linux-kernel - Re: [PATCHv4 0/3] Kernel Live Patching

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Message-ID: <alpine.LNX.2.00.1411261014510.23174@pobox.suse.cz>
Date:	Wed, 26 Nov 2014 10:18:24 +0100 (CET)
From:	Jiri Kosina <jkosina@...e.cz>
To:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
cc:	Seth Jennings <sjenning@...hat.com>,
	Josh Poimboeuf <jpoimboe@...hat.com>,
	Vojtech Pavlik <vojtech@...e.cz>,
	Steven Rostedt <rostedt@...dmis.org>,
	Petr Mladek <pmladek@...e.cz>, Miroslav Benes <mbenes@...e.cz>,
	Christoph Hellwig <hch@...radead.org>,
	Greg KH <gregkh@...uxfoundation.org>,
	Andy Lutomirski <luto@...capital.net>,
	live-patching@...r.kernel.org, x86@...nel.org, kpatch@...hat.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCHv4 0/3] Kernel Live Patching

On Wed, 26 Nov 2014, Masami Hiramatsu wrote:

> > Note to Steve:
> > Masami's IPMODIFY patch is heading for -next via your tree.  Once it arrives,
> > I'll rebase and make the change to set IPMODIFY.  Do not pull this for -next
> > yet.  This version (v4) is for review and gathering acks.
> 
> BTW, as we discussed IPMODIFY is an exclusive flag. So if we allocate 
> ftrace_ops for each function in each patch, it could be conflict each 
> other.

Yup, this corresponds to what Petr brought up yesterday. There are cases 
where all solutions (kpatch, kgraft, klp) would allocate multiple 
ftrace_ops for a single function entry (think of patching one function 
multiple times in a row).

So it's not as easy as just setting the flag.

> Maybe we need to have another ops hashtable to find such conflict and 
> new handler to handle it.

If I understand your proposal correctly, that would sound like a hackish 
workaround, trying to basically trick the IPMODIFY flag semantics you just 
implemented :)

What I'd propose instead is to make sure that we always have 
just a ftrace_ops per function entry, and only update the pointers there 
as necessary. Fortunately we can do the switch atomically, by making use 
of ->private.

-- 
Jiri Kosina
SUSE Labs
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