linux-kernel - Re: [PATCH v3 32/59] objtool: Allow STT_NOTYPE -> STT

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Date:   Wed, 21 Sep 2022 22:27:50 -0700
From:   Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>, linux-kernel@...r.kernel.org,
        x86@...nel.org, Linus Torvalds <torvalds@...ux-foundation.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Josh Poimboeuf <jpoimboe@...nel.org>,
        Andrew Cooper <Andrew.Cooper3@...rix.com>,
        Johannes Wikner <kwikner@...z.ch>,
        Alyssa Milburn <alyssa.milburn@...ux.intel.com>,
        Jann Horn <jannh@...gle.com>, "H.J. Lu" <hjl.tools@...il.com>,
        Joao Moreira <joao.moreira@...el.com>,
        Joseph Nuzman <joseph.nuzman@...el.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Juergen Gross <jgross@...e.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        K Prateek Nayak <kprateek.nayak@....com>,
        Eric Dumazet <edumazet@...gle.com>
Subject: Re: [PATCH v3 32/59] objtool: Allow STT_NOTYPE -> STT_FUNC+0
 tail-calls

On Thu, Sep 15, 2022 at 01:11:11PM +0200, Peter Zijlstra wrote:
> From: Peter Zijlstra <peterz@...radead.org>
> 
> Allow STT_NOTYPE to tail-call into STT_FUNC, per definition STT_NOTYPE
> is not a sub-function of the STT_FUNC.
> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> ---
>  tools/objtool/check.c |   29 ++++++++++++++++++++---------
>  1 file changed, 20 insertions(+), 9 deletions(-)
> 
> --- a/tools/objtool/check.c
> +++ b/tools/objtool/check.c
> @@ -1370,6 +1370,16 @@ static void add_return_call(struct objto
>  
>  static bool same_function(struct instruction *insn1, struct instruction *insn2)
>  {
> +	if (!insn1->func && !insn2->func)
> +		return true;
> +
> +	/* Allow STT_NOTYPE -> STT_FUNC+0 tail-calls */
> +	if (!insn1->func && insn1->func != insn2->func)
> +		return false;

Looks like this check is triggering below warning:

vmlinux.o: warning: objtool: ftrace_replace_code.cold+0x0: unreachable instruction

(.config is attached)

> +
> +	if (!insn2->func)
> +		return true;
> +
>  	return insn1->func->pfunc == insn2->func->pfunc;
>  }

View attachment ".config" of type "text/plain" (137862 bytes)