linux-kernel - Re: [PATCH] ftrace/x86: Fix function graph tracer reset path

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Date:	Mon, 16 May 2016 21:03:59 +0200
From:	Borislav Petkov <bp@...e.de>
To:	Namhyung Kim <namhyung@...nel.org>,
	Steven Rostedt <rostedt@...dmis.org>
Cc:	Masami Hiramatsu <mhiramat@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>, x86@...nel.org,
	"H.J. Lu" <hjl.tools@...il.com>
Subject: Re: [PATCH] ftrace/x86: Fix function graph tracer reset path

On Mon, May 16, 2016 at 11:24:53PM +0900, Namhyung Kim wrote:
> > -GLOBAL(ftrace_stub)
> > +/* This is weak to keep gas from relaxing the jumps */
> > +WEAK(ftrace_stub)
> >  	retq
> >  END(ftrace_caller)

You could also force the 5-byte jump. I guess you could also write
simply ".long 0" in there but this way it is more robust if someone
decides to add other stuff between the JMP and the ftrace_stub label.

---
diff --git a/arch/x86/kernel/mcount_64.S b/arch/x86/kernel/mcount_64.S
index ed48a9f465f8..b1db8a584c06 100644
--- a/arch/x86/kernel/mcount_64.S
+++ b/arch/x86/kernel/mcount_64.S
@@ -179,7 +179,9 @@ GLOBAL(ftrace_epilogue)
 
 #ifdef CONFIG_FUNCTION_GRAPH_TRACER
 GLOBAL(ftrace_graph_call)
-	jmp ftrace_stub
+	.byte 0xe9
+	.long ftrace_stub - 1f
+1:
 #endif
 
 GLOBAL(ftrace_stub)

-- 
Regards/Gruss,
    Boris.

SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nürnberg)
--