linux-kernel - Re: [PATCH 1/2] kprobes/x86: Use 5-byte NOP when the code might be modified by ftrace

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Message-ID: <20150220125223.GA28882@gmail.com>
Date:	Fri, 20 Feb 2015 13:52:23 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
Cc:	Petr Mladek <pmladek@...e.cz>,
	"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
Subject: Re: [PATCH 1/2] kprobes/x86: Use 5-byte NOP when the code might be
 modified by ftrace


* Masami Hiramatsu <masami.hiramatsu.pt@...achi.com> wrote:

> (2015/02/20 19:26), Ingo Molnar wrote:
> > 
> > * Petr Mladek <pmladek@...e.cz> 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.
> >>
> >> [...]
> > 
> > Had to drop this patch due to build failures on 32-bit x86:
> > 
> > arch/x86/kernel/kprobes/core.c:258:40: error: ‘MCOUNT_INSN_SIZE’ undeclared (first use in this function)
> 
> Oops, MCOUNT_INSN_SIZE actually depends on 
> CONFIG_FUNCTION_TRACER... I think we can use 5 instead of 
> that since we are copying NOP_ATOMIC5.

Or just make the define more widely available? It's not 
like the size changes from disabling the function tracer.

Thanks,

	Ingo
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