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

[Marco Elver]

On Wed, Aug 05, 2020 at 07:31PM +0200, Marco Elver wrote:
...
> Oh well, it seems that KCSAN on syzbot still crashes even with this
> "fix". It's harder to reproduce though, and I don't have a clear
> reproducer other than "fuzz the kernel" right now. I think the new IRQ
> state tracking code is still not compatible with KCSAN, even though we
> thought it would be. Most likely there are still ways to get recursion
> lockdep->KCSAN. An alternative would be to deal with the recursion
> like we did before, instead of trying to squash all of it. I'll try to
> investigate -- Peter, if you have ideas, help is appreciated.

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.

Thanks,
-- Marco

------ >8 ------

diff --git a/init/main.c b/init/main.c
index 15bd0efff3df..0873319dcff4 100644
--- a/init/main.c
+++ b/init/main.c
@@ -1041,6 +1041,22 @@ asmlinkage __visible void __init start_kernel(void)
 	sfi_init_late();
 	kcsan_init();
 
+	/* DEBUG CODE */
+	lockdep_assert_irqs_enabled(); /* Pass. */
+	{
+		unsigned long flags1;
+		raw_local_irq_save(flags1);
+		{
+			unsigned long flags2;
+			lockdep_assert_irqs_enabled(); /* Pass - expectedly blind. */
+			local_irq_save(flags2);
+			lockdep_assert_irqs_disabled(); /* Pass. */
+			local_irq_restore(flags2);
+		}
+		raw_local_irq_restore(flags1);
+	}
+	lockdep_assert_irqs_enabled(); /* FAIL! */
+
 	/* Do the rest non-__init'ed, we're now alive */
 	arch_call_rest_init();