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

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Date:	Thu, 27 Nov 2014 19:06:37 +0900
From:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
To:	Josh Poimboeuf <jpoimboe@...hat.com>
Cc:	Jiri Kosina <jkosina@...e.cz>, Seth Jennings <sjenning@...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

(2014/11/27 0:27), Josh Poimboeuf wrote:
> On Wed, Nov 26, 2014 at 10:18:24AM +0100, Jiri Kosina wrote:
>> 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 :)
> 
> I think Masami may be proposing something similar to what we do in
> kpatch today.  We have a single ftrace_ops and handler which is used for
> all functions.  The handler accesses a global hash of kpatch_func
> structs which is indexed by the original function's IP address.

Hmm, I think both is OK. kpatch method is less memory consuming and
will have a bigger overhead. However, as Steven talked at Plumbers Conf.,
he will introduce a direct code modifying interface for ftrace. After
that is introduced, we don't need to care about performance degradation
by patching :)

> It actually works out pretty well because it nicely encapsulates the
> knowledge about which functions are patched in a single place.  And it
> makes it easy to track function versions (for incremental patching and
> rollback).
> 
>> 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.
> 
> But how would you update multiple functions atomically, to enforce
> per-thread consistency?

At this point, both can do it atomically. We just need an atomic flag
for applying patches.

Thank you,


-- 
Masami HIRAMATSU
Software Platform Research Dept. Linux Technology Research Center
Hitachi, Ltd., Yokohama Research Laboratory
E-mail: masami.hiramatsu.pt@...achi.com


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