Re: KMSAN broken with lockdep again?

[Eric Biggers]

On Thu, Nov 17, 2022 at 02:46:29PM +0100, Alexander Potapenko wrote:
> On Wed, Nov 16, 2022 at 9:12 PM Eric Biggers <ebiggers@xxxxxxxxxx> wrote:
> >
> > Hi,
> >
> > I'm trying v6.1-rc5 with CONFIG_KMSAN, but the kernel continuously spams
> > "BUG: KMSAN: uninit-value in __init_waitqueue_head".
> >
> > I tracked it down to lockdep (CONFIG_PROVE_LOCKING=y).  The problem goes away if
> > I disable that.
> >
> > I don't see any obvious use of uninitialized memory in __init_waitqueue_head().
> >
> > The compiler I'm using is tip-of-tree clang (LLVM commit 4155be339ba80fef).
> >
> > Is this a known issue?
> >
> > - Eric
> 
> Thanks for flagging this!
> 
> The reason behind that is that under lockdep we're accessing the
> contents of wq_head->lock->dep_map, which KMSAN considers
> uninitialized.
> The initialization of dep_map happens inside kernel/locking/lockdep.c,
> for which KMSAN is deliberately disabled, because lockep used to
> deadlock in the past.
> 
> As far as I can tell, removing `KMSAN_SANITIZE_lockdep.o := n` does
> not actually break anything now (although the kernel becomes quite
> slow with both lockdep and KMSAN). Let me experiment a bit and send a
> patch.
> If this won't work out, we'll need an explicit call to
> kmsan_unpoison_memory() somewhere in lockdep_init_map_type() to
> suppress these reports.

Thanks.

I tried just disabling CONFIG_PROVE_LOCKING, but now KMSAN warnings are being
spammed from check_stack_object() in mm/usercopy.c.

Commenting out the call to arch_within_stack_frames() makes it go away.

- Eric