linux-kernel - Re: [PATCH v3 56/59] x86/ftrace: Remove ftrace

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Date:   Fri, 9 Dec 2022 10:41:17 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
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>,
        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>,
        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 56/59] x86/ftrace: Remove ftrace_epilogue()

On Thu, 15 Sep 2022 13:11:35 +0200
Peter Zijlstra <peterz@...radead.org> wrote:

> From: Peter Zijlstra <peterz@...radead.org>
> 
> Remove the weird jumps to RET and simply use RET.
> 
> This then promotes ftrace_stub() to a real function; which becomes
> important for kcfi.
> 
> 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>
> 

Moving my setup to a new machine, I found I had this patch queued and never
pushed out :-p

-- Steve

>From e55c8c52510be3ac996f3567e1d2dbdf76ed23de 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
+	RET
 SYM_FUNC_END(ftrace_caller);
 STACK_FRAME_NON_STANDARD_FP(ftrace_caller)
 
@@ -261,15 +263,9 @@ SYM_INNER_LABEL(ftrace_regs_caller_jmp, SYM_L_GLOBAL)
 	/* Restore flags */
 	popfq
 
-	/*
-	 * As this jmp to ftrace_epilogue can be a short jump
-	 * it must not be copied into the trampoline.
-	 * The trampoline will add the code to jump
-	 * to the return.
-	 */
 SYM_INNER_LABEL(ftrace_regs_caller_end, SYM_L_GLOBAL)
 	ANNOTATE_NOENDBR
-	jmp ftrace_epilogue
+	jmp 2f
 
 	/* Swap the flags with orig_rax */
 1:	movq MCOUNT_REG_SIZE(%rsp), %rdi
@@ -279,8 +275,10 @@ SYM_INNER_LABEL(ftrace_regs_caller_end, SYM_L_GLOBAL)
 	restore_mcount_regs 8
 	/* Restore flags */
 	popfq
+2:
 	UNWIND_HINT_FUNC
-	jmp	ftrace_epilogue
+	ENDBR
+	RET
 
 SYM_FUNC_END(ftrace_regs_caller)
 STACK_FRAME_NON_STANDARD_FP(ftrace_regs_caller)
-- 
2.35.1