linux-kernel - Re: [PATCH 1/2] ftrace/x86: Add frames pointers to trampoline as necessary

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Date:	Wed, 19 Nov 2014 19:26:48 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Steven Rostedt <rostedt@...dmis.org>
cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"H. Peter Anvin" <hpa@...or.com>, williams@...hat.com,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Namhyung Kim <namhyung@...nel.org>,
	Ingo Molnar <mingo@...hat.com>, x86@...nel.org
Subject: Re: [PATCH 1/2] ftrace/x86: Add frames pointers to trampoline as
 necessary

On Tue, 18 Nov 2014, Steven Rostedt wrote:
> From: "Steven Rostedt (Red Hat)" <rostedt@...dmis.org>
> 
> When CONFIG_FRAME_POINTERS are enabled, it is required that the
> ftrace_caller and ftrace_regs_caller trampolines set up frame pointers
> otherwise a stack trace from a function call wont print the functions
> that called the trampoline. This is due to a check in
> __save_stack_address():
> 
>  #ifdef CONFIG_FRAME_POINTER
> 	if (!reliable)
> 		return;
>  #endif
> 
> The "reliable" variable is only set if the function address is equal to
> contents of the address before the address the frame pointer register
> points to. If the frame pointer is not set up for the ftrace caller
> then this will fail the reliable test. It will miss the function that
> called the trampoline. Worse yet, if fentry is used (gcc 4.6 and
> beyond), it will also miss the parent, as the fentry is called before
> the stack frame is set up. That means the bp frame pointer points
> to the stack of just before the parent function was called.
> 
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: "H. Peter Anvin" <hpa@...or.com>
> Cc: x86@...nel.org
> Signed-off-by: Steven Rostedt <rostedt@...dmis.org>

Shouldn't this be tagged stable?

Acked-by: Thomas Gleixner <tglx@...utronix.de>

> ---
>  arch/x86/kernel/mcount_64.S | 41 +++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 41 insertions(+)
> 
> diff --git a/arch/x86/kernel/mcount_64.S b/arch/x86/kernel/mcount_64.S
> index 42f0cdd20baf..35a793fa4bba 100644
> --- a/arch/x86/kernel/mcount_64.S
> +++ b/arch/x86/kernel/mcount_64.S
> @@ -47,14 +47,51 @@ GLOBAL(\trace_label)
>  #endif
>  .endm
>  
> +#ifdef CONFIG_FRAME_POINTER
> +/*
> + * Stack traces will stop at the ftrace trampoline if the frame pointer
> + * is not set up properly. If fentry is used, we need to save a frame
> + * pointer for the parent as well as the function traced, because the
> + * fentry is called before the stack frame is set up, where as mcount
> + * is called afterward.
> + */
> +.macro create_frame parent rip
> +#ifdef CC_USING_FENTRY
> +	pushq \parent
> +	pushq %rbp
> +	movq %rsp, %rbp
> +#endif
> +	pushq \rip
> +	pushq %rbp
> +	movq %rsp, %rbp
> +.endm
> +
> +.macro restore_frame
> +#ifdef CC_USING_FENTRY
> +	addq $16, %rsp
> +#endif
> +	popq %rbp
> +	addq $8, %rsp
> +.endm
> +#else
> +.macro create_frame parent rip
> +.endm
> +.macro restore_frame
> +.endm
> +#endif /* CONFIG_FRAME_POINTER */
> +
>  ENTRY(ftrace_caller)
>  	ftrace_caller_setup ftrace_caller_op_ptr
>  	/* regs go into 4th parameter (but make it NULL) */
>  	movq $0, %rcx
>  
> +	create_frame %rsi, %rdi
> +
>  GLOBAL(ftrace_call)
>  	call ftrace_stub
>  
> +	restore_frame
> +
>  	MCOUNT_RESTORE_FRAME
>  
>  	/*
> @@ -105,9 +142,13 @@ ENTRY(ftrace_regs_caller)
>  	/* regs go into 4th parameter */
>  	leaq (%rsp), %rcx
>  
> +	create_frame %rsi, %rdi
> +
>  GLOBAL(ftrace_regs_call)
>  	call ftrace_stub
>  
> +	restore_frame
> +
>  	/* Copy flags back to SS, to restore them */
>  	movq EFLAGS(%rsp), %rax
>  	movq %rax, SS(%rsp)
> -- 
> 2.1.1
> 
> 
> 
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