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Subject: Re: [PATCH 0/3] livepatch: add shadow variable API
To: Miroslav Benes <mbenes@suse.cz>
References: <1496341526-19061-1-git-send-email-joe.lawrence@redhat.com>
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 <alpine.LSU.2.20.1706131217400.29266@pobox.suse.cz>
Cc: Jiri Kosina <jikos@kernel.org>, live-patching@vger.kernel.org,
        linux-kernel@vger.kernel.org, Josh Poimboeuf <jpoimboe@redhat.com>,
        Jessica Yu <jeyu@redhat.com>, Petr Mladek <pmladek@suse.com>
From: Joe Lawrence <joe.lawrence@redhat.com>
Organization: Red Hat
Message-ID: <8a4711b9-9536-3843-c737-50fab2536aa5@redhat.com>
Date: Tue, 13 Jun 2017 13:29:31 -0400
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On 06/13/2017 06:19 AM, Miroslav Benes wrote:
>> If you are referring to stacking livepatches ... to be honest I hadn't
>> thought of that scenario.  In that case, we might be able to get away
>> with pushing something like this into the hash:
>>
>>  klp #1:  klp_shadow_attach(ptr, "shadow_var", ...)
>>  klp #2:  klp_shadow_attach(ptr, "shadow_var_v2", ...)
> 
> I thought this was the reason to have a string there. Otherwise, a 
> pointer to original data would be enough, wouldn't it?

Well, one could attach multiple shadow variables to the same data
structure, ie, one for each new data element.  In the stacking case, you
might add a spinlock in patch 1, then a linked-list in patch 2.  Patched
codepaths would then use klp_shadow_get(obj, "spinlock") or
klp_shadow_get(obj, "list") as needed.

Versioning shadow variables would be a bit more involved.  You'd have to
figure out if you A) convert existing shadow variables to the new format
on livepatch module load, or B) convert on the fly, or C) handle none,
v1, and v2 instances of the shadow variables.  /head spins

To be honest, I don't think we've never needed anything beyond basic
shadow variables in kpatch, so I'm only speculating about their
potential (ab)uses :)  That said, since this patchset is introducing the
API, it would be good to be reasonably flexible.

-- Joe