linux-kernel - Re: [PATCHv4 2/3] kernel: add support for live patching

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Message-ID: <20141126154042.GB29079@treble.hsd1.ky.comcast.net>
Date:	Wed, 26 Nov 2014 09:40:42 -0600
From:	Josh Poimboeuf <jpoimboe@...hat.com>
To:	Miroslav Benes <mbenes@...e.cz>
Cc:	Seth Jennings <sjenning@...hat.com>, Jiri Kosina <jkosina@...e.cz>,
	Vojtech Pavlik <vojtech@...e.cz>,
	Steven Rostedt <rostedt@...dmis.org>,
	Petr Mladek <pmladek@...e.cz>,
	Christoph Hellwig <hch@...radead.org>,
	Greg KH <gregkh@...uxfoundation.org>,
	Andy Lutomirski <luto@...capital.net>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	live-patching@...r.kernel.org, x86@...nel.org, kpatch@...hat.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCHv4 2/3] kernel: add support for live patching

Hi Miroslav,

Just addressing one of your comments below.  I'll let Seth respond to
the others :-)

On Wed, Nov 26, 2014 at 03:19:17PM +0100, Miroslav Benes wrote:
> > +/**
> > + * struct klp_func - function structure for live patching
> > + * @old_name:	name of the function to be patched
> > + * @new_func:	pointer to the patched function code
> > + * @old_addr:	a hint conveying at what address the old function
> > + *		can be found (optional, vmlinux patches only)
> > + */
> > +struct klp_func {
> > +	/* external */
> > +	const char *old_name;
> > +	void *new_func;
> > +	/*
> > +	 * The old_addr field is optional and can be used to resolve
> > +	 * duplicate symbol names in the vmlinux object.  If this
> > +	 * information is not present, the symbol is located by name
> > +	 * with kallsyms. If the name is not unique and old_addr is
> > +	 * not provided, the patch application fails as there is no
> > +	 * way to resolve the ambiguity.
> > +	 */
> > +	unsigned long old_addr;
> 
> I wonder if we really need old_addr as an external field. I assume that 
> userspace tool in kpatch use it as a "hint" for kernel part and thus 
> kallsyms is not needed there (and it solves ambiguity problem as well). 
> But I am not sure if it is gonna be the same in upstream. When kernel is 
> randomized (CONFIG_RANDOMIZE_BASE is set to 'y', though default is 'n') 
> old_addr is not usable (and we throw it away in the code). Without 
> old_addr being set by the user we could spare some of code (calls to 
> klp_verify_vmlinux_symbol and such). 

Even with CONFIG_RANDOMIZE_BASE, the function offsets will be the same
regardless of the base address.  So we could still use old_addr to
determine the offset.

> So the question is whether future userspace tool in upstream would need it 
> and would use it. Please note that I do not mean it as a kpatch or kgraft 
> way to do things, I'm just not sure about old_addr being "public" and want 
> do discuss the options.
> 
> The ambiguity of symbols was discussed in some other thread in lkml in 
> october (I guess) with no conclusion IIRC...

We need to resolve ambiguity somehow, and old_addr is a way to do that.
Do you have any other ideas?

-- 
Josh
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