Re: [PATCH] x86/paravirt: Add missing noinstr to arch_local*() helpers

Marco Elver
2020-08-06T16:28:12Z

On Thu, 6 Aug 2020 at 15:17, Marco Elver <elver@google.com> wrote:
>
> On Thu, Aug 06, 2020 at 01:32PM +0200, peterz@infradead.org wrote:
> > On Thu, Aug 06, 2020 at 09:47:23AM +0200, Marco Elver wrote:
> > > Testing my hypothesis that raw then nested non-raw
> > > local_irq_save/restore() breaks IRQ state tracking -- see the reproducer
> > > below. This is at least 1 case I can think of that we're bound to hit.
> ...
> >
> > /me goes ponder things...
> >
> > How's something like this then?
> >
> > ---
> > include/linux/sched.h | 3 ---
> > kernel/kcsan/core.c | 62 ++++++++++++++++++++++++++++++++++++---------------
> > 2 files changed, 44 insertions(+), 21 deletions(-)
>
> Thank you! That approach seems to pass syzbot (also with
> CONFIG_PARAVIRT) and kcsan-test tests.
>
> I had to modify it some, so that report.c's use of the restore logic
> works and not mess up the IRQ trace printed on KCSAN reports (with
> CONFIG_KCSAN_VERBOSE).
>
> I still need to fully convince myself all is well now and we don't end
> up with more fixes. :-) If it passes further testing, I'll send it as a
> real patch (I want to add you as Co-developed-by, but would need your
> Signed-off-by for the code you pasted, I think.)

With CONFIG_PARAVIRT=y (without the notrace->noinstr patch), I still
get lockdep DEBUG_LOCKS_WARN_ON(!lockdep_hardirqs_enabled()), although
it takes longer for syzbot to hit them. But I think that's expected
because we can still get the recursion that I pointed out, and will
need that patch.

I also get some "BUG: MAX_LOCKDEP_CHAINS too low!" on syzbot (KCSAN is
not in the stacktrace). Although it may be unrelated:
https://lore.kernel.org/lkml/0000000000005613c705aaf88e04@google.com/
-- when are they expected?

Thanks,
-- Marco