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

[Vojtech Pavlik]

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.

-- 
Vojtech Pavlik
Director SUSE Labs