linux-kernel - Re: [PATCH v2 14/18] lib/test_lockup: fix kernel pointer check for separate address spaces

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Date:   Fri, 18 Feb 2022 08:15:42 +0100
From:   Arnd Bergmann <arnd@...nel.org>
To:     Christoph Hellwig <hch@....de>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        Linux API <linux-api@...r.kernel.org>,
        Arnd Bergmann <arnd@...db.de>,
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Subject: Re: [PATCH v2 14/18] lib/test_lockup: fix kernel pointer check for
 separate address spaces

On Fri, Feb 18, 2022 at 7:35 AM Christoph Hellwig <hch@....de> wrote:
>
> On Wed, Feb 16, 2022 at 02:13:28PM +0100, Arnd Bergmann wrote:
> > From: Arnd Bergmann <arnd@...db.de>
> >
> > test_kernel_ptr() uses access_ok() to figure out if a given address
> > points to user space instead of kernel space. However on architectures
> > that set CONFIG_ALTERNATE_USER_ADDRESS_SPACE, a pointer can be valid
> > for both, and the check always fails because access_ok() returns true.
> >
> > Make the check for user space pointers conditional on the type of
> > address space layout.
>
> What is this code even trying to do?  It looks extremly broken.

As I understand it, this is only meant for debugging, and the module contains
intentionally broken lock usage to test whether the watchdog and lockup
detection in the kernel is able to find them.

I did not try that hard to understand how it works though.

      Arnd