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

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:

quoted

On Thu, Jan 21, 2021 at 10:37:21PM +0530, Naresh Kamboju wrote:

quoted

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

[...]

quoted

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 (local)

Very good, then my workaround (shown below for Naresh's ease of testing)
is only a short-term workaround.  Yay!  ;-)

							Thanx, Paul

------------------------------------------------------------------------

diff --git a/mm/slab_common.c b/mm/slab_common.c
index cefa9ae..a8375d1 100644
--- a/mm/slab_common.c
+++ b/mm/slab_common.c

@@ -550,7 +550,8 @@ bool kmem_valid_obj(void *object)
 {
 	struct page *page;
 
-	if (!virt_addr_valid(object))
+	/* Some arches consider ZERO_SIZE_PTR to be a valid address. */
+	if (object < (void *)PAGE_SIZE || !virt_addr_valid(object))
 		return false;
 	page = virt_to_head_page(object);
 	return PageSlab(page);