rcu-torture: Internal error: Oops: 96000006

[Mark Rutland]

On Thu, Jan 21, 2021 at 01:43:14PM -0800, Paul E. McKenney wrote:
> On Thu, Jan 21, 2021 at 09:31:10PM +0000, Will Deacon wrote:
> > 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/ecbc7651-82c4-6518-d4a9-dbdbdf833b5b@arm.com
> 
> Very good, then my workaround (shown below for Naresh's ease of testing)
> is only a short-term workaround.  Yay!  ;-)

Hopefully, though we might need to check other architectures beyond
arm64, ppc, and x86, to be certain!

Is there any other latent use of virt_addr_valid() that needs this
semantic? If so we'll probably want to backport the changes to arm64's
implementation, at least for v5.10.

Vincenzo, would you mind taking a look?

Thanks,
Mark.