linux-kernel - Re: [PATCH 2/5] ftrace: use code patching for ftrace graph tracer

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Message-Id: <20081125213546.ff4eddf4.akpm@linux-foundation.org>
Date:	Tue, 25 Nov 2008 21:35:46 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Frederic Weisbecker <fweisbec@...il.com>,
	containers@...ts.osdl.org,
	Sukadev Bhattiprolu <sukadev@...ux.vnet.ibm.com>,
	"Serge E. Hallyn" <serue@...ibm.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Steven Rostedt <srostedt@...hat.com>,
	Harvey Harrison <harvey.harrison@...il.com>
Subject: Re: [PATCH 2/5] ftrace: use code patching for ftrace graph tracer

On Wed, 26 Nov 2008 00:16:24 -0500 Steven Rostedt <rostedt@...dmis.org> wrote:

> From: Steven Rostedt <rostedt@...dmis.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.

> +		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.

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

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