Re: [RFC PATCH 6/9] livepatch: create per-task consistency model

[Josh Poimboeuf]

On Thu, Feb 12, 2015 at 02:16:07PM +0100, Jiri Kosina wrote:
> On Thu, 12 Feb 2015, Peter Zijlstra wrote:
> 
> > > The short answer is: I need a way to ensure that a task isn't sleeping
> > > on any of the functions we're trying to patch.  If it's not, then I can
> > > switch the task over to start using new versions of functions.
> > > 
> > > Obviously, there are many more details than that.  If you have specific
> > > questions I can try to answer them.
> > 
> > How can one task run new and another task old functions? Once you patch
> > any indirect function pointer any task will see the new call.
> 
> Patched functions are redirected through ftrace trampoline, and decision 
> is being made there which function (old or new) to redirect to.
> 
> Function calls through pointer always go first to the original function, 
> and get redirected from its __fentry__ site.
> 
> Once the system is in fully patched state, the overhead of the trampoline 
> is reduced (no expensive decision-making to be made there, etc) to 
> minimum.
> 
> Sure, you will never be on a 100% of performance of the unpatched kernel 
> for redirected functions, the indirect call through the trampoline will 
> always be there (although ftrace with dynamic trampolines is really 
> minimizing this penalty to few extra instructions, one extra call and one 
> extra ret being the expensive ones).
> 
> > And what's wrong with using known good spots like the freezer?
> 
> It has undefined semantics when it comes to what you want to achieve here.
> 
> Say for example you have a kernel thread which does something like
> 
> while (some_condition) {
> 	ret = foo();
> 	...
> 	try_to_freeze();
> 	...
> }
> 
> and you have a livepatch patching foo() and changing its return value 
> semantics. Then freezer doesn't really help.

Don't we have the same issue with livepatch?  For example:

while (some_condition) {
	ret = foo();
	...
	schedule(); <-- switch to the new universe while it's sleeps
	...
	// use ret in an unexpected way
}

I think it's not really a problem, just something the patch author needs
to be aware of regardless.  It should be part of the checklist.  You
always need to be extremely careful when changing a function's return
semantics.

IIRC, when I looked at the freezer before, the biggest problems I found
were that it's too disruptive to the process, and that not all kthreads
are freezable.  And I don't see anything inherently safer about it
compared to just stack checking.

-- 
Josh
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