Subject: Re: rcu-torture: Internal error: Oops: 96000006

From: Will Deacon <[email-masked]>
Date: Thu, 21 Jan 2021 21:31:10 +0000

On Thu, Jan 21, 2021 at 10:55:21AM -0800, Paul E. McKenney wrote:
> On Thu, Jan 21, 2021 at 10:37:21PM +0530, Naresh Kamboju wrote:
> > While running rcu-torture test on qemu_arm64 and arm64 Juno-r2 device
> > the following kernel crash noticed. This started happening from Linux next
> > next-20210111 tag to next-20210121.
> > 
> > metadata:
> >   git branch: master
> >   git repo: https://gitlab.com/Linaro/lkft/mirrors/next/linux-next
> >   git describe: next-20210111
> >   kernel-config: https://builds.tuxbuild.com/1muTTn7AfqcWvH5x2Alxifn7EUH/config
> > 
> > output log:
> > 
> > [  621.538050] mem_dump_obj() slab test: rcu_torture_stats =
> > ffff0000c0a3ac40, &rhp = ffff800012debe40, rhp = ffff0000c8cba000, &z
> > = ffff8000091ab8e0
> > [  621.546662] mem_dump_obj(ZERO_SIZE_PTR):
> > [  621.546696] Unable to handle kernel NULL pointer dereference at
> > virtual address 0000000000000008

[...]

> Huh.  I am relying on virt_addr_valid() rejecting NULL pointers and
> things like ZERO_SIZE_PTR, which is defined as ((void *)16).  It looks
> like your configuration rejects NULL as an invalid virtual address,
> but does not reject ZERO_SIZE_PTR.  Is this the intent, given that you
> are not allowed to dereference a ZERO_SIZE_PTR?
> 
> Adding the ARM64 guys on CC for their thoughts.

Spooky timing, there was a thread _today_ about that:

https://lore.kernel.org/r/[email-masked]

Will