linux-kernel - Re: [PATCH v2 29/62] objtool: Mark prefix functions

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Message-ID: <20250526104355.GM24938@noisy.programming.kicks-ass.net>
Date: Mon, 26 May 2025 12:43:55 +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 29/62] objtool: Mark prefix functions

On Fri, May 09, 2025 at 01:16:53PM -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                 | 5 +++++
>  tools/objtool/include/objtool/elf.h | 6 ++++++
>  3 files changed, 12 insertions(+), 2 deletions(-)
> 
> diff --git a/tools/objtool/check.c b/tools/objtool/check.c
> index a2a025ec57e8..6b2e57d9aaf8 100644
> --- a/tools/objtool/check.c
> +++ b/tools/objtool/check.c
> @@ -3601,8 +3601,7 @@ 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..9a1fc0392b7f 100644
> --- a/tools/objtool/elf.c
> +++ b/tools/objtool/elf.c
> @@ -442,6 +442,11 @@ 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) && sym->len == 16 &&

Where did that 'sym->len == 16' thing come from? I mean, they are, but
the old code didn't assert that.

I would rather objtool issue a warn if not 16, but still consider these
as prefix.

> +	    (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;