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: <20150220133440.GK5136@pathway.suse.cz>
Date:	Fri, 20 Feb 2015 14:34:40 +0100
From:	Petr Mladek <pmladek@...e.cz>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	"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

On Fri 2015-02-20 13:52:23, Ingo Molnar wrote:
> 
> * 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.

I'll use 5 for now.

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

I would do this in a separate patch that would consolidate all
NOP_ATOMIC5 users: ftrace, jump_labels, and kprobe. The number
5 is hardcoded more times in arch/x86/kernel/kprobes/core.c.
I'll put it into my TODO list.

Best Regards,
Petr
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