linux-kernel - Re: [patch 36/38] x86/ftrace: Make it call depth tracking aware

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Date:   Tue, 19 Jul 2022 10:46:48 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        LKML <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>,
        Pawan Gupta <pawan.kumar.gupta@...ux.intel.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>
Subject: Re: [patch 36/38] x86/ftrace: Make it call depth tracking aware

On Mon, Jul 18, 2022 at 05:01:23PM -0400, Steven Rostedt wrote:

> From 533f10bd48ffbc4ee5d2a07f0a7fe99aeb1c823a Mon Sep 17 00:00:00 2001
> From: "Steven Rostedt (Google)" <rostedt@...dmis.org>
> Date: Mon, 18 Jul 2022 16:01:07 -0400
> Subject: [PATCH] ftrace/x86: Remove jumps to ftrace_epilogue
> 
> The jumps to ftrace_epilogue were done as a way to make sure all the
> function tracing trampolines ended at the function graph trampoline, as
> the ftrace_epilogue was the location that it would handle that.
> 
> With the advent of function graph tracer now being just one of the
> callbacks of the function tracer there is no more requirement that all
> trampolines go to a single location.
> 
> Remove the jumps to the ftrace_epilogue and replace them with return
> statements.
> 
> Note, the ftrace_epilogue can probably be renamed to ftrace_stub and the
> weak logic for that could probably be removed. But lets leave that as a
> separate change.
> 
> Signed-off-by: Steven Rostedt (Google) <rostedt@...dmis.org>
> ---
>  arch/x86/kernel/ftrace_64.S | 16 +++++++---------
>  1 file changed, 7 insertions(+), 9 deletions(-)
> 
> diff --git a/arch/x86/kernel/ftrace_64.S b/arch/x86/kernel/ftrace_64.S
> index dfeb227de561..8f225fafa5fb 100644
> --- a/arch/x86/kernel/ftrace_64.S
> +++ b/arch/x86/kernel/ftrace_64.S
> @@ -173,7 +173,9 @@ SYM_INNER_LABEL(ftrace_call, SYM_L_GLOBAL)
>  SYM_INNER_LABEL(ftrace_caller_end, SYM_L_GLOBAL)
>  	ANNOTATE_NOENDBR
>  
> -	jmp ftrace_epilogue
> +	UNWIND_HINT_FUNC
> +	ENDBR

Only the RET should do I think; you definitely don't need an ENDBR here
nor do you need to override the unwind hint. Lemme try..

Yeah, the below is sufficient:

diff --git a/arch/x86/kernel/ftrace_64.S b/arch/x86/kernel/ftrace_64.S
index dfeb227de561..d6679b65b6f2 100644
--- a/arch/x86/kernel/ftrace_64.S
+++ b/arch/x86/kernel/ftrace_64.S
@@ -172,8 +172,7 @@ SYM_INNER_LABEL(ftrace_call, SYM_L_GLOBAL)
 	 */
 SYM_INNER_LABEL(ftrace_caller_end, SYM_L_GLOBAL)
 	ANNOTATE_NOENDBR
-
-	jmp ftrace_epilogue
+	RET
 SYM_FUNC_END(ftrace_caller);
 STACK_FRAME_NON_STANDARD_FP(ftrace_caller)
 
@@ -269,7 +268,7 @@ SYM_INNER_LABEL(ftrace_regs_caller_jmp, SYM_L_GLOBAL)
 	 */
 SYM_INNER_LABEL(ftrace_regs_caller_end, SYM_L_GLOBAL)
 	ANNOTATE_NOENDBR
-	jmp ftrace_epilogue
+	RET
 
 	/* Swap the flags with orig_rax */
 1:	movq MCOUNT_REG_SIZE(%rsp), %rdi
@@ -280,7 +279,7 @@ SYM_INNER_LABEL(ftrace_regs_caller_end, SYM_L_GLOBAL)
 	/* Restore flags */
 	popfq
 	UNWIND_HINT_FUNC
-	jmp	ftrace_epilogue
+	RET
 
 SYM_FUNC_END(ftrace_regs_caller)
 STACK_FRAME_NON_STANDARD_FP(ftrace_regs_caller)