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

Marco Elver
2020-08-07T09:01:44Z

On Thu, 6 Aug 2020 at 18:06, Marco Elver <elver@google.com> wrote:
> 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.)

I let it run on syzbot through the night, and it's fine without
PARAVIRT (see below). I have sent the patch (need your Signed-off-by
as it's based on your code, thank you!):
https://lkml.kernel.org/r/20200807090031.3506555-1-elver@google.com

> 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.

Never mind, I get these warnings even if I don't turn on KCSAN
(CONFIG_KCSAN=n). Something else is going on with PARAVIRT=y that
throws off IRQ state tracking. :-/

Thanks,
-- Marco