linux-kernel - Re: [PATCH v5 0/3] livepatch: introduce atomic replace

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Date:   Wed, 31 Jan 2018 17:09:21 -0500
From:   Joe Lawrence <joe.lawrence@...hat.com>
To:     Petr Mladek <pmladek@...e.com>,
        Evgenii Shatokhin <eshatokhin@...tuozzo.com>
Cc:     Jason Baron <jbaron@...mai.com>, linux-kernel@...r.kernel.org,
        live-patching@...r.kernel.org, jpoimboe@...hat.com,
        jeyu@...nel.org, jikos@...nel.org, mbenes@...e.cz
Subject: Re: [PATCH v5 0/3] livepatch: introduce atomic replace

On 01/30/2018 09:03 AM, Petr Mladek wrote:
> On Fri 2018-01-26 14:29:36, Evgenii Shatokhin wrote:
>>
>> In my experience, it was quite convenient sometimes to just "replace all
>> binary patches the user currently has loaded with this single one". No
>> matter what these original binary patches did and where they came from.
> 
> To be honest, I would feel better if the livepatch framework is
> more safe. It should prevent breaking the system by a patch
> that atomically replaces other random patches that rely on callbacks.
> 
> Well, combining random livepatches from random vendors is a call
> for troubles. It might easily fail when two patches add
> different changes to the same function.
> 
> I wonder if we should introduce some tags, keys, vendors. I mean
> to define an identification or dependencies that would say that some
> patches are compatible or incompatible.
> 
> We could allow to livepatch one function by two livepatches
> only if they are from the same vendor. But then the order still
> might be important. Also I am not sure if this condition is safe
> enough.
> 
> One the other hand, we could handle callbacks like the shadow
> variables. Every shadow variable has an ID. We have an API to
> add/read/update/remove them. We might allow to have more
> callbacks with different IDs. Then we could run callbacks
> only for IDs that are newly added or removed. Sigh, this would
> be very complex implementation as well. But it might make
> these features easier to use and maintain.

Interesting ideas, but I wonder if this could be accomplished at the
livepatch module level?  ie, leave it to kpatch-build (or a livepatch
equivalent) to produce a module that does this automatically.  I guess
it would then be completely opt-in checking, but transfers the
complexity out of the kernel livepatching core.

I don't see a simple way to provide flexibility of when/if calling
redundant callbacks without making the code even more complicated.  I
don't have any use-cases off hand that would require such features, but
I guess if I did absolutely needed them, I might be inclined to say
prepare ahead of time and write callbacks so that they may be disabled
externally -- either by a new livepatch module's init() or some other
means.  Then whatever crazy policy I need is contained to my modules,
not provided or enforced by the kernel.

-- Joe