linux-kernel - Re: FInal kprobes rollup patches

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Message-Id: <1197927385.23402.41.camel@brick>
Date:	Mon, 17 Dec 2007 13:36:25 -0800
From:	Harvey Harrison <harvey.harrison@...il.com>
To:	Masami Hiramatsu <mhiramat@...hat.com>
Cc:	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...e.hu>,
	LKML <linux-kernel@...r.kernel.org>,
	Maneesh Soni <maneesh@...ux.vnet.ibm.com>,
	srinivasa@...ibm.com, Jim Keniston <jkenisto@...ibm.com>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Masami Hiramatsu <hiramatu@....hitachi.co.jp>,
	Rusty Lynch <rusty.lynch@...el.com>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Keshavamurthy Anil S <anil.s.keshavamurthy@...el.com>
Subject: Re: FInal kprobes rollup patches

On Mon, 2007-12-17 at 16:28 -0500, Masami Hiramatsu wrote:
> Hi Harvey,
> If you mention about a relative jump which is inserted by
> resume_execution(), I think you might misunderstand that relative jump.
> 
> The size of that relative jump, which will be embedded by kprobe-booster, is
> 5-bytes(not 1 byte). So it needs 5 bytes space.
> And we decided not to expand MAX_INSN_SIZE when we developed the booster.
> The reasons are:
>  - it is supplemental feature(just accelerating kprobes), if we have no space,
>    we can disable it.
>  - 5 bytes are big enough compared with 15(=MAX_INSN_SIZE)
>  - the lengths of most of instructions are less than 10 bytes.
> 
> Additionally, MAX_INSN_SIZE is used in kernel/kprobes.c to allocate an
> instruction buffer which will be assigned to p->ainsn.insn. Since the
> instruction buffer size is MAX_INSN_SIZE, you can not copy instructions
> more than MAX_INSN_SIZE.
> 
> BTW, in my patch, I unified MAX_INSN_SIZE to bigger one(16).
> I think it is enough for us.
> 

I went with 15 in mine, I thought it made the code a little more
readable, but I will defer if you think 16 is better.  If you want me
to send the whole series to you, let me know.

I just sent out a series of 4 patches equivalent to your patches 1-4/6
but based on my already unified kprobes.c/h, You may want to check your
handling of restored registers in trampoline_probe_handler which I found
when rebasing yours on top of my cleanups.  Not sure if this is
important, but it was a difference I found.

X86_32:
regs->cs = __KERNEL_CS | get_kernel_rpl();

yours:
regs->cs = __KERNEL_CS;

Harvey

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