 .LANCHOR0 + 48
000c 0000000000000033 R_RISCV_RELAX                         48

0044 0000061700000023 R_RISCV_ADD32   0000000000000048 pe_head_start + 0
0044 000dd5b900000027 R_RISCV_SUB32   0000000000000002 _start + 0

But it is a bit off-topic:/

Regards,
laokz