Re: [PATCH v1 2/2] perf/core: Fake regs for leaked kernel samples

Peter Zijlstra
2020-08-11T08:46:09Z

On Tue, Aug 11, 2020 at 04:31:10PM +0800, Jin, Yao wrote:
> Hi Peter,
>
> On 8/11/2020 3:59 PM, Peter Zijlstra wrote:
> > On Tue, Aug 11, 2020 at 03:50:43PM +0800, Jin, Yao wrote:
> > > Could I post v2 which basically refers to your patch but removes some
> > > conditions since I see some issues in test if we use these conditions.
> > >
> > > 1. Remove '!event->attr.exclude_hv || !event->attr.exclude_host ||
> > > !event->attr.exclude_guest' at the entry of sanitize_sample_regs().
> > >
> > > 2. Remove '!attr.exclude_hv || !attr.exclude_host || !attr.exclude_guest'
> > > at the perf_event_open syscall entry.
> >
> > exclude_host, maybe -- due to the dodgy semantics of it, but the others
> > should definitely be there.
> >
>
> exclude_guest and exclude_hv are tricky too.
>
> If we do 'perf record -e cycles:u' in both host and guest, we can see:
>
> event->attr.exclude_guest = 0
>
> thus sanitize_sample_regs() returns regs directly even if exclude_kernel = 1.
>
> And in guest, exclude_hv = 0, it's out of my expectation too.

I'm confused, how can 'perf record -e cycles:u' _ever_ have
exclude_guest=0, exclude_hv=0 ? That simply makes no sense and is utterly
broken.

You explicitly ask for userspace-only, reporting hypervisor or guest
events is a straight up bug.