linux-kernel - Re: [PATCH v2] livepatch: old_name.number scheme in livepatch sysfs directory

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Date:	Wed, 4 Nov 2015 10:17:52 -0600
From:	Chris J Arges <chris.j.arges@...onical.com>
To:	Josh Poimboeuf <jpoimboe@...hat.com>,
	Miroslav Benes <mbenes@...e.cz>
Cc:	live-patching@...r.kernel.org, jeyu@...hat.com,
	Seth Jennings <sjenning@...hat.com>,
	Jiri Kosina <jikos@...nel.org>,
	Vojtech Pavlik <vojtech@...e.com>, linux-api@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] livepatch: old_name.number scheme in livepatch sysfs
 directory



On 11/04/2015 10:03 AM, Josh Poimboeuf wrote:
> On Wed, Nov 04, 2015 at 10:52:52AM +0100, Miroslav Benes wrote:
>> On Tue, 3 Nov 2015, Josh Poimboeuf wrote:
>>>> Object entry would be empty for not loaded object. I would not 
>>>> dare to propose to remove such object entries. It would make things worse. 
>>>
>>> Why would removing an empty object entry make things worse?
>>
>> I think it all comes down to a question whether the sysfs entries say what 
>> a patch is capable to patch or what this patch is currently patching in 
>> the system. I am inclined to the former so the removal would make me 
>> nervous. But I am not against the second approach. We are still in testing 
>> mode as far as sysfs is concerned so we can try even harsh changes and see 
>> how it's gonna go.
> 
> I see your point.  This approach only describes what is patched now, but
> it doesn't describe what *will* be patched.  Ideally we could find a way
> to describe both.
> 
> Speaking of harsh changes, here's an idea.
> 
> What if we require the patch author to supply the value of 'n' instead
> of supplying the symbol address?  We could get rid of 'old_addr' as an
> input in klp_func and and replace it with 'old_sympos' which has the
> value of 'n'.  Or alternatively we could require old_name to be of the
> format "func,n".

I like the idea of old_sympos better than modifying the string.
In addition if no old_sympos is specified then it should default to 0,
since this will probably be the more common case.

> 
> That would uniquely identify each patched function, even _before_ the
> object is loaded.
> 
> It would also fix another big problem we have today, where there's no
> way to disambiguate duplicate symbols in modules, for both function
> addresses and for relocs.
> 
> It would simplify the code in other places as well: no special handling
> for kASLR, no need for klp_verify_vmlinux_symbol() vs
> klp_find_object_symbol().
> 
> A drawback is that it requires the patch author to do a little more due
> diligence when filling out klp_func.  But we already require them to be
> careful.
> 
> Thoughts?
> 

I'll hold off on my v3 for now. Very interesting discussion : ).
--chris

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