linux-kernel - Re: [PATCH -rc] [BUGFIX] x86: fix kernel_trap

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Message-ID: <20090512112300.GL7601@sgi.com>
Date:	Tue, 12 May 2009 06:23:00 -0500
From:	Robin Holt <holt@....com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Masami Hiramatsu <mhiramat@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>,
	systemtap@...rces.redhat.com,
	Harvey Harrison <harvey.harrison@...il.com>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Jan Blunck <jblunck@...e.de>,
	Christoph Hellwig <hch@...radead.org>
Subject: Re: [PATCH -rc] [BUGFIX] x86: fix kernel_trap_sp()

On Mon, May 11, 2009 at 03:24:07PM -0700, Linus Torvalds wrote:
> 
> 
> On Mon, 11 May 2009, Masami Hiramatsu wrote:
> >
> > Use &regs->sp instead of regs for getting the top of stack in kernel mode.
> > (on x86-64, regs->sp always points the top of stack)
> 
> Ack. 
> 
> That said, we have only _one_ use of this "kernel_trap_sp()" in the whole 
> kernel, and that use is actually fairly odd too, in that it does it 
> _before_ checking that it's in kernel mode.
> 
> Admittedly it will then only _use_ the value after it has checked that 
> things are in kernel mode, but it all boils down to "ok, that's pretty 
> odd".
> 
> So how about fixing that, and also fixing the naming of the function. Call 
> it "kernel_stack_pointer()" to match its more widely used sibling function
> "user_stack_pointer()".
> 
> IOW, something like this?
> 
> 		Linus
> 
> ---
>  arch/x86/include/asm/ptrace.h |    7 ++++---
>  arch/x86/oprofile/backtrace.c |    2 +-
>  2 files changed, 5 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/include/asm/ptrace.h b/arch/x86/include/asm/ptrace.h
> index e304b66..624f133 100644
> --- a/arch/x86/include/asm/ptrace.h
> +++ b/arch/x86/include/asm/ptrace.h
> @@ -187,14 +187,15 @@ static inline int v8086_mode(struct pt_regs *regs)
>  
>  /*
>   * X86_32 CPUs don't save ss and esp if the CPU is already in kernel mode
> - * when it traps.  So regs will be the current sp.
> + * when it traps.  The previous stack will be directly underneath the saved
> + * registers, and 'sp/ss' won't even have been saved. Thus the '&regs->sp'.
>   *
>   * This is valid only for kernel mode traps.
>   */
> -static inline unsigned long kernel_trap_sp(struct pt_regs *regs)
> +static inline unsigned long kernel_stack_pointer(struct pt_regs *regs)

Why not have it return an unsigned long *?
>  {
>  #ifdef CONFIG_X86_32
> -	return (unsigned long)regs;
> +	return (unsigned long)(&regs->sp);
>  #else
>  	return regs->sp;
>  #endif
> diff --git a/arch/x86/oprofile/backtrace.c b/arch/x86/oprofile/backtrace.c
> index 04df67f..044897b 100644
> --- a/arch/x86/oprofile/backtrace.c
> +++ b/arch/x86/oprofile/backtrace.c
> @@ -76,9 +76,9 @@ void
>  x86_backtrace(struct pt_regs * const regs, unsigned int depth)
>  {
>  	struct frame_head *head = (struct frame_head *)frame_pointer(regs);
> -	unsigned long stack = kernel_trap_sp(regs);
>  
>  	if (!user_mode_vm(regs)) {
> +		unsigned long stack = kernel_stack_pointer(regs);

Make this an unsigned long *?

>  		if (depth)
>  			dump_trace(NULL, regs, (unsigned long *)stack, 0,

Then get rid of the cast?

Robin
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