linux-kernel - Re: [PATCH 3/3] objtool: optimize add_dead

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Date:   Tue, 21 Apr 2020 13:14:38 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Sami Tolvanen <samitolvanen@...gle.com>
Cc:     Josh Poimboeuf <jpoimboe@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] objtool: optimize add_dead_ends for split sections

On Tue, Apr 21, 2020 at 11:07:24AM -0700, Sami Tolvanen wrote:
> When running objtool on vmlinux.o compiled with -ffunction-sections,
> .rela.discard.(un)reachable often contains relocations that point to
> a different section. Instead of iterating through the list of all
> instructions each time, store a pointer to the last instruction of
> each section when decoding instructions.
> 
> Signed-off-by: Sami Tolvanen <samitolvanen@...gle.com>

Reviewed-by: Kees Cook <keescook@...omium.org>

-Kees

> ---
>  tools/objtool/check.c | 43 ++++++++++++++++++++++++++-----------------
>  tools/objtool/check.h |  4 ++++
>  tools/objtool/elf.h   |  1 +
>  3 files changed, 31 insertions(+), 17 deletions(-)
> 
> diff --git a/tools/objtool/check.c b/tools/objtool/check.c
> index 4770fb07b365..7d4104de0a5e 100644
> --- a/tools/objtool/check.c
> +++ b/tools/objtool/check.c
> @@ -237,6 +237,7 @@ static void clear_insn_state(struct insn_state *state)
>  static int decode_instructions(struct objtool_file *file)
>  {
>  	struct section *sec;
> +	struct section_info *sec_info = NULL;
>  	struct symbol *func;
>  	unsigned long offset;
>  	struct instruction *insn;
> @@ -253,6 +254,8 @@ static int decode_instructions(struct objtool_file *file)
>  		    strncmp(sec->name, ".discard.", 9))
>  			sec->text = true;
>  
> +		insn = NULL;
> +
>  		for (offset = 0; offset < sec->len; offset += insn->len) {
>  			insn = malloc(sizeof(*insn));
>  			if (!insn) {
> @@ -279,6 +282,17 @@ static int decode_instructions(struct objtool_file *file)
>  			nr_insns++;
>  		}
>  
> +		if (insn) {
> +			sec_info = malloc(sizeof(*sec_info));
> +			if (!sec_info) {
> +				WARN("malloc failed");
> +				return -1;
> +			}
> +
> +			sec_info->last_insn = insn;
> +			sec->section_info = sec_info;
> +		}
> +
>  		list_for_each_entry(func, &sec->symbol_list, list) {
>  			if (func->type != STT_FUNC || func->alias != func)
>  				continue;
> @@ -312,7 +326,6 @@ static int add_dead_ends(struct objtool_file *file)
>  	struct section *sec;
>  	struct rela *rela;
>  	struct instruction *insn;
> -	bool found;
>  
>  	/*
>  	 * By default, "ud2" is a dead end unless otherwise annotated, because
> @@ -338,15 +351,13 @@ static int add_dead_ends(struct objtool_file *file)
>  		if (insn)
>  			insn = list_prev_entry(insn, list);
>  		else if (rela->addend == rela->sym->sec->len) {
> -			found = false;
> -			list_for_each_entry_reverse(insn, &file->insn_list, list) {
> -				if (insn->sec == rela->sym->sec) {
> -					found = true;
> -					break;
> -				}
> -			}
> +			struct section_info *sec_info = (struct section_info *)
> +				rela->sym->sec->section_info;
> +
> +			if (sec_info)
> +				insn = sec_info->last_insn;
>  
> -			if (!found) {
> +			if (!insn) {
>  				WARN("can't find unreachable insn at %s+0x%x",
>  				     rela->sym->sec->name, rela->addend);
>  				return -1;
> @@ -380,15 +391,13 @@ static int add_dead_ends(struct objtool_file *file)
>  		if (insn)
>  			insn = list_prev_entry(insn, list);
>  		else if (rela->addend == rela->sym->sec->len) {
> -			found = false;
> -			list_for_each_entry_reverse(insn, &file->insn_list, list) {
> -				if (insn->sec == rela->sym->sec) {
> -					found = true;
> -					break;
> -				}
> -			}
> +			struct section_info *sec_info = (struct section_info *)
> +				rela->sym->sec->section_info;
> +
> +			if (sec_info)
> +				insn = sec_info->last_insn;
>  
> -			if (!found) {
> +			if (!insn) {
>  				WARN("can't find reachable insn at %s+0x%x",
>  				     rela->sym->sec->name, rela->addend);
>  				return -1;
> diff --git a/tools/objtool/check.h b/tools/objtool/check.h
> index bc78eca7982e..353677ec85d4 100644
> --- a/tools/objtool/check.h
> +++ b/tools/objtool/check.h
> @@ -47,6 +47,10 @@ struct instruction {
>  	struct orc_entry orc;
>  };
>  
> +struct section_info {
> +	struct instruction *last_insn;
> +};
> +
>  struct objtool_file {
>  	struct elf *elf;
>  	struct list_head insn_list;
> diff --git a/tools/objtool/elf.h b/tools/objtool/elf.h
> index ebbb10c61e24..98f2b41d18e4 100644
> --- a/tools/objtool/elf.h
> +++ b/tools/objtool/elf.h
> @@ -40,6 +40,7 @@ struct section {
>  	int idx;
>  	unsigned int len;
>  	bool changed, text, rodata;
> +	void *section_info;
>  };
>  
>  struct symbol {
> -- 
> 2.26.1.301.g55bc3eb7cb9-goog
> 

-- 
Kees Cook