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

[Marco Elver]

On Fri, Aug 07, 2020 at 12:35PM +0200, Jürgen Groß wrote:
> On 07.08.20 11:50, Marco Elver wrote:
> > On Fri, Aug 07, 2020 at 11:24AM +0200, Jürgen Groß wrote:
> > > On 07.08.20 11:01, Marco Elver wrote:
> > > > On Thu, 6 Aug 2020 at 18:06, Marco Elver <elver@xxxxxxxxxx> wrote:
> > > > > On Thu, 6 Aug 2020 at 15:17, Marco Elver <elver@xxxxxxxxxx> wrote:
> > > > > > On Thu, Aug 06, 2020 at 01:32PM +0200, peterz@xxxxxxxxxxxxx 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@xxxxxxxxxx
> > > > 
> > > > > 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. :-/
> > > 
> > > What are the settings of CONFIG_PARAVIRT_XXL and
> > > CONFIG_PARAVIRT_SPINLOCKS in this case?
> > 
> > I attached a config.
> > 
> > 	$> grep PARAVIRT .config
> > 	CONFIG_PARAVIRT=y
> > 	CONFIG_PARAVIRT_XXL=y
> > 	# CONFIG_PARAVIRT_DEBUG is not set
> > 	CONFIG_PARAVIRT_SPINLOCKS=y
> > 	# CONFIG_PARAVIRT_TIME_ACCOUNTING is not set
> > 	CONFIG_PARAVIRT_CLOCK=y
> 
> Anything special I need to do to reproduce the problem? Or would you be
> willing to do some more rounds with different config settings?

I can only test it with syzkaller, but that probably doesn't help if you
don't already have it set up. It can't seem to find a C reproducer.

I did some more rounds with different configs.

> I think CONFIG_PARAVIRT_XXL shouldn't matter, but I'm not completely
> sure about that. CONFIG_PARAVIRT_SPINLOCKS would be my primary suspect.

Yes, PARAVIRT_XXL doesn't make a different. When disabling
PARAVIRT_SPINLOCKS, however, the warnings go away.

Thanks,
-- Marco