Date: Wed, 26 Nov 2008 11:46:58 -0500 (EST)
From: Steven Rostedt <rostedt@goodmis.org>
To: Andrew Morton <akpm@linux-foundation.org>
cc: linux-kernel@vger.kernel.org, Ingo Molnar <mingo@elte.hu>,
       Frederic Weisbecker <fweisbec@gmail.com>, containers@lists.osdl.org,
       Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com>,
       "Serge E. Hallyn" <serue@us.ibm.com>,
       "Eric W. Biederman" <ebiederm@xmission.com>,
       Steven Rostedt <srostedt@redhat.com>,
       Harvey Harrison <harvey.harrison@gmail.com>
Subject: Re: [PATCH 2/5] ftrace: use code patching for ftrace graph tracer
In-Reply-To: <20081125213546.ff4eddf4.akpm@linux-foundation.org>
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References: <20081126051622.134970943@goodmis.org> <20081126051709.774546196@goodmis.org> <20081125213546.ff4eddf4.akpm@linux-foundation.org>
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On Tue, 25 Nov 2008, Andrew Morton wrote:

> On Wed, 26 Nov 2008 00:16:24 -0500 Steven Rostedt <rostedt@goodmis.org> wrote:
> 
> > From: Steven Rostedt <rostedt@goodmis.org>
> > 
> > Impact: more efficient code for ftrace graph tracer
> > 
> > This patch uses the dynamic patching, when available, to patch
> > the function graph code into the kernel.
> > 
> > This patch will ease the way for letting both function tracing
> > and function graph tracing run together.
> > 
> > ...
> >
> > +static int ftrace_mod_jmp(unsigned long ip,
> > +			  int old_offset, int new_offset)
> > +{
> > +	unsigned char code[MCOUNT_INSN_SIZE];
> > +
> > +	if (probe_kernel_read(code, (void *)ip, MCOUNT_INSN_SIZE))
> > +		return -EFAULT;
> > +
> > +	if (code[0] != 0xe9 || old_offset != *(int *)(&code[1]))
> 
> erk.  I suspect that there's a nicer way of doing this amongst our
> forest of get_unaligned_foo() interfaces.  Harvey will know.

Hmm, I may be able to make a struct out of code.

  struct {
	unsigned char op;
	unsigned int  offset;
  } code __attribute__((packed));

Would that look better?

> 
> > +		return -EINVAL;
> > +
> > +	*(int *)(&code[1]) = new_offset;
> 
> Might be able to use put_unaligned_foo() here.
> 
> The problem is that these functions use sizeof(*ptr) to work out what
> to do, so a cast is still needed.  A get_unaligned32(ptr) would be
> nice.  One which takes a void* and assumes CPU ordering.

Is there a correctness concern here? This is arch specific code, so I'm
not worried about other archs.

-- Steve

> 
> > +	if (do_ftrace_mod_code(ip, &code))
> > +		return -EPERM;
> > +
> > +	return 0;
> > +}
> > +
> 
> 
> 
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