Re: [PATCH v4 13/15] livepatch: change to a per-task consistency model

From: Josh Poimboeuf <jpoimboe@redhat.com>
To: Miroslav Benes <mbenes@suse.cz>
Cc: Jessica Yu <jeyu@redhat.com>, Jiri Kosina <jikos@kernel.org>,
	Petr Mladek <pmladek@suse.com>,
	linux-kernel@vger.kernel.org, live-patching@vger.kernel.org,
	Michael Ellerman <mpe@ellerman.id.au>,
	Heiko Carstens <heiko.carstens@de.ibm.com>,
	x86@kernel.org, linuxppc-dev@lists.ozlabs.org,
	linux-s390@vger.kernel.org, Vojtech Pavlik <vojtech@suse.com>,
	Jiri Slaby <jslaby@suse.cz>,
	Chris J Arges <chris.j.arges@canonical.com>,
	Andy Lutomirski <luto@kernel.org>, Ingo Molnar <mingo@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Kamalesh Babulal <kamalesh@linux.vnet.ibm.com>,
	Balbir Singh <bsingharora@gmail.com>
Subject: Re: [PATCH v4 13/15] livepatch: change to a per-task consistency model
Date: Mon, 6 Feb 2017 09:58:28 -0600	[thread overview]
Message-ID: <20170206155828.7mivuyzdtre4dato@treble> (raw)
In-Reply-To: <alpine.LSU.2.20.1702031724040.28531@pobox.suse.cz>

On Fri, Feb 03, 2017 at 05:41:28PM +0100, Miroslav Benes wrote:
> 
> Petr has already mentioned majority of things I too found out, so only 
> couple of nits...
> 
> > diff --git a/Documentation/ABI/testing/sysfs-kernel-livepatch b/Documentation/ABI/testing/sysfs-kernel-livepatch
> > index da87f43..24b6570 100644
> > --- a/Documentation/ABI/testing/sysfs-kernel-livepatch
> > +++ b/Documentation/ABI/testing/sysfs-kernel-livepatch
> > @@ -25,6 +25,14 @@ Description:
> >  		code is currently applied.  Writing 0 will disable the patch
> >  		while writing 1 will re-enable the patch.
> >  
> > +What:		/sys/kernel/livepatch/<patch>/transition
> > +Date:		May 2016
> 
> 'May 2016' looks strange, but maybe nobody cares about it...

Will update the date to Feb 2017 and also the affected kernel version to
4.12.

> 
> > diff --git a/Documentation/livepatch/livepatch.txt b/Documentation/livepatch/livepatch.txt
> > index 7f04e13..fb00d66 100644
> > --- a/Documentation/livepatch/livepatch.txt
> > +++ b/Documentation/livepatch/livepatch.txt
> >  4. Livepatch module
> > @@ -134,7 +242,7 @@ Documentation/livepatch/module-elf-format.txt for more details.
> >  
> >  
> >  4.2. Metadata
> > -------------
> > +-------------
> 
> klp_func and klp_patch have new members - immediate. Should be documented 
> here in "4.2. Metadata" section.

Agreed.

> The section also contains this text under klp_patch bullet. It seems 
> oudated:
> 
> "Also if a more complex consistency model is supported then a selected 
> unit (thread, kernel as a whole) will see the new code from the entire 
> patch only when it is in a safe state."
> 
> We now have a more complex consistency model.

Agreed.

> And finally, the section "Limitations" has this text under the first 
> bullet:
> 
>   + The patch must not change the semantic of the patched functions.
> 
>     The current implementation guarantees only that either the old
>     or the new function is called. The functions are patched one
>     by one. It means that the patch must _not_ change the semantic
>     of the function.
> 
> I think it is confusing. The consistency model allows us to change the 
> semantic of a function. To certain degree. Of course, there are cases that 
> cannot be patched, or have to be patched carefully. For example if a 
> function takes a lock by calling foo_lock(), foo_lock() is not on a stack 
> afterwards. Then the locking semantics may be changed with a livepatch. 
> One has to make sure to patch also the caller foo_lock() to enforce the 
> consistency. And so on... But I do not consider a limitation of livepatch. 
> It is a feature of the consistency model, which is weaker than kGraft's or 
> kpatch's (or stronger. It depends on your point of view.)
> 
> So, I propose to remove this text and better describe the properties of 
> the consistency model above in the section 3. Maybe a quote from an old 
> mail thread (Nov 2014) would be sufficient. I don't remember what was 
> mentioned and what not.
> 
> What do you think?

I'll remove the above limitation.

I'm not sure how to improve the consistency model section.  It already
has at least some mentions of changed function semantics and locking
semantics.  I'll leave it alone for now, unless you have a specific
suggestion.

> > diff --git a/include/linux/livepatch.h b/include/linux/livepatch.h
> > index 6602b34..ed90ad1 100644
> > --- a/include/linux/livepatch.h
> > +++ b/include/linux/livepatch.h
> > @@ -68,7 +92,7 @@ struct klp_func {
> >   * @funcs:	function entries for functions to be patched in the object
> >   * @kobj:	kobject for sysfs resources
> >   * @mod:	kernel module associated with the patched object
> > - * 		(NULL for vmlinux)
> > + *		(NULL for vmlinux)
> 
> This looks superfluous.

This is a minor whitespace fix -- remove a space before tab.  I figured
I'd go ahead and fix it since I'm already changing some of the
surrounding code.

> (checking my notes)... and that's it. Aside from the discussion in Petr's 
> subthread it looks good to me. Great job.

Thanks!

-- 
Josh