linux-kernel - Re: [RFC PATCH v1.9 12/14] livepatch: create per-task consistency model

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Message-ID: <20160405135321.y4hmoghfa2ukwj5w@treble.redhat.com>
Date:	Tue, 5 Apr 2016 08:53:21 -0500
From:	Josh Poimboeuf <jpoimboe@...hat.com>
To:	Vojtech Pavlik <vojtech@...e.com>
Cc:	Miroslav Benes <mbenes@...e.cz>, Jiri Kosina <jikos@...nel.org>,
	Jessica Yu <jeyu@...hat.com>, linux-kernel@...r.kernel.org,
	live-patching@...r.kernel.org
Subject: Re: [RFC PATCH v1.9 12/14] livepatch: create per-task consistency
 model

On Tue, Apr 05, 2016 at 01:36:34PM +0200, Vojtech Pavlik wrote:
> On Mon, Apr 04, 2016 at 01:33:07PM -0500, Josh Poimboeuf wrote:
> > On Mon, Apr 04, 2016 at 08:27:59PM +0200, Vojtech Pavlik wrote:
> > > On Mon, Apr 04, 2016 at 01:21:38PM -0500, Josh Poimboeuf wrote:
> > > 
> > > > Hm, can you explain why it should be copied from the parent?
> > > > 
> > > > I'm thinking the above code is correct for today, but it should still be
> > > > changed to be more future-proof.
> > > > 
> > > > Here's my thinking:
> > > > 
> > > > A forked task starts out with no stack, so if I understand correctly, it
> > > > can safely start out in the goal universe, regardless of which universe
> > > > its parent belongs to.
> > > > 
> > > > However, the current ret_from_fork code is a mess, and Andy Lutomirski
> > > > has mentioned that he would like to give newly forked tasks a proper
> > > > stack such that instead of jumping to ret_from_fork, they would just
> > > > return from schedule().  In that case, it would no longer be safe to
> > > > start the new task in the goal universe because it could be "sleeping"
> > > > on a to-be-patched function.
> > > > 
> > > > So for proper future proofing, newly forked tasks should be started in
> > > > the initial universe (rather than starting in the goal universe or
> > > > inheriting the parent's universe).  They can then be transitioned over
> > > > to the goal universe like any other task.  How does that sound?
> > > 
> > > How could a newly forked task start in the old universe if its parent
> > > has already been migrated? Any context it inherits will already be from
> > > the new universe.
> > 
> > Can you be more specific about "context" and why inheritance of it would
> > be a problem?
> 
> Currently a forked task starts out with no stack, and as such it can
> start in the goal universe.
> 
> If we create a synthetic stack, then we may need to start in the initial
> universe, as the synthetic stack would likely be created using initial
> universe return addresses. 
> 
> If we simply copy the stack of the parent process, which is in my
> opionion the safest way, as it places little assumptions on the
> compiler, then it may contain either old or new addresses
> and we need to copy the universe flag along.

That all makes sense.  I'll change it to inherit the parent's universe
for now.

-- 
Josh