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

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Message-ID: <20160516151357.73fca76a@gandalf.local.home>
Date:	Mon, 16 May 2016 15:13:57 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Borislav Petkov <bp@...e.de>
Cc:	Namhyung Kim <namhyung@...nel.org>,
	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, 16 May 2016 21:03:59 +0200
Borislav Petkov <bp@...e.de> wrote:

> 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
>  

I actually thought about this first, but I thought it rather a hack
(although one could argue all of function tracing is a hack ;-) But as
the "weak" call was used to fix one location, why not use it here too.
Being consistent, and also making sure all calls to ftrace_stub do the
same.

-- Steve