linux-kernel - Re: [PATCH v3 29/64] objtool: Mark prefix functions

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Message-ID: <20250627103158.GV1613200@noisy.programming.kicks-ass.net>
Date: Fri, 27 Jun 2025 12:31:58 +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>,
	Dylan Hatch <dylanbhatch@...gle.com>
Subject: Re: [PATCH v3 29/64] objtool: Mark prefix functions

On Thu, Jun 26, 2025 at 04:55:16PM -0700, Josh Poimboeuf wrote:
> In preparation for the objtool klp diff subcommand, introduce a flag to
> identify __pfx_*() and __cfi_*() functions in advance so they don't need
> to be manually identified every time a check is needed.
> 
> Signed-off-by: Josh Poimboeuf <jpoimboe@...nel.org>
> ---
>  tools/objtool/check.c               | 3 +--
>  tools/objtool/elf.c                 | 4 ++++
>  tools/objtool/include/objtool/elf.h | 6 ++++++
>  3 files changed, 11 insertions(+), 2 deletions(-)
> 
> diff --git a/tools/objtool/check.c b/tools/objtool/check.c
> index 80bafcdb42af..55cc3a2a21c9 100644
> --- a/tools/objtool/check.c
> +++ b/tools/objtool/check.c
> @@ -3564,8 +3564,7 @@ static int validate_branch(struct objtool_file *file, struct symbol *func,
>  
>  		if (func && insn_func(insn) && func != insn_func(insn)->pfunc) {
>  			/* Ignore KCFI type preambles, which always fall through */
> -			if (!strncmp(func->name, "__cfi_", 6) ||
> -			    !strncmp(func->name, "__pfx_", 6))
> +			if (is_prefix_func(func))
>  				return 0;
>  
>  			if (file->ignore_unreachables)
> diff --git a/tools/objtool/elf.c b/tools/objtool/elf.c
> index 59568381486c..1bb86151243a 100644
> --- a/tools/objtool/elf.c
> +++ b/tools/objtool/elf.c
> @@ -442,6 +442,10 @@ static void elf_add_symbol(struct elf *elf, struct symbol *sym)
>  	elf_hash_add(symbol, &sym->hash, sym->idx);
>  	elf_hash_add(symbol_name, &sym->name_hash, str_hash(sym->name));
>  
> +	if (is_func_sym(sym) &&
> +	    (strstarts(sym->name, "__pfx_") || strstarts(sym->name, "__cfi_")))
> +		sym->prefix = 1;
> +
>  	if (is_func_sym(sym) && strstr(sym->name, ".cold"))
>  		sym->cold = 1;
>  	sym->pfunc = sym->cfunc = sym;
> diff --git a/tools/objtool/include/objtool/elf.h b/tools/objtool/include/objtool/elf.h
> index f41496b0ad8f..842faec1b9a9 100644
> --- a/tools/objtool/include/objtool/elf.h
> +++ b/tools/objtool/include/objtool/elf.h
> @@ -72,6 +72,7 @@ struct symbol {
>  	u8 frame_pointer     : 1;
>  	u8 ignore	     : 1;
>  	u8 cold		     : 1;
> +	u8 prefix	     : 1;
>  	struct list_head pv_target;
>  	struct reloc *relocs;
>  	struct section *group_sec;
> @@ -229,6 +230,11 @@ static inline bool is_local_sym(struct symbol *sym)
>  	return sym->bind == STB_LOCAL;
>  }
>  
> +static inline bool is_prefix_func(struct symbol *sym)
> +{
> +	return is_func_sym(sym) && sym->prefix;
> +}

func_is_prefix() ?

Also, since we only ever set sym->prefix when is_func_sym(), this helper
could avoid checking that again.