linux-kernel - Re: [PATCH v2] kprobes/x86: Use 5-byte NOP when the code might be modified by ftrace

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Date:	Wed, 18 Feb 2015 22:22:05 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Petr Mladek <pmladek@...e.cz>
Cc:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Ingo Molnar <mingo@...e.hu>,
	"David S. Miller" <davem@...emloft.net>,
	Anil S Keshavamurthy <anil.s.keshavamurthy@...el.com>,
	Ananth NMavinakayanahalli <ananth@...ibm.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Jiri Kosina <jkosina@...e.cz>, linux-kernel@...r.kernel.org,
	Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH v2] kprobes/x86: Use 5-byte NOP when the code might be
 modified by ftrace


* Petr Mladek <pmladek@...e.cz> wrote:

> On Tue 2015-02-03 16:41:39, Masami Hiramatsu wrote:
> > (2015/02/03 2:48), Petr Mladek wrote:
> > > can_probe() checks if the given address points to the beginning of
> > > an instruction. It analyzes all the instructions from the beginning
> > > of the function until the given address. The code might be modified
> > > by another Kprobe. In this case, the current code is read into a buffer,
> > > int3 breakpoint is replaced by the saved opcode in the buffer, and
> > > can_probe() analyzes the buffer instead.
> > > 
> > > There is a bug that __recover_probed_insn() tries to restore
> > > the original code even for Kprobes using the ftrace framework.
> > > But in this case, the opcode is not stored. See the difference
> > > between arch_prepare_kprobe() and arch_prepare_kprobe_ftrace().
> > > The opcode is stored by arch_copy_kprobe() only from
> > > arch_prepare_kprobe().
> > > 
> > > This patch makes Kprobe to use the ideal 5-byte NOP when the code
> > > can be modified by ftrace. It is the original instruction, see
> > > ftrace_make_nop() and ftrace_nop_replace().
> > > 
> > > Note that we always need to use the NOP for ftrace locations. Kprobes
> > > do not block ftrace and the instruction might get modified at anytime.
> > > It might even be in an inconsistent state because it is modified step
> > > by step using the int3 breakpoint.
> > > 
> > > The patch also fixes indentation of the touched comment.
> > > 
> > > Note that I found this problem when playing with Kprobes. I did it
> > > on x86_64 with gcc-4.8.3 that supported -mfentry. I modified
> > > samples/kprobes/kprobe_example.c and added offset 5 to put
> > > the probe right after the fentry area:
> > > 
> > > --- cut ---
> > >  static struct kprobe kp = {
> > >  	.symbol_name	= "do_fork",
> > > +	.offset = 5,
> > >  };
> > > --- cut ---
> > > 
> > > Then I was able to load kprobe_example before jprobe_example
> > > but not the other way around:
> > > 
> > > $> modprobe jprobe_example
> > > $> modprobe kprobe_example
> > > modprobe: ERROR: could not insert 'kprobe_example': Invalid or incomplete multibyte or wide character
> > > 
> > > It did not make much sense and debugging pointed to the bug
> > > described above.
> > > 
> > 
> > This looks good to me :)
> > 
> > Acked-by: Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
> > 
> > Ingo, could you merge this as an urgent fix?
> 
> Please, wait a bit, see below.

Please [re]send the final patch with Masami's Ack added and 
me Cc:-ed.

Thanks,

	Ingo
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