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

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Message-Id: <1219270138.21386.73.camel@pasglop>
Date:	Thu, 21 Aug 2008 08:08:58 +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 17:43 -0400, Steven Rostedt wrote:
> 
> 
> On Thu, 21 Aug 2008, Benjamin Herrenschmidt wrote:
> 
> > 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 ?
> 
> This is the x86 version, sync_core should be fine.

Oh oops, I didn't look well enough :-)

Ben.

> -- Steve
> 
> > 
> > > 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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