linux-kernel - Re: [RFC][PATCH 4/5] ftrace/x86_32: Clean up ftrace_regs

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Date:   Wed, 15 Mar 2017 16:13:43 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Andy Lutomirski <luto@...capital.net>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [RFC][PATCH 4/5] ftrace/x86_32: Clean up ftrace_regs_caller

On Wed, Mar 15, 2017 at 03:55:31PM -0400, Steven Rostedt wrote:
> From: "Steven Rostedt (VMware)" <rostedt@...dmis.org>
> 
> When ftrace_regs_caller was created, it was designed to preserve flags as
> much as possible as it needed to act just like a breakpoint triggered on the
> same location. But the design is over complicated as it treated all
> operations as modifying flags. But push, mov and lea do not modify flags.
> This means the code can become more simplified by allowing flags to be
> stored further down.
> 
> Making ftrace_regs_caller simpler will also be useful in implementing fentry
> logic.
> 
> Signed-off-by: Steven Rostedt (VMware) <rostedt@...dmis.org>
> ---
>  arch/x86/kernel/ftrace_32.S | 45 +++++++++++++++++++++++++--------------------
>  1 file changed, 25 insertions(+), 20 deletions(-)
> 
> diff --git a/arch/x86/kernel/ftrace_32.S b/arch/x86/kernel/ftrace_32.S
> index 73d61a62649d..8ca33d9806ac 100644
> --- a/arch/x86/kernel/ftrace_32.S
> +++ b/arch/x86/kernel/ftrace_32.S
> @@ -55,23 +55,30 @@ WEAK(ftrace_stub)
>  END(ftrace_caller)
>  
>  ENTRY(ftrace_regs_caller)
> -	pushf	/* push flags before compare (in cs location) */
> -
>  	/*
>  	 * i386 does not save SS and ESP when coming from kernel.
>  	 * Instead, to get sp, &regs->sp is used (see ptrace.h).
>  	 * Unfortunately, that means eflags must be at the same location
>  	 * as the current return ip is. We move the return ip into the
> -	 * ip location, and move flags into the return ip location.
> -	 */
> -	pushl	4(%esp)				/* save return ip into ip slot */
> +	 * regs->ip location, and move flags into the return ip location.
> +         */
> +	pushl	$__KERNEL_CS
> +	pushl	4(%esp)				/* Save the return ip */
> +
> +	/* temporarily save flags in the orig_ax location */
> +	pushf
>  
> -	pushl	$0				/* Load 0 into orig_ax */
>  	pushl	%gs
>  	pushl	%fs
>  	pushl	%es
>  	pushl	%ds
>  	pushl	%eax
> +
> +	/* move flags into the location of where the return ip was */
> +	movl	5*4(%esp), %eax
> +	movl	$0, (%esp)			/* Load 0 into orig_ax */

This writes a 0 into regs->ax instead of regs->orig_ax.

-- 
Josh