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

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Message-ID: <20250526182719.GR24938@noisy.programming.kicks-ass.net>
Date: Mon, 26 May 2025 20:27:19 +0200
From: Peter Zijlstra <peterz@...radead.org>
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>,
	laokz <laokz@...mail.com>, 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 Fri, May 09, 2025 at 01:17:16PM -0700, Josh Poimboeuf wrote:
> diff --git a/tools/objtool/arch/x86/decode.c b/tools/objtool/arch/x86/decode.c
> index cdf385e54c69..ae4f83fcbadf 100644
> --- a/tools/objtool/arch/x86/decode.c
> +++ b/tools/objtool/arch/x86/decode.c
> @@ -95,6 +95,46 @@ s64 arch_insn_adjusted_addend(struct instruction *insn, struct reloc *reloc)
>  	return phys_to_virt(addend);
>  }
>  
> +static void scan_for_insn(struct section *sec, unsigned long offset,
> +			  unsigned long *insn_off, unsigned int *insn_len)
> +{
> +	unsigned long o = 0;
> +	struct insn insn;
> +
> +	while (1) {
> +
> +		insn_decode(&insn, sec->data->d_buf + o, sec_size(sec) - o,
> +			    INSN_MODE_64);
> +
> +		if (o + insn.length > offset) {
> +			*insn_off = o;
> +			*insn_len = insn.length;
> +			return;
> +		}
> +
> +		o += insn.length;
> +	}
> +}
> +
> +u64 arch_adjusted_addend(struct reloc *reloc)
> +{
> +	unsigned int type = reloc_type(reloc);
> +	s64 addend = reloc_addend(reloc);
> +	unsigned long insn_off;
> +	unsigned int insn_len;
> +
> +	if (type == R_X86_64_PLT32)
> +		return addend + 4;
> +
> +	if (type != R_X86_64_PC32 || !is_text_sec(reloc->sec->base))
> +		return addend;
> +
> +	scan_for_insn(reloc->sec->base, reloc_offset(reloc),
> +		      &insn_off, &insn_len);
> +
> +	return addend + insn_off + insn_len - reloc_offset(reloc);
> +}

This looks like a rather expensive proposition; it will have to decode
the section nr_reloc times.

Does it not make more sense to fully decode the section like 'normal' ?