Re: [PATCH v2 01/11] x86: kcov: disable instrumentation of arch/x86/kernel/tsc.c

[Peter Zijlstra]

On Fri, Jun 27, 2025 at 12:51:47PM +0200, Alexander Potapenko wrote:
> On Fri, Jun 27, 2025 at 9:59 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> >
> > On Thu, Jun 26, 2025 at 03:41:48PM +0200, Alexander Potapenko wrote:
> > > sched_clock() appears to be called from interrupts, producing spurious
> > > coverage, as reported by CONFIG_KCOV_SELFTEST:
> >
> > NMI context even. But I'm not sure how this leads to problems. What does
> > spurious coverage even mean?
> 
> This leads to KCOV collecting slightly different coverage when
> executing the same syscall multiple times.
> For syzkaller that means higher chance to pick a less interesting
> input incorrectly assuming it produced some new coverage.
> 
> There's a similar discussion at
> https://lore.kernel.org/all/20240619111936.GK31592@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/T/#u

Clearly I'm not remembering any of that :-)

Anyway, looking at kcov again, all the __sanitize_*() hooks seem to have
check_kcov_mode(), which in turn has something like:

 if (!in_task() ..)
   return false;

Which should be filtering out all these things, no? If this filter
'broken' ?