Re: Dealing with the NMI mess

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Jul 24, 2015 at 03:21:28PM +0200, Willy Tarreau wrote:
> On Fri, Jul 24, 2015 at 09:03:42AM -0400, Steven Rostedt wrote:
> > On Fri, 24 Jul 2015 14:43:04 +0200
> > Peter Zijlstra <peterz@infradead.org> wrote:
> > 
> >  
> > > > I'm not too familiar with how to use hw breakpoints, but I'm guessing
> > > > (correct me if I'm wrong) that breakpoints on code that trigger when
> > > > executed, but watchpoints on data trigger when accessed. Then
> > > > copy_from_user_inatomic() would only trigger on watchpoints (it's not
> > > > executing that code, at least I hope it isn't!), and those wont bother
> > > > us.
> > > 
> > > These things can be: RW, W, X.
> > > 
> > > Sure, hitting a user X watchpoint is going to be 'interesting', but its
> > > fairly easy to hit a RW one.
> > 
> > But do we care if we do hit one? The return from the #DB handler can
> > use a RET. Right?

Look at do_debug(), it has lovely bits like:

	preempt_conditional_sti();

in it, we do _NOT_ want to be re-enabling interrupts if we're called
from an !IF context, that'd be _bad_.

> My understanding is that by using RET we can't set the RF flag and #DB
> will immediately strike again when the operation is attempted again. Thus
> we have to completely disable the breakpoints on leaving after the first
> one strikes, resulting in some userland breakpoints being missed. Maybe
> it can be accepted as a limitation when perf is running. I don't know if
> the output of perf is that relevant when a debugger is present BTW.

The patch I posted will re-enable the breakpoints before returning to
userspace. So userspace will only 'miss' events generated by the kernel.

Missing reads from the kernel is not a problem -- and maybe even
expected, but certainly unavoidable.

Missing updates from the kernel might be a problem, you'd get a variable
change content even though you have a W watchpoint on it, that'd be
surprising.

Then again, I suppose we can argue the variable changed through another
mapping and watchpoints work on the virtual address, so tough cookies or
somesuch -- the kernel could in fact do this on highmem kernel anyway.