linux-kernel - Re: [PATCH] ftrace: x86 use copy to and from user functions

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Message-Id: <1219268378.21386.69.camel@pasglop>
Date:	Thu, 21 Aug 2008 07:39:38 +1000
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH] ftrace: x86 use copy to and from user functions

On Wed, 2008-08-20 at 12:55 -0400, Steven Rostedt wrote:
> The modification of code is performed either by kstop_machine, before
> SMP starts, or on module code before the module is executed. There is
> no reason to do the modifications from assembly. The copy to and from
> user functions are sufficient and produces cleaner and easier to read
> code.
> 
> Thanks to Benjamin Herrenschmidt for suggesting the idea.

Haven't we lost the dcache/icache synchronisation somewhere ?

> Signed-off-by: Steven Rostedt <srostedt@...hat.com>
> ---
>  arch/x86/kernel/ftrace.c |   38 +++++++++++++-------------------------
>  1 file changed, 13 insertions(+), 25 deletions(-)
> 
> Index: linux-tip.git/arch/x86/kernel/ftrace.c
> ===================================================================
> --- linux-tip.git.orig/arch/x86/kernel/ftrace.c	2008-08-20 12:39:41.000000000 -0400
> +++ linux-tip.git/arch/x86/kernel/ftrace.c	2008-08-20 12:40:17.000000000 -0400
> @@ -11,6 +11,7 @@
>  
>  #include <linux/spinlock.h>
>  #include <linux/hardirq.h>
> +#include <linux/uaccess.h>
>  #include <linux/ftrace.h>
>  #include <linux/percpu.h>
>  #include <linux/init.h>
> @@ -60,11 +61,7 @@ notrace int
>  ftrace_modify_code(unsigned long ip, unsigned char *old_code,
>  		   unsigned char *new_code)
>  {
> -	unsigned replaced;
> -	unsigned old = *(unsigned *)old_code; /* 4 bytes */
> -	unsigned new = *(unsigned *)new_code; /* 4 bytes */
> -	unsigned char newch = new_code[4];
> -	int faulted = 0;
> +	unsigned char replaced[MCOUNT_INSN_SIZE];
>  
>  	/*
>  	 * Note: Due to modules and __init, code can
> @@ -72,29 +69,20 @@ ftrace_modify_code(unsigned long ip, uns
>  	 *  as well as code changing.
>  	 *
>  	 * No real locking needed, this code is run through
> -	 * kstop_machine.
> +	 * kstop_machine, or before SMP starts.
>  	 */
> -	asm volatile (
> -		"1: lock\n"
> -		"   cmpxchg %3, (%2)\n"
> -		"   jnz 2f\n"
> -		"   movb %b4, 4(%2)\n"
> -		"2:\n"
> -		".section .fixup, \"ax\"\n"
> -		"3:	movl $1, %0\n"
> -		"	jmp 2b\n"
> -		".previous\n"
> -		_ASM_EXTABLE(1b, 3b)
> -		: "=r"(faulted), "=a"(replaced)
> -		: "r"(ip), "r"(new), "c"(newch),
> -		  "0"(faulted), "a"(old)
> -		: "memory");
> -	sync_core();
> +	if (__copy_from_user(replaced, (char __user *)ip, MCOUNT_INSN_SIZE))
> +		return 1;
>  
> -	if (replaced != old && replaced != new)
> -		faulted = 2;
> +	if (memcmp(replaced, old_code, MCOUNT_INSN_SIZE) != 0)
> +		return 2;
>  
> -	return faulted;
> +	WARN_ON_ONCE(__copy_to_user((char __user *)ip, new_code,
> +				    MCOUNT_INSN_SIZE));
> +
> +	sync_core();
> +
> +	return 0;
>  }
>  
>  notrace int ftrace_update_ftrace_func(ftrace_func_t func)

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