linux-kernel - Re: [RFC] kreplace: Rebootless kernel updates

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Message-ID: <4926C838.8080105@redhat.com>
Date:	Fri, 21 Nov 2008 09:39:52 -0500
From:	Masami Hiramatsu <mhiramat@...hat.com>
To:	Nikanth Karthikesan <knikanth@...e.de>
CC:	linux-kernel@...r.kernel.org, ananth@...ibm.com,
	davem@...emloft.net, contact@...lice.com, jbarnold@...lice.com,
	tabbott@...lice.com, wdaher@...lice.com, andersk@...lice.com
Subject: Re: [RFC] kreplace: Rebootless kernel updates

Hi Nikanth,

Nikanth Karthikesan wrote:
> This RFC patch adds support for limited form of rebootless kernel patching 
> even without building the entire kernel.
> 
> When looking for a shortcut to avoid the rebuild/reboot cycle when hacking the  
> kernel - the ksplice[1] was posted. This patch extends kprobes to do something 
> similar, which would require even lesser time to _experiment_ with the running 
> kernel. 
> 
> This small patch extends jprobes so that the jprobe's handler is executed but 
> skips executing the actual function. But this has its own limitations such as 
> Cannot access symbols not exported for modules (ofcourse hacks like 
> pointers[2] can be used.), problems related to return values[3], etc... This 
> is currently a x86_64 only _hack_.

Hmm,
Would you like to replace a function to another function?
If so, AFAIK, you can do that with kprobe and below pre_handler.
(see booster enabled path in setup_singlestep())

pre_handler(...)
{
	reset_current_kprobe();	/* this kprobe doesn't need any more */
	regs->ip = new_function;	/* change IP to new function */
	preempt_enable_no_resched();	/* recover preempt count */
	return 1;	/* No need to setup singlestep */
}

Thank you,


-- 
Masami Hiramatsu

Software Engineer
Hitachi Computer Products (America) Inc.
Software Solutions Division

e-mail: mhiramat@...hat.com

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