Re: [PATCHv4 0/3] Kernel Live Patching

Masami Hiramatsu
2014-11-27T10:07:10Z

(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@hitachi.com